Introduction
Over the past two centuries, the law of naval warfare has undergone moments of partial codification, expansion, fragmentation and exceptionalism. As will be outlined in this article, the “classic” or “traditional” law of naval warfare that existed at the time of the 1909 London Declaration concerning the Laws of Naval WarFootnote 1 and 1913 Manual of the Laws of Naval War (Oxford Manual)Footnote 2 was relatively restrained when it came to targeting. While lawfully targeting enemy belligerent warships outside of neutral waters was always foreseen within the law, the treatment of merchant ships was much more nuanced. The “traditional” law allowed for direct attacks against enemy or neutral merchant vessels only when they were directly supporting the naval hostilities of the belligerent, thus requiring a significant level of involvement. Beyond this narrow scope, neutral merchant vessels engaged in actions in support of a belligerent were principally subject only to capture, while enemy merchant vessels were, under the traditional law of naval warfare, always liable to capture irrespective of their activities at sea. Of course, methods of naval warfare including the lawful exercise of visit and search, such as for contraband control or blockade enforcement, did allow for belligerents to use force up to and including rendering a vessel incapable of navigation or sinking it if it persistently refused to stop upon being summoned or actively resisted boarding. However, sinking a vessel in such circumstances was effectively a last resort.Footnote 3
The experience of the First World War shattered this classic framework. Belligerent governments expanded considerably the range of goods deemed contraband (expansions that were constantly supported by their prize courts) and took more lethal actions against merchant ships in support of the war effort.Footnote 4 Following the First World War, an attempt was made in the 1936 Procès-Verbal Relating to the Rules of Submarine Warfare Set Forth in Part IV of the Treaty of London of 22 April 1930 (1936 Procès-Verbal)Footnote 5 to reaffirm the restraint to be applied to the destruction of merchant vessels and to preserve humanitarian considerations. Despite this, the Second World War saw an unprecedented expansion of attacks against merchant vessels that clearly went beyond the “classic” or “traditional” law of naval warfare. Arguments in support of this expansion included reprisal action,Footnote 6 grounds that merchant ships had received orders to ram submarines,Footnote 7 the declaration of large submarine operational zones where authorization was provided to sink all enemy merchant ships, and broad views of assimilation of both enemy and merchant ships into the naval war effort.Footnote 8 The war in practice saw a policy that began to look a lot like sinking merchant ships on sight.
In the aftermath of the two World Wars and the adoption of the United Nations (UN) Charter, determining the exact rules for conducting hostilities at sea became a particular challenge. Although eminent scholars in the 1950s still articulated it in its “classic” or “traditional” terms,Footnote 9 it remained unclear whether the law had in fact evolved particularly as a result of the decisive experience of the Second World War. Of note, around the same time, the International Committee of the Red Cross (ICRC) observed that issues related to the safety of civilian navigation required further study by States and suggested that the outcome of such studies might, “at the appropriate moment, … be appended as an annex to the present rules and might deal more particularly with the safety of civil navigation”. Those “present rules” would later become the 1977 Additional Protocol I (AP I).Footnote 10
In the end, however, no such annex to AP I on the safety of civilian navigation was adopted. The distinction between the land and maritime domains – largely due to IHL’s historical development alongside prize law and the rules of maritime neutrality – was particularly apparent during the AP I negotiations. States disagreed over how far the rules on the conduct of hostilities – particularly those found in Part IV, Section I (Articles 48–67) of AP I – should apply at sea. Article 49(3) reflected this tension, limiting the application of these rules to “sea warfare that may affect the civilian population, individual civilians or civilian objects on land. They further apply to all attacks from the sea … against objectives on land.” The Diplomatic Conference’s reticence to address the issue of naval warfare in a comprehensive manner can be understood considering the unsettled state of the law at the time.Footnote 11
It was this legal uncertainty that led to the drafting of the San Remo Manual on International Law Applicable to Armed Conflicts at Sea (San Remo Manual) in the late 1980s and early 1990s. Against this backdrop, the San Remo Manual drafters’ deliberate albeit cautious decision to draw upon AP I targeting concepts can be understood and assessed.Footnote 12 If the law of naval warfare had in fact changed as a result of the Second World War experience to permit sinking merchant vessels solely for their potential role in the war effort, then importing AP I’s targeting concepts would have undoubtedly imposed important humanitarian limits on such attacks. However, if the Second World War experience was sui generis and the “traditional” or “classic” law of naval warfare was to remain the original reference point, then combining AP I’s comparatively broader definition of what counts as a “military objective” with traditional law of naval warfare rules may not have represented as significant a humanitarian development as the San Remo Manual drafters might have assumed.Footnote 13
Nearly fifty years after its adoption, the full extent of AP I’s influence on the law of naval warfare remains uncertain. Although frequently invoked in academic commentary, its precise impact has yet to be subjected to comprehensive and systematic assessment.Footnote 14 Although not all States are party to AP I, and scholarly debate persists as to the extent to which its rules on the conduct of hostilities extend to naval warfare that does not affect civilians ashore, many States nevertheless regard its provisions as highly relevant to the maritime domain. Many of these rules are considered reflective of customary international law, applying across all domains of warfare.Footnote 15 Moreover, due to the multi-domain nature of contemporary armed conflict, questions arise as to the extent to which developments in customary international humanitarian law (IHL) governing the conduct of hostilities in other domains – such as land, air, cyber, and outer space – may influence the rules governing the conduct of hostilities in the maritime domain.
Since the adoption of AP I and the publication of the San Remo Manual, State practice and the outputs of expert bodiesFootnote 16 have revealed diverse interpretations and applications of the treaty and customary rules governing hostilities at sea, particularly those stemming from the principles of distinction (i.e., both people and objects), proportionality and precaution.
Meanwhile, the maritime domain – a busy global commons – has seen dramatic changes in how conflicts are, and will in the future be, waged within it. For example, merchant shipping, now the backbone of the global economy, looks nothing like it did at the end of the Second World War. It is now highly globalized, deeply interconnected, and less tied to any single State.Footnote 17 Means and methods of naval warfare have also evolved: technological advancements now allow for merchant vessels to be directly attacked or incidentally harmed more cost-effectively, over greater distances and with far greater destructive force. These changes have increased risks for civilian mariners and heightened the stakes for maritime commerce during armed conflict.
Yet even as the need to think through the importance of ensuring legal and practical protection for merchant vessels during armed conflict at sea becomes increasingly vital, their ongoing protection must be considered in light of the long-standing practice of States. Throughout history, warring parties have relied on merchant shipping to support their war efforts – not only by transporting troops and military logistics or even taking “direct part in hostilities”,Footnote 18 but also to search for, collect and care for the wounded, sick, shipwrecked and dead, as well as to deliver humanitarian assistance and conduct evacuation operations. These practices are all likely to persist in future conflicts at sea, particularly in large-scale conflicts.Footnote 19 The challenge therefore lies in striking a careful balance between protecting merchant vessels and their crews and passengers, as well as the populations that depend on them, while facing the contemporary realities of modern naval warfare and ensuring that lawful military goals can be meaningfully attained.
This article traces the development of the law of naval warfare, examines the impact of AP I on the traditional law of naval warfare and analyzes some consequences of the San Remo Manual’s adoption of AP I as the benchmark for attacks against merchant vesselsFootnote 20 at sea. It argues for preserving the traditional law governing the use of force against merchant vessels in contexts such as visit and search, contraband control and blockade enforcement, highlighting the unique historical legacy of such methods.
The article will extensively engage with scenarios in which merchant vessels persistently refuse to stop upon being duly summoned or actively resist visit and search. In such scenarios, the precautions in attack under Article 57 of AP I largely reflected in San Remo Manual Rule 46 – especially the target verification requirements – cannot, in practice, be fully satisfied. In most cases, only a “reasonable suspicion” exists as to the vessel’s cargo, and any assessment of the anticipated military advantage of an attack will always remain inherently speculative. Moreover, the mere attempt to breach, or even the successful breach of, a blockade does not in itself satisfy the definition of a military objective under either treaty or customary international law, and vessels making (or succeeding in) such an attempt may not be made the object of direct attack simply for doing so. They may, however, be subjected to escalating force under the “traditional” law of naval warfare to compel compliance – including, if necessary, sinking – provided that sufficient opportunity is given for the vessel to submit to boarding. In this way, the “traditional” law preserves humanitarian considerations in its very requirements.
This article contends that there is an urgent need for greater clarity from States as to how the principles of distinction, proportionality and precaution are to be interpreted and applied in contemporary naval warfare, particularly as regards merchant vessels, cargo, civilian crew and passengers. Efforts to enhance protections for civilian navigation, including through the adoption of AP I targeting frameworks, have not been fully articulated, either in the San Remo Manual or by States themselves.
Only a limited number of national military manuals appear to address maritime operations in situations of armed conflict. Those that do generally offer sparse guidance; concepts drawn from AP I are invoked in a general manner, without sufficient explanation of their specific implications at sea. This article explores some of these uncertainties and identifies key areas in which States should articulate a considered and coherent position on the interpretation and application of the rules governing the conduct of hostilities in naval warfare today.
The “traditional” or “classic” law of naval warfare
Sources
Prior to the adoption of AP I in 1977, international armed conflict at sea – including aerial operations directed against maritime targets and producing no effects ashore – was governed by the pre-existing law of naval warfare. This traditional law was comprised of an intricate patchwork of treaty and customary international law, much of which is rooted in developments from the late nineteenth and early twentieth centuries.
Despite a long list of relevant instruments,Footnote 21 there has never been a comprehensive multilateral treaty on the conduct of all aspects of naval warfare, and the exact rules have always been subject to much debate between States. Many existing treaties have been ratified by only a limited number of States,Footnote 22 and this reflects, in part, the historical context in which these instruments were first adopted: colonial structures shaping international participation at the time excluded many people and political entities from international legal processes and from recognition as sovereign States.Footnote 23 Some instruments have since been superseded by more recent agreementsFootnote 24 or have never entered into force.Footnote 25
Except for the universally ratified Geneva Convention II of 1949Footnote 26 (which addresses the protection of wounded, sick and shipwrecked members of armed forces at sea), no multilateral treaty specifically focused on naval hostilities has been adopted since the 1930s, when the 1930 London Treaty – now no longer in force, save for the 1936 Procès-Verbal – was concluded. Although the 1909 London Declaration on the Laws of Naval WarFootnote 27 was signed by the mainly European countries that negotiated it, no State ever ratified it, so it never entered into force; despite this, the unratified London Declaration is said to have reflected the customary law of the time and is therefore influential.Footnote 28 Similarly, the 1913 Oxford ManualFootnote 29 provides useful commentary on what was understood to reflect customary international law at that time.Footnote 30
Main rules
In terms of the conduct of hostilities at sea, the collective influence of these traditional sources of law allowed for belligerent naval forces to capture and attack the warships and what we would now understand to be the auxiliaries of an enemy belligerent State.Footnote 31 However, to the extent that a warship or naval auxiliary constituted a cartel vessel (for example, vessels that “act as bearers of a flag of truce”)Footnote 32 or had surrendered,Footnote 33 it could not be attacked.
Traditionally, the primary operational objective of belligerent naval operations against merchant vessels was capture under prize law. The traditional law also allowed belligerent naval forces to stop and board any merchant vessel at sea (beyond territorial water limits of neutral States) to ascertain the status of the vessel and its cargo.Footnote 34 Enemy-flagged merchant ships, or ships otherwise found to have enemy character, were subject to capture at any time, regardless of the nature of their cargo;Footnote 35 however, there were treaty rules that exempted certain enemy-flagged vessels from capture, such as small coastal vessels and vessels engaged in religious, scientific or philanthropic missions. Hospital ships were also exempt from capture (provided that these vessels were employed in their innocent roles).Footnote 36
Neutral-flagged merchant vessels could also be subject to capture under prize law. The London Declaration distinguishes between two levels of “unneutral service” that permit such capture, categorized by the gravity of the conduct involved.Footnote 37 The first level involves a single, isolated act of unneutral service, which renders the vessel liable to capture under the rules of visit and search concerning the carrying of contraband.Footnote 38 In such cases, the vessel retains its neutral character and continues to benefit from the legal protections afforded to neutral vessels;Footnote 39 capture is permitted only during the specific voyage on which the unneutral service is being performed. Examples of conduct falling into this category include transport undertaken specifically to carry members of the enemy’s armed forces or to transmit intelligence on their behalf, or, where known to the owner, charterer or master, the carriage of an enemy military detachment or individuals who, during the voyage, directly assist enemy military operations.
The second, more serious level arises when a neutral vessel is continuously employed in the service of the enemy.Footnote 40 In such cases, that neutral vessel and its cargo was to be condemned and, in a general way, was to receive the same treatment as would be applicable to her if she were an enemy merchant vessel – namely, capture and subsequent condemnation (i.e., subject to forfeiture under prize proceedings).Footnote 41 This category applies when a vessel is under the orders or control of an agent placed on board by the enemy government, under the exclusive employment of the enemy government, exclusively engaged at the time in the transport of enemy troops or in the transmission of intelligence, or “tak[ing] a direct part in the hostilities”.Footnote 42
Some scholars have argued that by the end of the Second World War, State practice had expanded both the scope of what constituted unneutral service and the severity of its consequences. On this view, certain forms of conduct, particularly those within the second category described above, could, where necessary, render neutral vessels liable to attack and destruction on sight.Footnote 43 In 1955, Tucker asserted that the differing levels of unneutral service needed to be maintained, albeit with some accommodation of the experience of the Second World War:
The most serious forms of unneutral service occur when neutral merchant vessels (or neutral private aircraft) directly participate in the military operations of a belligerent, either by entering into the actual hostilities or by serving in any capacity as a naval or military auxiliary to belligerent forces (e. g., as colliers, troopships; laying of mines, reconnoitring). In performing these acts neutral merchant vessels (and aircraft) are considered to acquire an enemy character and must bear the same treatment accorded to enemy warships (and military aircraft). As such they are always liable to capture and – if necessary – to attack and destruction on sight.
The general principle involved is reasonably clear, and no attempt need be made to enumerate all of the acts that may result in this assimilation to an enemy’s armed forces. It is not the mere fact of assisting a belligerent that permits this severe treatment. Nor is it simply the consideration that the belligerent exercises a close control and direction over the neutral merchant vessel. The decisive consideration is rather that the services rendered are in direct support of the belligerent’s military operations. It is this support, leading as it does to the identification of the neutral merchant vessel (or aircraft) with the belligerent’s naval or military forces, that permits a treatment similar to that meted out to these forces.Footnote 44
Similarly, in 1967, drawing upon the 1909 London Declaration, Colombos drew a distinction between “grave” acts of unneutral service and “less grave” acts, noting that in the former case, neutral merchant ships were liable to attack, whereas in the latter they were subject to the same “treatment as an enemy merchant vessel”, namely capture.Footnote 45 In both Tucker and Colombos’ accounts, there is little discussion on the acts themselves that would situate an enemy or neutral merchant vessel within either category, but emphasis was placed upon the directness of the engagement in hostilities.
Meanwhile, a belligerent warship could also capture a neutral vessel that was engaged in an activity which, while not per se unneutral service, the belligerent was still lawfully entitled to prevent. This occurred if the vessel were suspected of carrying contraband or breaching a blockade.Footnote 46
Hence, when it comes to merchant vessels (enemy and neutral), the primary operational focus for the classic law of naval warfare was upon captureFootnote 47 – but to secure an actual condemnation of a merchant vessel, a hearing would have to be conducted by a prize court in the belligerent State. That court would consider the evidence, decide whether the vessel in question did in fact (1) have enemy character, (2) carry contraband or (3) seek to breach a blockade, and determine whether the vessel and/or cargo should be condemned or released. Having enemy character or seeking to breach a blockade were each sufficient grounds for the vessel and all its cargo to be condemned, while the penalty for carriage of contraband was more variable.Footnote 48
In exceptional circumstances, where operational imperatives demanded, a captured vessel could be sunk at the venue of capture (and hence not returned to a home port), provided that the passengers, the crew and the ship’s papers were placed in a position of safety.Footnote 49 This was much more controversial in the case of a neutral vessel, given the less certain grounds for condemnation.Footnote 50 As a result, the London Declaration allowed only narrow exceptions for destruction of captured neutral vessels based upon operational necessities.Footnote 51 The justification for such sinking would then need to be reviewed by a prize court to determine the lawfulness of the action, and compensation may be payable to the ship owner in circumstances where such sinking was not lawfully justified.Footnote 52
There remained uncertainty in the traditional law of naval warfare about whether a neutral merchant vessel which refused to submit to search obtained “enemy character” as a result.Footnote 53 In cases of “forcible resistance”, the London Declaration provided that the vessel and cargo would be subject to condemnation as if they had enemy character.Footnote 54
The traditional law of naval warfare governing when and how force could be used against merchant vessels
If a merchant vessel refused to submit to boarding, a belligerent could use force up to and including sinking that vessel. This applied, for example, to enemy merchant vessels actively resisting visit and search, as well as to neutral merchant vessels that had been suspected of carrying contraband or violating a legally established blockade and had refused belligerent orders to stop.Footnote 55 This principle was expressly reflected in the 1936 Procès-Verbal,Footnote 56 which carried over a key provision of the lapsed 1930 London Treaty (Article 22):
… except in the case of persistent refusal to stop on being duly summoned, or of active resistance to visit or search, a warship, whether surface vessel or submarine, may not sink or render incapable of navigation a merchant vessel without having first placed passengers, crew and ship’s papers in a place of safety.Footnote 57
Drafted long before modern notions such as AP I’s “military objective” concept, Article 22 remains in force for fifty-one States,Footnote 58 including several major naval powers, some of which rely on it to justify action against merchant vessels resisting lawful orders.Footnote 59 Its customary nature is also recognized in both the San Remo Manual and other academic manuals.Footnote 60
This rule, however, must be understood in its proper context. The primary aim of any interaction between belligerent warship and merchant vessel was capture. Accordingly, belligerents were permitted to use only “a degree of force necessary to compel the neutral vessel to submit to visit and search” if it refused to do so.Footnote 61
Importantly, the reference to “rendering [a vessel] incapable of navigation” as outlined in Article 22 did not denote an attack in the sense understood under the rules on the conduct of hostilities as codified in AP I since 1977. Rather, it referred to the use of forceful measures designed to halt the vessel’s movement in order to enable its capture. Even the sinking of a vessel was conceived within a graduated enforcement framework: the belligerent was first obliged to attempt to stop and seize the vessel, with sinking reserved as a last resort (i.e., the most extreme step on an escalating continuum of prize options).
Hence, Article 32 of the 1913 Oxford Manual detailed the steps that a belligerent warship needed to undertake to compel a merchant vessel to stop, including firing a “charge of powder” and then firing rounds across its bow. The terms “persistent refusal to stop” and “active resistance to visit and search” are not defined, but it is clear from both the language of the Oxford Manual and academic commentaryFootnote 62 that there were escalatory steps required before any decision could be made to sink the vessel or render it incapable of navigation.Footnote 63
In sum, the traditional law accepted that a belligerent would not incur responsibility for sinking or disabling a vessel – even one carrying passengers and crew – provided the vessel had persistently refused to stop upon being duly summoned or the crew had actively resisted, and that all prior, escalatory measures had been exhausted. Such action was lawful only when it was the last remaining means of prize law enforcement available.
A turning tide: AP I’s impact on the traditional law of naval warfare
The 1974–77 Diplomatic Conference and other preparatory work
Throughout the various preparatory meetings for AP I involving governmental and other experts, as well as the 1974–77 Diplomatic Conference itself, divergent views emerged regarding the extent to which the Protocol should apply to naval warfare. A principal source of contention concerned the scope of the section establishing the general rules for the protection of civilians against the effects of hostilities (later codified as Part IV, Section I, Articles 48–67).
There was unanimous agreement that Part IV, Section I should at least apply to military operations on land and to operations from the sea and air against persons and objects on land, particularly bombardments.Footnote 64 Beyond this, disagreement arose. States with significant naval forces warned that attempting a revision of existing treaty and customary law of naval warfare would be dangerous.Footnote 65 They also warned that, because the preparatory work by government experts had not focused specifically on naval or aerial warfare, any action by the Diplomatic Conference – whether deliberate or inadvertent – that affected these rules could render this part of the new Protocol unacceptable to States.Footnote 66
It was specifically noted by the UK representative that due to inconsistencies with instruments such as the 1930 London TreatyFootnote 67 and its 1936 Procès-Verbal,Footnote 68 as well as applicable customary international law, there was a “substantial contradiction” between the existing rules governing hostilities at sea and the proposed protections under AP I.Footnote 69 The UK representative noted that while more comprehensive protection might be afforded to merchant ships through a future change in the law, “a great deal of preparatory work would first be required to resolve the contradictions”.Footnote 70 Similarly, the Finnish delegate observed that the
contemporary rules of air and sea warfare were different from those of land warfare, especially where the protection of the civilian population was concerned, and the Conference was not in a position to alter them even if it found them unsatisfactory.Footnote 71
When the ICRC prepared its 1973 draft to serve as a basis for discussions at the forthcoming Diplomatic Conference, it took note of the above constraints and the fact that its call some two decades earlier to examine the legal protection of civilian shipping during armed conflict at sea had gone unheeded by States.Footnote 72 As such, the ICRC’s draft confined the application of Part IV, Section I to “any land, air, or sea warfare that may affect the civilian population, individual civilians, and civilian objects on land”.Footnote 73
In presenting that proposal, the ICRC implored that, although its draft addressed only civilians and civilian objects on land, if the Diplomatic Conference proved unable to make meaningful progress towards “resolving the problems of humanitarian rules at sea and in the air, it would be desirable for it at least to adopt a resolution inviting parties to a conflict to apply those rules by analogy”.Footnote 74
Relevantly, several delegations – including some undertaking rapid expansion of their naval forces – disagreed with the circumscribed application as proposed in the ICRC’s draft, and as such the matter was extensively debated within Committee III and its Working Group.Footnote 75 These delegations strongly preferred deleting the words “on land” in Article 49(3), arguing that Part IV, Section I of AP I should apply to all naval and aerial warfare to the extent that its provisions offered greater protection to civilians than existing rules of law – for example, in situations such as blockades and the sinking of merchant vessels which affect civilians at sea (including crews and passengers) and on land.Footnote 76 The delegations’ aim in this regard was that protection under this section “would be as broad as possible, embracing the protection of the civilian population, individual civilians and civilian objects at sea and in the air”.Footnote 77 Conversely, other delegates, particularly from the naval powers at the time, insisted on retaining the words “on land” to ensure that this section of AP I would not apply to operations such as blockades and attacks on merchant ships, which they contended were governed by separate legal regimes like the law of prize, under which the sub-regimes for blockade and visit and search operate.Footnote 78
What eventually became Article 49(3)–(4) of AP I reflects this distinction by limiting the scope of Part IV, Section I (Articles 48–67) to naval warfare that may affect civilian populations, individual civilians, and civilian objects on land, while also preserving existing treaty and customary law rules designed to protect civilians and civilian objects at sea:
3. The Provisions of this Section apply to any land, air or sea warfare which may affect the civilian population, individual civilians or civilian objects on land. They further apply to all attacks from the sea or from the air against objectives on land but do not otherwise affect the rules of international law applicable in armed conflict at sea or in the air.
4. The provisions of this Section are additional to the rules concerning humanitarian protection … in other international agreements binding upon the High Contracting Parties, as well as to other rules of international law relating to the protection of civilians and civilian objects on land, at sea or in the air against the effects of hostilities.Footnote 79
As explained in the ICRC’s 1987 Commentary on AP I Article 49,
[i]n general the delegates at the Diplomatic Conference were guided by a concern not to undertake a revision of the rules applicable to armed conflict at sea or in the air. This is why the words “on land” were retained and a second sentence clearly indicating that the Protocol did not change international law applicable in such situations was added.
This concern is understandable: after all, the conditions of sea warfare were radically transformed during the Second World War and in subsequent conflicts. It is therefore difficult to determine exactly which are the rules that still apply. …
Admittedly both sea and air warfare are subject to restrictions imposed by treaties of general application … but there are hardly any specific rules relating to sea or air warfare, and insofar as they do exist, they are controversial or have fallen into disuse.Footnote 80
Effect of the AP I Article 49(3) limitation
The retaining of the “on land” limitation in Article 49(3) of AP I means that Articles 48–67 do not apply (as a matter of treaty law) except in two situations: (1) sea or air warfare that may affect the civilian population, individual civilians, or the civilian population on land, and (2) attacks directed against objects on land.
Notably, Articles 48–67 of AP I deal mostly with rules specific to targeting and other aspects of the conduct of hostilities a number of which are considered to reflect customary international law today.Footnote 81 Such rules include, inter alia, rules stemming from the principles of distinction,Footnote 82 proportionalityFootnote 83 and precaution,Footnote 84 and also rules on non-defended localities,Footnote 85 demilitarized zonesFootnote 86 and civil defence.Footnote 87
Effect of the AP I Article 49(4) “without prejudice” clause
Article 49(4) of AP I sought to clarify the relationship between Section I of Part IV and the pre-existing rules of customary and treaty law protecting civilians against the effects of hostilities at sea. Section I of Part IV “provides additional protections to supplement, but not replace, or diminish existing ones. It thus reaffirms and clarifies existing protections and creates some new ones.”Footnote 88 This is an important consideration in the context of the protection of merchant vessels;Footnote 89 for instance, in its 1987 Commentary on Article 49 of AP I, the ICRC noted that
[w]hile paragraph 3 seems to limit the prohibitions laid down in this Section to objectives on land, paragraph 4 leaves the applicability of both treaty rules and customary rules unimpaired, insofar as such rules are aimed at protecting civilians and civilian objects in air and sea warfare, and this paragraph even tends to supplement these rules. This aspect is confirmed by the provisions of Article 57 (“Precautions in attack”), of which paragraph 4 urges Parties to the conflict to take all reasonable precautions to avoid losses of civilian lives and damage to civilian objects in the conduct of military operations at sea or in the air.Footnote 90
Despite this clarification, contemporary academic debate – and more importantly State practice – remains sparse concerning the precise interpretation and application of Article 49(4) of AP I, especially in the maritime context and in its relationship to Articles 49(3) and 57(4) of AP I (the latter of which is discussed in the next section). Greater clarity from States on this issue would therefore be highly desirable.
Article 57 of AP I: Precautions in the conduct of military operations at sea
Article 57 of AP I prescribes the legal requirements for precautions in attack.Footnote 91 It appears in Section I of Part IV of AP I and is therefore, following Article 49(3), generally considered not to apply to sea warfare not affecting civilians or civilian objects on land – at least as a matter of treaty law. The inclusion of paragraph 4 of Article 57, however, reflects a conscious decision by delegates at the Diplomatic Conference to preserve the relevance of pre-1977 precautionary obligations protecting civilians in the conduct of military operations at sea and in the air, notwithstanding these general limitations. As such, Article 57(4) is explicitly relevant to such military operations. It states that
[i]n the conduct of military operations at sea or in the air, each Party to the conflict shall, in conformity with its rights and duties under the rules of international law applicable in armed conflict, take all reasonable precautions to avoid losses of civilian lives and damage to civilian objects.
This specific formulation is noteworthy in the naval warfare context, where many military operations do not amount to an “attack” in the technical legal sense under IHL as per Article 49(1) of AP I but nevertheless give rise to risk of harm to civilian lives and civilian objects.Footnote 92 For example, it has been suggested that the term “military operations” in the sense of Article 57(4) encompasses all acts associated with belligerent interference with enemy or neutral commerce, including capture, seizure, visit and search, destruction, pursuit of blockade runners, and other actions taken to enforce a naval blockade.Footnote 93
Article 57(4) appears to set up a distinct precautionary standard due to its reference to “all reasonable” precautions, as opposed to the requirement to take “all feasible” precautions found elsewhere in Article 57. Shortly after its adoption, the authors of the original ICRC Commentary on AP I surmised that “reasonable” is “undoubtedly slightly different from and a little less far-reaching than the expression ‘take all feasible precautions’ used in paragraph 2”, acknowledging that “as the nuance is tenuous, the purpose of the provision appears to be to reaffirm the rules that exist to protect civilians in such situations”.Footnote 94 Beyond that, there was little articulation of the precise interpretation and application of Art 57(4) in the maritime context, and the ICRC Commentary considered it “regrettable” that the delegates at the Diplomatic Conference did not provide “practical examples” of what was in their contemplation at the time.Footnote 95
This absence of guidance, in turn, gave rise – shortly after its adoption – to divergent scholarly views regarding the purpose and legal effect of Article 57(4). Some characterized the provision as primarily exhortatory and unnecessary, arguing that the provision itself has little, if any, independent legal effect because it merely restates existing pre-1977 law applicable to naval and air warfare rather than creating new obligations. In their view, Article 57(4) was included simply to “satisfy those delegations which felt uneasy about the limitation of the scope of this Section of Protocol I to the effects of attacks on objectives on land”.Footnote 96 Professor Elmar Rauch (whose views are discussed in more detail below) argued that Article 57(4) was “superfluous, unnecessary and confusing” and had “no legal effect at all”.Footnote 97 He stated that no attempt should be made to reconcile it with the rest of the treaty, since it exemplifies “extremely bad draftsmanship” and creates a clear tension with Article 49(4).Footnote 98
Others did not seem to agree entirely that Article 57(4) is completely without independent meaning and effect. For instance, a US government official reacting to such criticisms noted that the emphasis in Article 57(4) on precautionary measures in the conduct of military operations at sea is appropriate since “Art. 49 (3) does not include within the scope of Protocol I naval operations which have only an indirect effect on civilians on land”.Footnote 99
AP I rules with direct application to naval warfare
Despite the circumscribed scope of Part IV, Section I, other provisions of AP I outside Articles 48–67 remain applicable to armed conflict at sea by way of treaty law for States party to the Protocol. Many of these provisions are also considered to reflect customary international law.Footnote 100
Take, for example, Part III on means and methods of warfare, which reaffirms the basic rule that the right of warring parties to choose their means or methods of warfare is not unlimited, and includes the prohibition against the employment of weapons, projectiles and material and methods of warfare of a nature to cause superfluous injury and unnecessary suffering.Footnote 101 Although the military objectives attacked during armed conflict at sea are primarily platforms, this prohibition nevertheless applies to protect individuals aboard those platforms. Moreover, means and methods of naval warfare which are intended, or may be expected, to cause widespread, long-term and severe damage to the natural environment are also prohibited.Footnote 102
Part III also includes the obligation to review new weapons, means and methods of warfare to determine whether they would be prohibited by AP I or other rules of international law in all or some circumstances, and this prohibition applies equally to naval weapons, means or methods intended for use at sea.Footnote 103 The prohibition against perfidy,Footnote 104 which is especially important for the use of deception in naval warfare, likewise also applies at sea and must be read in conjunction with the rules on recognized emblemsFootnote 105 and emblems of nationality (including the specific carve-out regarding the use of false flags in naval warfare).Footnote 106
The denial of quarter – ordering that there will be no survivors, threatening the adversary with this or conducting hostilities on this basis – is also prohibited.Footnote 107 This obligation must indeed be read in conjunction with the Geneva Convention II (GC II) obligations requiring belligerents, after each engagement and without delay, to take all possible measures to search for and collect the shipwrecked, wounded and sick, to protect them against pillage and ill-treatment, to ensure their adequate care, and to search for the dead and prevent their being despoiled.Footnote 108
Beyond Part III on means and methods of warfare, other rules of AP I are applicable and very relevant for naval warfare, including the revised and updated rules on humanitarian access.Footnote 109 These rules are especially important for the conduct of blockades and other blockade-like measures. In addition, the fundamental guarantees set out in AP I for persons in the hands of a party to the conflict apply equally in armed conflict at sea; at a minimum, these guarantees are likely applicable with respect to the crews of merchant vessels that find themselves in the power of a party to the conflict.Footnote 110 What is more, AP I defines key terms relevant to the wounded, sick and shipwrecked at seaFootnote 111 and extends the protection of GC II to all civilians who are wounded, sick or shipwrecked,Footnote 112 and to other medical ships and craft not mentioned in GC II.Footnote 113
Finally, as noted in the previous section, Articles 48–67 of AP I apply to sea warfare that “may affect” the civilian population, individual civilians or civilian objects on land. To understand the scope of Article 49(3), key terms must be interpreted. For instance, the phrase “sea warfare” raises the question of whether it covers only attacks in the sense of Article 49(1), or also other naval operations.Footnote 114 Similarly, “may affect” prompts consideration of what type and likelihood of effects on civilians are legally relevant: for example, does it refer solely to effects which qualify as incidental loss of civilian life, injury to civilians or damage to civilian objects, or does it encompass effects that are broader?
In practice, the prevailing view amongst States and scholars suggests that Article 54 of AP I, which prohibits starvation of civilians as a method of warfare, applies to its full extent vis-à-vis naval operations that affect the civilian population on land, including blockades and blockade like measuresFootnote 115 and, depending on how the category is defined, the interception of contraband.Footnote 116
Some scholars have suggested that in addition to naval bombardment of land targets, Articles 48–67 of AP I would also apply “to naval operations in … close proximity to the coastline that may inflict harm to protected persons and objects on land”.Footnote 117 This would not, however, include the “following operations if they are not anticipated to affect protected persons and objects on land: land-to-sea, sea-to-sea, air-to-sea, sea-to-air, and air-to-air”.Footnote 118
As the next section details, interpretive challenges surrounding Articles 49(3)–(4) and 57(4) of AP I emerged shortly after the Protocol’s adoption and remained prominent during the drafting of the San Remo Manual in the late 1980s and early 1990s. Many of these questions remain unresolved today.
Divergent interpretations of Articles 49 and 57(4) of AP I: Rauch versus Meyrowitz in the 1980s
The precise effect of AP I on the rules governing the conduct of hostilities at sea became the subject of debate shortly after the Protocol’s adoption.Footnote 119 Notably, some scholars contended that despite the Diplomatic Conference’s efforts to ensure that AP I would not inadvertently alter the “traditional” law of naval warfare, certain changes did occur, though opinions differed regarding the scope and significance of these changes.
At one end of the spectrum stood Dr Elmar Rauch, a leading proponent of the view that AP I had, albeit unintentionally, significantly altered the traditional law of naval warfare, including the law of blockade, contraband control, submarine warfare and naval mine warfare. In the early 1980s, Rauch ignited a provocative debate, much of which is detailed in a comprehensive report that he authored.Footnote 120 At the other end of the spectrum stood scholars such as Dr Henri Meyrowitz who firmly rejected Rauch’s assertions.Footnote 121
The mid-1980s scholarship of Rauch and Meyrowitz was both timely and influential. Their sharply contrasting views were a notable reference point shaping the debate and guiding experts attempting to reconcile the implications of AP I with the “traditional” law of naval warfare during the drafting of the original San Remo Manual. It is therefore useful to summarize their principal arguments before examining how the Manual ultimately addressed these issues.
Rauch’s interpretation
Among Rauch’s main concerns was that Article 49(3)–(4) of AP I was open to differing, even contradictory, interpretations.Footnote 122 He conceded that since the word “attack” is found in the title of the article and is a key term in this section – and Article 49 defines both the term and its scope – it might seem logical to begin interpretation with the final sentence of Article 49(3), despite the article’s introductory wording.Footnote 123 From that perspective, and for the sake of consistency, the section’s provisions would apply only to attacks launched from the sea against military objectives on land, leaving the existing rules of naval warfare intact.Footnote 124
Rauch argued, however, that civilians are affected by far more than naval bombardment alone.Footnote 125 He observed that in both World Wars, the impact of bombardments was “rather negligible” compared with the effects of naval blockades and contraband control, which had far greater consequences for the daily lives of millions.Footnote 126 On this basis, he argued that the object and purpose of AP I – to enhance protection of the civilian population – required its provisions to be applied more broadly.Footnote 127 Rauch found support for this broader interpretation in the first sentence of Article 49(3), which refers to “sea warfare” rather than “attacks”. He understood this to encompass all belligerent actions involving interference with neutral or enemy shipping, such as the capture, seizure or destruction of vessels or goods at sea, and blockade enforcement.Footnote 128 At the same time, “sea warfare”, in his view, also included naval bombardments of military objectives on land, since these too could cause incidental harm to civilians.Footnote 129
Rauch conceded that the Diplomatic Conference may have intended a narrower reading of the phrase “sea warfare which may affect the civilian population … on land” given the apparent illogic of extending “they further apply” to all attacks from the sea.Footnote 130 He remained, however, unconvinced by this reading. Even if the final sentence of Article 49(3) were, in his words, “a merely obscurantistic tautology”, it does not explain why the caveat “but do not otherwise affect the rules of international law applicable in armed conflict at sea” is attached specifically to the part of the rule concerning attacks, rather than to the broader clause addressing the effects of sea warfare on civilians on land.Footnote 131 Rauch argued that even if the Working Group and Committee III appeared inclined to read the second sentence of Article 49(3) as merely clarifying the first sentence (i.e. limiting Section I of Part IV to naval attacks on land-based targets), such an interpretation is untenable today, as it would be “tantamount to a treaty interpretation contralegem”.Footnote 132
Rauch argued that any coherent interpretation of Article 49 must reconcile the exception in the final sentence of paragraph 3 with the provisions of paragraph 4.Footnote 133 In this regard, he asserted that under the general principles of treaty interpretation, the travaux préparatoires cannot override the plain text of the article itself – particularly when a restrictive interpretation, grounded in preparatory work completed two years prior to adoption, would directly conflict with paragraph 4.Footnote 134 In his view, the 1936 Procès-Verbal was one treaty that contained important rules designed for humanitarian protection, meaning that they fell within Article 49(4) of AP I, and he asserted that the International Military Tribunal had confirmed the binding nature of these rules during the trial of Karl Dönitz.Footnote 135 On this basis, Rauch proposed the following construction of Article 49:
Good faith interpretation, in accordance with the ordinary meaning to be given to the terms of Protocol I in their context and in the light of its object and purpose, suggests the following construction of Art. 49 of Protocol I: The provisions of Section I of Part IV
– apply to all acts of naval warfare which may affect the civilian population, individual civilians or civilian objects on land, including attacks from the sea against military objectives on land and measures against the maritime commerce of the enemy whether under its own flag or under neutral flags;
– apply to all other acts of naval warfare as far as the effects of hostilities on civilians and civilian objects on land, at sea, or in the air are concerned, as regulated by conventional or customary international humanitarian law, including collateral damage of operations against enemy warships and effects of belligerent acts against merchant ships;
– do not affect other rules of naval warfare, including belligerent action against the enemy’s fleet or other military objectives at sea or in the superjacent airspace and attacks from land against ships at sea or aircraft in the superjacent airspace.Footnote 136
For Rauch, it was “beyond doubt” that Article 54 of AP I – which prohibits the starvation of civilians and relatedly protects objects indispensable to their survival – had modified the traditional rules of naval warfare.Footnote 137 Specifically, it rendered blockades aimed at starving civilians unlawful, broadened the categories of supplies exempt from being treated as contraband, prohibited the capture or destruction of enemy merchant vessels and their cargo when carrying goods essential for civilian survival, and redefined unneutral service by stipulating that participation in, or acceptance of, convoys transporting such goods – whether directed by the enemy government or undertaken voluntarily – could no longer be regarded as a hostile or unfriendly act.Footnote 138
As briefly mentioned earlier in this article, Rauch was rather critical of Article 57(4) of AP I, claiming that it was “superfluous, unnecessary and confusing” and had “no legal effect at all”.Footnote 139 As noted above, he asserted that no attempt should be made to reconcile the article with the rest of the treaty, since it represented “extremely bad draftsmanship” and created an obvious tension with Article 49(4) of AP I:
As far as the reasonable precautions are concerned, Art. 57 par. 4 requires them “in conformity with” the existing international law. There is only one interpretation admissible, i.e. to read this provision as a mere repetition of an existing body of law. But if this is the case, then we have an evident contradiction at hand between this stipulation and Art. 49 par. 4 which speaks of “additional” provisions to customary and conventional rules relating to the protection of civilians and civilian objects at sea. Normally, one would not expect a codification conference to draft rules which do not have a meaning of their own. And an additional treaty rule stands on its own, even if previous conventional rights and obligations are repealed. In contrast, the mere reference in Art. 57 par. 4 makes all precautionary measures dependent upon the law as it stood before 1977. It seems ludicrous to formally declare in the basic rule that all provisions of this Section are additional and at the same time couch the important stipulation that way.Footnote 140
Rauch was also critical of Solf and other commentators for suggesting that Article 57(4) was included simply to “satisfy those delegations which felt uneasy about the limitations of the scope of this Section of Protocol I to the effects of attacks on objectives on land”.Footnote 141 Rauch considered this to be a misguided explanation that contributed “not only to a wrong conception of Art. 49 par. 3 by some delegates at the Diplomatic Conference, but also to a complete neglect of Art. 49 par. 4”,Footnote 142 which in his view affirmed the application of the 1936 Procès-Verbal. The paradox, in his view, was as follows:
[A] meaningless provision was inserted (Art. 57 par. 4) in order to quiet concerns about the limited scope of another provision (Art. 49 par. 3) whereas the problem was addressed in quite an unlimited fashion in a different stipulation (Art. 49 par. 4) which should have calmed down any uneasiness about the scope of application of Protocol I with respect to naval warfare.Footnote 143
Meyrowitz’s interpretation
Scholars such as Dr Henri Meyrowitz firmly rejected arguments like Rauch’s that Article 49(3) had inadvertently and fundamentally changed the traditional law of naval warfare.Footnote 144 Meyrowitz also disagreed with Rauch’s claim that Article 49(4) implied a reaffirmation, in its entirety, of the 1936 Procès-Verbal. Although Meyrowiz acknowledged that AP I reinforced humanitarian protections – especially regarding starvation and humanitarian relief – he disagreed with Rauch as to the extent to which Articles 54(1) and 70 modified the rules relating to blockade, prize law and contraband.
First, Meyrowitz argued that Article 49(3) is not an independent provision but merely refers back to the rules contained in the relevant section of AP I.Footnote 145 In Meyrowitz’s view, with the exception of Article 54(1), the provisions of this section of AP I do not apply to naval warfare. While Article 70 – like Article 54(1) – has effectively modified certain traditional rules concerning blockades, contraband, booty and capture, it does not alter the underlying principle that this section of AP I remains inapplicable to naval operations.Footnote 146
Second, Meyrowitz posited that a strict literal interpretation of Article 49 produces an obscure and absurd result – namely, the overhaul (“renversement”) of the law of naval warfare.Footnote 147 Accordingly, he argued that the recourse to the preparatory works is necessary to interpret the provision in accordance with the Vienna Convention on the Law of Treaties.Footnote 148
Third, Meyrowitz asserted that the phrase “may affect” in Article 49(3) is overly broad and too vague, rendering it an inadequate criterion for determining the application of Section I of Part IV of AP I to the full range of maritime warfare activities.Footnote 149 While the second sentence of the provision could, in theory, operate independently, Meyrowitz contended that this would only be relevant in exceptional cases.Footnote 150 One such case involves a lawful attack – land, air or naval – against an object at sea that incidentally harms civilians or damages civilian objects on land, in which case Article 51(5)(b) would apply.Footnote 151 Another example is an attack on a lawful military objective such as a large oil tanker, in geographical conditions such that the attack would cause, on land, “non-traumatic” indirect harm to the civilian population, for instance through environmental damage affecting public health or survival, as addressed in Article 55(1).Footnote 152
Meyrowitz critiqued Rauch’s broader interpretation of “sea warfare”,Footnote 153 which encompasses both violent acts (such as attacks and destruction causing physical (“traumatic”) harm, including death, injury or destruction) and non-violent actions (such as prize measures, blockades, contraband control and the capture of vessels) which may affect civilians on land through indirect, “non-traumatic” effects (such as economic deprivation affecting civilian survival).Footnote 154 He argued that AP I already permits violent acts capable of causing serious “non-traumatic effects” for civilians, even absent “traumatic effects”. As illustrations, he cited attacks on oil installations or measures intended to deprive the civilian population of fuel and petroleum essential for transport, agriculture and the food industry.Footnote 155
In sum, with respect to Article 49(3), Meyrowitz argued that a single sentence in a multilateral convention cannot transform an entire body of customary international law – especially where it implies a complete overhaul that the AP I drafters explicitly sought to avoid.Footnote 156 In his view, a fundamental distinction between the laws of land and naval warfare persists. The latter has historically regulated not only hostilities against enemy warships, but also interactions with enemy and neutral merchant shipping and maritime commerce more broadly. It has given rise to a distinct set of institutions that are unknown in land warfare: visit and search, capture, prize, blockade and contraband control. According to Meyrowitz, one cannot simply transpose the rules developed for land warfare into the maritime sphere.Footnote 157
With regard to Article 49(4), Meyrowitz challenged Rauch’s assertion that the phrase “other international agreements” should be read as encompassing, inter alia, the 1936 Procès-Verbal, and that the instrument is thereby implicitly confirmed by AP I.Footnote 158 In Meyrowitz’s view, Article 49(4) cannot be interpreted as reviving or restoring treaties that have fallen into desuetude or that have been overtaken by subsequent State practice – circumstances he believed accurately described the status of the 1936 Procès-Verbal, which in his view had been distorted by the conduct of belligerents during the Second World War.Footnote 159
In response to Meyrowitz’s and others’ critiques of Rauch’s broad reading of Article 49, Rauch acknowledged that in his comprehensive report he had “tried to walk the fine line between being thought-provoking and provocative”.Footnote 160 He emphasized that the “manifold problems” created by AP I are persistent and “can’t be swept under the carpet”, and that legal difficulties cannot be resolved by ignoring established rules of treaty interpretation.Footnote 161 Recognizing the complexity of the issues at hand, he noted that multiple, sometimes contradictory views regarding the interpretation of Article 49 of AP I are valid, as illustrated by Meyrowitz.Footnote 162
Opening Pandora’s box at sea: Unresolved interpretive challenges in AP I
Many of the questions that were first raised by Rauch regarding the construction of Article 49(3)–(4) of AP I, including as regards its relationship with Article 57(4), remain unresolved and would most certainly benefit from further clarification, particularly by States. How should Article 49(3) be understood today in light of modern naval capabilities and civilian reliance on maritime trade? Relatedly, which types of naval operations are likely to affect civilians on land, and what kinds of effects are legally relevant? The rationale for explicitly addressing military operations at sea in Article 57(4), given its placement in Section I, also warrants closer examination, as do the legal consequences of Article 57(4) in connection with Article 49(3)–(4). Finally, considering Article 49(4), it remains necessary to clarify which precautions from the traditional law of naval warfare – such as those discussed earlier in this paper – continue to apply at sea, particularly with respect to merchant vessels.
Unsurprisingly, Rauch was critical of the Diplomatic Conference for having
opened a Pandora’s box of difficulties for the rules and customs of naval warfare. It can only be hoped that the lacunae and ambiguities … will not have to be ironed out by State practice, i.e., by actual combat at sea. The battle should be fought with pencil and ink among those in charge of translating Protocol I … into understandable guidelines and clear advice for the seagoing officer. In doing so, they will have to avoid both extremes, adopting neither the “war-monger approach” nor the overambitious and exaggerated humanitarianism, in order to achieve a delicate balance of accommodation between military needs and humanitarian considerations.Footnote 163
Ultimately, it was the Group of Experts responsible for drafting the original San Remo Manual that undertook the painstaking task of reconciling developments in instruments such as AP I with the traditional law of naval warfare, striving to balance military necessity with humanitarian considerations. Given the intrinsic uncertainties and interpretive difficulties at the time, their efforts to translate new AP I concepts into practical guidance for naval operators and their lawyers were both impressive and commendable – even if at times not entirely conclusive, as will be made clear in the next section.
The San Remo Manual: Harmonizing AP I targeting rules with the traditional law of naval warfare, and implications for practice
The Group of Experts preparing the San Remo Manual confronted the relationship between AP I and the traditional law of naval warfare directly at the 1989 Bochum Round Table, which focused on “The Military Objective and the Principle of Distinction in the Law of Naval Warfare”. The central question was whether, and to what extent, the rules on the principle of distinction – particularly the relatively new “military objective” concept in Article 52 of AP I – could be transposed to the maritime domain.
In his comprehensive report, Rapporteur Commander William Fenrick of the Canadian Ministry of Defence examined the historical background, relevant treaty law and relevant State practice, and offered several recommendations on the way forward.Footnote 164 He observed that the proper interpretation of Article 49(3) of AP I – particularly the meaning of the expression “may affect the civilian population, individual civilians or civilian objects on land” – was contentious.Footnote 165 In his view, Part IV, Section I clearly regulated naval shore bombardment conducted by ships or aircraft; by contrast, attacks against objects on or under the sea would not have a direct effect on the civilian population on land.Footnote 166
Fenrick did nevertheless specify Article 54 of AP I as one provision in this section that “might have an impact, after a fashion, on naval targeting”,Footnote 167 and suggested that it could require “consideration of traditional commerce interdiction measures such as blockade and contraband”.Footnote 168 He argued that
Art 54(1) [of AP I] would prohibit a naval blockade to prevent the import of foodstuffs in order to hasten the end of an armed conflict through starvation of the civilian population. Further objects indispensable to the survival of the civilian population must not be declared or treated as conditional contraband even if they are used by the enemy in direct support of military action, provided that their seizure and condemnation may leave the civilian population with such inadequate supplies as to cause its starvation or force its movement. …As a result of Art. 54, it may be necessary for parties to Protocol I to exempt from attack ships known to be carrying exclusively objects indispensable to the survival of the civilian population even if such ships breach a blockade or are so unwise as to refuse to stop for visit and search.Footnote 169
More broadly, Fenrick proposed that although most of Part IV, Section I of AP I was “not, strictly speaking, applicable to the targeting of objects on, over or under the seas, it does contain some concepts and texts which might eventually be considered suitable for incorporation in the law of naval warfare”.Footnote 170 In particular, he highlighted the definition of military objective in Article 52(2) and the principle of proportionality embedded in Articles 51 and 57.Footnote 171
These questions were carried forward at the 1990 Toulon Round Table on Methods and Means of Combat in Naval Warfare.Footnote 172 At Toulon, the discussion shifted from the conceptual scope of AP I to the practical implications of applying its rules to traditional naval warfare means and methods. Participants examined the extent of convergence and divergence between land and naval warfare and considered which, if any, aspects of AP I on precautions and proportionality were suitable for incorporation into the new San Remo Manual.
A central focus of discussions was the degree of alignment between land and naval targeting rules. Rapporteur G. J. F. van Hegelsom from the Netherlands Ministry of Defence’s Directorate of Legal Affairs presented an introductory report which, among other things, recalled the debates sparked by Rauch and Meryowitz. Hegelsom observed that previous discussions of the Group of Experts in both Madrid and Bochum had demonstrated that “the distinction between land warfare and naval warfare is not as stringent as the eminent lawyer [Meyrowitz] suggests”.Footnote 173 Specifically, Hegelsom noted that
although the provisions of that section [Part IV, Section I] were developed with only land warfare in mind and therefore presented some interpretational difficulties if applied to naval warfare, the absolute character of the distinction advocated by Meyrowitz was not warranted: the standards underlying the regulation of belligerent behaviour were a codification of customary law and therefore were already applicable in naval warfare. Moreover, the provisional concluding document of the Bochum Round Table relies heavily on the provisions of Additional Protocol I in assessing the roles of the principle of distinction and of the military objective in armed conflict at sea.
…
Although the maritime environment may have specific characteristics which should be taken into account when developing modern rules of naval warfare, it is submitted that the absolute distinction advocated by Meyrowitz should not be accepted. General principles and codified customary law as adopted in Additional Protocol I provide useful guidelines for the identification of rules applicable to naval warfare.Footnote 174
Since the experts concluded that certain land targeting concepts could be transposed to the maritime domain, the next section examines how the Group of Experts sought to harmonize core AP I concepts on the conduct of hostilities – particularly the definitions of military objective, precautions in attack and proportionality – with the traditional law of naval warfare, as well as the practical implications of this harmonization and the questions that remain unresolved today.
The San Remo Manual’s fusing of traditional naval warfare targeting rules with the “new” AP I concept of military objective
After extensive debate in Bochum, the experts concluded that the foundational IHL principle of distinction and the concept of military objective, as codified in AP I, are applicable in naval warfare. They reaffirmed the obligation to distinguish between civilians and combatants, and between civilian objects and military objectives, and that attacks are to be strictly limited to military objectives.Footnote 175 Following the Rapporteur’s suggestion, they agreed that Article 52 of AP I was suitable for incorporation. Much of the subsequent discussion in Bochum then appeared to focus on specific vessels exempt from attack, as well as the circumstances under which merchant vessels – both enemy and neutral – may qualify as a military objective or otherwise be attacked.
Consequently, the San Remo Manual adopted Rule 40, which reflects verbatim the two-pronged test – both cumulative and temporal – set out in Article 52(2) of AP I for determining what qualifies as a military objective.Footnote 176 San Remo Manual Rules 60 (“Enemy Merchant Vessels”)Footnote 177 and 67 (“Neutral Merchant Vessels”)Footnote 178 then provided so-called domain-specific circumstances in which certain conduct by enemy and neutral merchant vessels may satisfy the military objective criteria. These domain-specific rules were adopted with the intention to “limit the lawfulness of attacks to merchant vessels which directly help the military action of the enemy”.Footnote 179 This decision to incorporate the concept of military objective was taken despite the prevailing view at the time that, under treaty law, it did not formally apply to naval warfare lacking effects on civilians or civilian objects ashore, at least for States Parties to AP I by virtue of Article 49(3).Footnote 180
Relatedly, it is noteworthy that the experts decided to expressly incorporate the presumption of civilian status in cases of doubt reflected in AP I.Footnote 181 Although formulated in the context of exempt vessels and aircraft, the inclusion of this rule reaffirmed the importance of directing attacks against vessels only where there is concrete evidence demonstrating that they are making an effective contribution to the enemy’s military action.
In effect, the San Remo Manual fused “traditional” law of naval warfare rules governing the use of force against vessels with what was at the time the “new” military objective concept from Article 52 of AP I. However, this integration was not entirely conclusive – understandably, given the complex and unresolved debates at the Bochum Round Table, particularly concerning the legal basis for attacks against (especially neutral) merchant vessels.Footnote 182
In relation to enemy belligerent warships and the modern concept of auxiliary vessels,Footnote 183 for example, application of the “military objective” criteria in Article 52(2) of AP I appears straightforward. By their “nature” or “use”, such vessels make an effective contribution to the enemy’s military action, and an attack against them may reasonably be expected to offer a definite military advantage, assuming that the temporal requirement of “in the circumstances ruling at the time” is also satisfied.
Meanwhile, enemy and neutral merchant vessels that are “taking a direct part in hostilities”Footnote 184 or engaging in other forms of unneutral service – traditionally subject to capture, or in exceptional cases, to attack when direct engagement with belligerent forces is more grave – are now generally regarded under modern law as “effectively contributing to the military action” of a belligerent. Consequently, they may be treated as military objectives as per the Article 52(2) criteria and are therefore subject to the fundamental rules of precaution and proportionality in attack.Footnote 185 There is little discussion of this transition in most manuals, though the English version of the German Handbook helpfully notes:
Today there is agreement that the prohibition of attacks on enemy merchant vessels and enemy aircraft does not apply if they are effectively contributing to the enemy’s military activities and their destruction or neutralization by other means constitutes a definite military advantage in the circumstances ruling at the time. Therefore, the definition of military objectives is applied to these vehicles too.Footnote 186
The legacy of Article 22 of the 1936 London Procès-Verbal
While many forms of unneutral service can be easily mapped onto the framework of Article 52(2) of AP I, the historical legacy of Article 22 of the 1936 Procès-Verbal complicates the picture.Footnote 187 Article 22 permits a belligerent warship to employ graduated force against a merchant vessel that persistently refuses an order to stop or actively resists visit and search, including, if necessary, rendering it incapable of navigation or sinking it. The provision remains in force for fifty-one States (including several naval powers) and finds expression in state military manualsFootnote 188 as well as the San Remo Manual and more recent Newport Manual on the Law of Naval Warfare (Newport Manual).Footnote 189
This legacy authority predates the modern AP I concept of “military objective” and operates on a different logic. Under Article 22 of the 1936 Procès-Verbal, force is triggered by the refusal to stop or active resistance rather than verified contribution to the enemy’s military action or the anticipated advantage to be gained from attacking a fleeing or resisting ship. In these circumstances, such assessments can only ever be speculative. Target verification in the sense contemplated by the principle of precautions in attack will often be impossible as a belligerent cannot be expected to have sufficient knowledge of a vessel’s purpose, use or cargo; indeed, the very purpose of visit and search is to resolve such uncertainty. In such cases, there is at most a reasonable suspicion that the vessel is carrying contraband, or conjecture regarding the reasons for a breach or attempted breach of a blockade (i.e., San Remo Manual Rule 67(a)). Nonetheless, as an expression of the traditional law of naval warfare, the entitlement to use force in these circumstances is generally considered to remain valid. For example, the 2013 Norwegian Manual justifies this approach by stating: “The reason the merchant vessel must then be regarded as a lawful target is that it is impossible to check whether it is carrying contraband, participating in hostilities etc.”Footnote 190
Structural ambiguity between Rules 40–41 and Rules 60 and 67 of the San Remo Manual
While it was understood that vessels resisting visit, search or capture would not qualify as military objectives strictu sensu,Footnote 191 the experts nevertheless chose to fuse the traditional law of naval warfare basis to use force against merchant vessels with the AP I concept of “military objective”, resulting in a formulation in the San Remo Manual that retains considerable textual ambiguity. This is particularly evident when San Remo Manual Rules 40 and 41 – reflecting the two-part definition of military objectives in Article 52 of AP I and the requirement that attacks be strictly limited to military objectives – are read alongside the conduct-based categories set out in Rules 60 (“Enemy Merchant Vessels”) and 67 (“Neutral Merchant Vessels”). At least two interpretations seem possible:
• Interpretation 1: On one view, San Remo Manual Rules 60 and 67 operate as independent and supplementary grounds for attack under the rules governing the conduct of hostilities at sea. This approach effectively imports traditional grounds for using force during prize-law enforcement against merchant vessels alongside the AP I Article 52 framework. Applying this view, it is arguable that including San Remo Manual Rule 40 effectively broadened the traditional grounds for attacking merchant vessels that existed prior to AP I, despite historical caution in the “traditional” law against attacking both enemy and neutral merchant ships rather than merely capturing them, except for when they are performing “grave acts” of unneutral service.Footnote 192
• Interpretation 2: Alternatively, San Remo Manual Rules 60 and 67 may be understood as merely identifying situations in which a vessel may qualify as a military objective, without dispensing with the two-part test in San Remo Manual Rule 40 and Article 52 of AP I.
As drafted, the current structure of the San Remo Manual tends to support the second reading. The repeated renvoi to Rule 40, before and after the conduct-based lists, seems structurally significant;Footnote 193 it signals that all attacks, including those against vessels which refuse to stop or actively resist, remain subject to the two-part military objective test – namely, that they make an effective contribution to military action and that their destruction offers a definite military advantage in the circumstances ruling at the time.
The language of San Remo Manual Rule 60 is also noteworthy. It states that specified conduct “may render” a vessel a military objective, without necessarily implying automatic targetability. It seems that Rules 59 and 61 still demand a contextual assessment consistent with Rule 40; meanwhile, Rule 67 is framed in protective terms stating that a neutral merchant vessel “may not be attacked unless” certain conditions are met. When read alongside Rule 68 (which makes a renvoi to Rule 40), a literal interpretation might suggest that Rule 67 imposes cumulative or limiting conditions. Under this reading, a neutral vessel would need to satisfy both the specific criteria listed in Rule 67 and the general definition of a military objective under Rule 40 (as reinforced by Rule 68), rather than Rule 67 serving as a set of illustrative examples akin to Rule 60.
National military manuals and academic manuals since publication of the San Remo Manual
A cursory review of military and academic manuals does not necessarily resolve these ambiguities. A few illustrative, non-exhaustive examples are provided below.
The 2025 Norwegian Manual tends to support the first interpretation. It states that the reason why a merchant vessel which sails in convoy under the protection of enemy warships or enemy aircraft is regarded as a lawful target is because by sailing under the protection of enemy warships, it clearly demonstrates that active resistance will be made against visit, search or capture.Footnote 194 Similarly, the US Manual specifies that enemy merchant vessels will constitute a military objective that can be attacked with or without prior warning, in circumstances such as persistently refusing to stop when duly summoned or actively resisting visit, search or capture.Footnote 195 Of note also, the US Manual asserts that merchant vessels which constitute military objectives are not considered to be merchant vessels in the sense of the 1936 Procès-Verbal rule.Footnote 196
Interestingly, the New Zealand Manual, in the context of blockade enforcement, explicitly treats clear resistance against interception and capture as satisfying the AP I Article 52 test, reasoning that the destruction of such vessels provides a definite military advantage by preserving the effectiveness of the blockade.Footnote 197 The Manual goes on to assert, however, that the fact that a vessel is liable to attack does not necessarily justify an attack likely to cause it to be sunk, destroyed or severely damaged. The principle of proportionality still applies, and in most cases it will be sufficient to disable the vessel or (where possible) board it – by force if necessary.Footnote 198
By contrast, the second interpretation appears to be supported by States such as the UK and Denmark. For example, the UK Manual adapts the conduct-specific factors by expanding the chapeau in San Remo Manual Rules 60 and 67 as follows (emphasis added):Footnote 199
• Paragraph 13.41 (adapting Rule 60) states: “The following activities may, depending on the circumstances, render enemy merchant vessels military objectives: …”.
• Paragraph 13.47 (adapting Rule 67) similarly begins: “Merchant vessels flying the flag of neutral states may only be attacked if they fall within the definition of military objectives. They may, depending on the circumstances, become military objectives if they …”.
Similarly, the Danish Manual provides a non-exhaustive list of activities that may render enemy and neutral merchant vessels military objectives, mirroring San Remo Manual Rules 60 and 67, but also makes a renvoi back to its own general chapter on “Military Objectives” applicable across all domains, stating that
even though participation in these activities would render a vessel a military objective in the vast majority of cases, it must always be determined by an assessment of the specific circumstances, whether the criteria applicable to military objectives as set forth in Chapter 8 [“Military Objectives”] are met.Footnote 200
The Central African Republic (CAR) Manual adopts a similar approach, stating that only vessels qualifying as military objectives may be attacked. However, it then introduces its examples with the wording “À titre indicatif, on citera”, which translates roughly as “By way of indication” or “By way of example”. Unlike the San Remo Manual, it does not provide an extensive list of illustrative cases. Instead, it limits itself to four examples: vessels sailing in convoy, vessels transporting goods of military importance, vessels carrying members of the enemy’s armed forces, and vessels breaching a blockade.Footnote 201 The Manual then goes on to clarify that nevertheless, in case of doubt, a ship may be intercepted and searched in order to determine its status; if it refuses to stop or resists visit and search, it may be destroyed after prior warning.Footnote 202 Togo’s military manual adopts a similar approach.Footnote 203
The Sri Lankan Manual largely reflects the language and approach of the San Remo Manual. For example, it first states that “[e]nemy merchant vessels may only be attacked if they are military objectives”,Footnote 204 mirroring Rule 59 of the San Remo Manual. In the following paragraph, the Sri Lankan Manual incorporates the Rule 40 test for determining military objectives and then closely reproduces Rule 60. Manuals from Türkiye, Peru, Belgium and Russia adopt very similar approaches to Sri Lanka.Footnote 205 Although all of these manuals assert that a merchant vessel must qualify as a military objective within the meaning of IHL before it may be attacked, they provide no further clarification or explanation beyond this.
Although civil aircraft rules in the San Remo Manual fall outside the scope of our article, they are instructive. In general, attacks against civil aircraft – enemyFootnote 206 or neutralFootnote 207 – must satisfy the military objective definition under San Remo Manual Rule 40 and Article 52(2) of AP I. Some San Remo Manual provisions (Rules 63(d)–(e) and 70(a)) suggest that attacks may be permitted under aerial-domain-specific circumstances even when the full two-part test cannot be satisfied.Footnote 208 The HPCR Manual on International Law Applicable to Air and Missile Warfare (HPCR Manual) builds on this approach but goes further by emphasizing that the list of example circumstances in which a civil aircraftFootnote 209 or civil airlinerFootnote 210 may qualify as a military objective are merely illustrative rather than exhaustive.Footnote 211 Even when an example applies, such aircraft do not automatically become military objectives unless they satisfy the two-part AP I Article 52 test.Footnote 212 This approach lends support to the second interpretation described above.
In the present authors’ view, even if the second interpretation is accepted – namely, that San Remo Manual Rules 60 and 67 identify when a vessel may, depending on the circumstances, qualify as a military objective, without overriding the two-part test in Rule 40 and AP I Article 52 – this does not preclude actions (other than attack) against non-compliant vessels that do not satisfy the AP I Article 52 test. Such actions must, however, rest on a separate legal basis. That basis is to be found in earlier treaty and customary law, including the 1936 Procès-Verbal.Footnote 213 As mentioned earlier, that earlier treaty and customary law emphasizes the use of escalated force to secure capture, which remains the primary objective; authority to sink a merchant vessel that cannot be captured must therefore remain exceptional and narrowly confined.Footnote 214
Use of force against non-compliant merchant vessels: Hostilities or prize law enforcement?
Uncertainty remains as to how States interpret and apply both treaty and customary rules derived from the principle of distinction, particularly in relation to merchant vessels. Today, the term “military objective” (whether understood in its AP I Article 52 sense, or in its much narrower pre-1977 sense under the traditional law of naval warfare) is generally invoked to satisfy distinction requirements prior to launching an attack.
Where a vessel qualifies as a military objective under Article 52 of AP I and Rule 40 of the San Remo Manual, it may be subject to “attack” within the contemporary meaning of IHL and subject to the conduct of hostilities paradigm. The San Remo Manual identifies certain conduct that may, depending on circumstances, satisfy this test: engaging in belligerent acts on behalf of the enemy (Rules 60(a) and 67(b)), being incorporated into the enemy’s intelligence-gathering system (Rules 60(c) and 67(d)), or otherwise making an effective contribution to military action, such as by transporting military materials (Rules 60(g) and 67(f)). However, State practice provides scant guidance on the threshold of conduct required; for example, it seems highly unlikely that a merchant vessel merely reporting the sighting or movements of belligerent vessels or aircraft that it happens to encounter while engaged in normal operations would, by those actions alone, qualify as being part of the enemy’s intelligence-gathering system,Footnote 215 but there is little clear State affirmation on where the line should be drawn.
Even greater uncertainty arises where force is employed against a vessel that does not meet the Article 52 definition but persistently refuses to stop or actively resists visit and search under the traditional law of naval warfare. Should the use of force against such vessels properly be understood as an “attack” under the rules governing the conduct of hostilities or as an enforcement action under prize law? Without clarity on such foundational issues, there are some conceptual and practical challenges regarding the implementation of proportionality and precautions in attack that are likely to endure (see next section).
Another unresolved question that would benefit from further clarification by States concerns what conduct amounts to “persistent refusal to stop”, “intentional and clear resistance” or “active resistance” in the context of contemporary naval warfare. Overt acts such as a merchant ship firing at or ramming a pursuing belligerent warship (as occurred during the Second World War) or firing upon a blockading force would clearly qualify.Footnote 216 Likewise, enemy merchant vessels “sailing under the convoy of enemy warships or military aircraft” are said to indicate that active resistance will be made against visit, search or capture.Footnote 217 However, questions arise regarding how this concept translates to today’s maritime operating environment, especially when compared with historical practice.Footnote 218 For example, according to New Zealand, “clear resistance” means that the “the ship acts in a manner that has, or may have, an impeding effect on the intercepting forces. A mere change of course to escape is not sufficient.”Footnote 219 Beyond such cases, contemporary State practice seems to offer little clarity.
As evidenced from the above, many contemporary military manuals and expert bodies, following the San Remo Manual, assume that any use of force at sea against merchant vessels which actively resist or persistently refuse to stop must fit within an AP I targeting framework while at the same time emphasizing escalatory force logic.Footnote 220 However, the present authors contend that framing prize-law enforcement through the lens of AP I targeting law may be misplaced. Under contemporary IHL, a merchant vessel that does not meet the definition of a military objective cannot lawfully be “attacked”, although traditional prize law provides a lawful framework for graduated enforcement. Conflating these paradigms affects practical application of core obligations including target verification, proportionality and precautionary measures (some of which are further discussed below). A more coherent approach situates such operations within a distinct enforcement logic rooted in prize law: an historic, escalatory framework designed to compel compliance, not to destroy enemy military objectives at the first instance.
Closely connected is the question of boarding operations against merchant vessels that encounter armed resistance from either the crew or passengers.Footnote 221 When naval forces respond to such resistance, does the conduct of hostilities paradigm apply, or does something akin to a law enforcement paradigm govern the use of force and subsequent detention?Footnote 222 State manuals generally do not address this issue, though such questions were considered in the various inquiries that were produced in relation to the 31 May 2010 Gaza flotilla incident as well as by the International Criminal Court prosecutor at the time – albeit with contrasting findings.Footnote 223
The San Remo Manual’s import of land-based requirements from Article 57 of AP I
With respect to precautions at sea, Rapporteur Hegelsom recommended at the Toulon Round Table that the Group of Experts adopt the standards set out in Article 57 of AP I, notwithstanding their technical inapplicability as a matter of treaty law. He stated:
Belligerents should take reasonable precautions to ascertain whether they are attacking a legitimate military objective and whether the attack will not entail collateral damage which is excessive in relation to the concrete and direct military advantage anticipated. … Paragraph 2 of Article 57 of Additional Protocol I would seem a sound precedent for precautionary measures.
…
In light of the concern not to unduly venture in the law of naval warfare but still wishing to retain precautionary measures in naval warfare, the delegates at the Diplomatic Conference added a special paragraph to the Article [Article 57(4)], explicitly meant for naval and aerial warfare:
…
Doctrine would seem to be unanimous with respect to the thesis that Article 57(4) is merely declaratory and does not have any practical effect on the law of naval warfare. Is there a great difference between the obligations under paragraph 2 and paragraph 4 in practice? Up till now, no commentaries have addressed this issue. Upon closer analysis and bearing in mind the findings of the Bochum table with regard to the principle of distinction, one may effectively wonder whether there has been a specific reason, save for the general feeling mentioned above that the Diplomatic Conference did not wish to amend the law of naval warfare unintentionally, for the inclusion of paragraph 4. The declarations made by several countries upon ratification of Additional Protocol I with regard to the terms “military advantage”, “feasible”, and the responsibility of commanders in attack, would however also be useful in the naval environment.Footnote 224
Following discussion, the Group of Experts agreed with Hegelsom’s recommendation to use Article 57 of AP I as the basis for precautions in attack, aligning with Fenrick’s earlier suggestion at Bochum the previous year. Accordingly, Rule 46 of the San Remo Manual invokes a “simplified” version of Article 57(2), using the “all feasible” and “everything feasible” wording, and imposes obligations with respect to target verification, choice of means and methods, and conducting proportionality assessments during attacks.Footnote 225
The difficulty in clearly defining what maritime forces must do to uphold the rules stemming from the principles of precaution and proportionality was something discussed during the original process and is understandable given the complex realities at sea. Today’s merchant vessels can carry up to 20,000 containers – often with the specific details of the cargo being unknown even to the ship’s master – and may enter operational areas with limited warning and incomplete information available to the belligerent as to their cargo and intent. These conditions pose significant risks to civilian shipping and create acute operational dilemmas for naval forces. Such realities appear to have driven the development of the rules of naval warfare along a different path than those governing land warfare, and consequently, applying land-based precautionary criteria in a maritime context can present distinct and complex challenges.
National military manuals and academic manuals since publication of the San Remo Manual
Despite these complexities, Rule 46 of the San Remo Manual (reflective largely of Article 57 AP I) reflects the understanding that proportionality and precautions must be observed in maritime warfare, affirming that the core protective aims of AP I extend beyond the land domain.
In the years since the adoption of the San Remo Manual, numerous States have continued to reaffirm the importance of the Rule 46 (reflecting Article 57) precautions in attack and proportionality rules underscoring a crucial principle that even in circumstances where certain AP I treaty rules governing the conduct of hostilities may be formally inapplicable to naval hostilities which do not affect civilians on land, the underlying rules continue to shape the conduct of hostilities at sea.
As to which standard (i.e., “all reasonable” or “all feasible”) for precautions in attack at sea applies, similar to Rapporteur Hegelsom, the ICRC’s current legal position is that the distinction between “all reasonable” and “all feasible” is so highly nuanced as to be of little practical relevance, and the applicable legal regime is operationally the same. Similar views have been expressed by other expert bodies such as the International Law Association Study Group on the Conduct of Hostilities in the 21st Century (ILA Study Group)Footnote 226 and the authors of the HPCR ManualFootnote 227 and the Tallinn Manual 2.0 on the International Law applicable to Cyber Operations (Tallinn Manual 2.0).Footnote 228
A handful of State military manuals also seem to share a similar view. For example, the UK Manual, in its “Maritime Warfare” chapter, closely follows Rule 46 of the San Remo Manual in its section on “Precautions in Attack”.Footnote 229
Similarly, the 2013 edition of the Norwegian Manual notes that “the rules presented in other chapters of this manual also apply to maritime operations, in particular the rules on the conduct of attacks”; as a result, the Manual does not elaborate on precautions within its naval warfare chapter. Nevertheless, the Manual includes two relevant references that align with the wording of Article 57: the requirement that “all feasible precautions shall be taken to avoid or reduce incidental injury or damage to civilians and civilian objects”,Footnote 230 and the obligation “to take all feasible precautions to verify that a vessel is a lawful target”.Footnote 231
The Danish Manual notes that “[a]s regards attacks on objectives at sea or in the air, AP I merely states that the parties must take all reasonable precautions to avoid losses of civilian lives and damage to civilian objects”, explicitly referencing Article 57(4) of AP I. In explaining the practical application of this provision, the Danish Manual makes express reference to Rule 46 of the San Remo Manual, highlighting the requirement to take “all feasible” measures/precautions and directing readers to the San Remo Manual’s general chapter on “Military Objectives”, which elaborates on what it means in practice to “do everything feasible”.Footnote 232
Some States assert that land-based rules on proportionality and precautions apply at sea without explaining their practical application, even as they acknowledge that the maritime context may differ. Such an approach seems to be the one taken by the Canadian Manual, the New Zealand Manual, the Turkish Manual, the Togo Manual, and the CAR Manual.Footnote 233 Of note, however, Togo and the CAR’s manuals both emphasize that attacks against merchant ships and civilian aircraft without military significance must be avoided.Footnote 234 The New Zealand Manual states that “[a]ll orders of this manual apply with necessary modifications to armed conflict in the maritime or air environments”,Footnote 235 but while it references “necessary modification”, Chapter 10 (titled “Further Rules in the Maritime and Air Environment”) does not appear to indicate that such modifications affect the application of precautionary measures.
According to the authors of the Newport Manual, “[t]he standard of ‘reasonable’ precautions in attack at sea under Article 57(4) AP I suggests that it is a lower bar to attack than the ‘feasible’ precautions for attacks on land”.Footnote 236 In a separate article, one of the Newport Manual’s lead authors suggests possible substantive obligations under this provision in the context of naval warfare:
Reasonable precautions at sea might include such tactics as: (a) issuing warnings through radio communication or visual signals before engaging a suspicious vessel; (b) diverting suspect vessels away from conflict areas through the establishment of maritime zones or when setting a naval blockade; (c) conducting boarding operations before escalating to the use of force against a suspected ship; or (d) postponing or aborting an attack if civilian small craft or neutral vessels unexpectedly enter the engagement zone.Footnote 237
Proportionality and precautions in attack regarding vessels at sea: Who and what must be taken into account?
A major area of ambiguity concerns whether proportionality and precautions in attack must account for persons and objects on board a vessel that qualifies as a “military objective” under the San Remo Manual (Rule 40/AP I Article 52).
San Remo Manual Rule 46(a)–(b) focuses on the identification of civilian objects “present in an area of attack” and limiting attacks to “military objectives”, while Rule 46(c)–(d) outlines the obligation to avoid or at least minimize “collateral casualties”Footnote 238 and the prohibition against disproportionate attacks. Paragraph 46.5 of the San Remo Manual commentary (known as the “explanation”) illustrates this point through the sinking of the passenger liner Lusitania, which, despite carrying a significant cargo of munitions, resulted in the deaths of 1,198 passengers and crew – a loss retrospectively deemed disproportionate by the Group of Experts.Footnote 239 This example suggests that persons and objects on board a vessel – even one qualifying as a military objective – must be taken into account in proportionality assessments. At the same time, passenger liners have generally been treated as a special class of “exempt vessel” requiring a more comprehensive proportionality analysis, although the San Remo Manual does not explicitly explain this reasoning.
It is reasonable to ask whether passenger liners are the only vessels that merit such special consideration. The records of the Diplomatic Conference and the original 1987 ICRC Commentary on AP I provide little guidance on these questions, particularly when compared with scenarios involving incidental effects on people and objects near vessels under attack. For example, with respect to Article 57(4), the ICRC Commentary notes:
[O]ne could conceive that hostilities between adverse fleets could endanger the civilian population, with or without the intervention of the airforce, because missiles could miss their target or civilian ships or aircraft could get mixed up in the battle. Similarly, fighting between adverse military aircraft could have incidental repercussions on the civilian population – for example, when a crippled aircraft crashes. It should be noted that “all reasonable precautions” must be taken, which is undoubtedly slightly different from and a little less far-reaching than the expression “take all feasible precautions”, used in paragraph 2. As the nuance is tenuous, the purpose of the provision appears to be to reaffirm the rules that exist to protect civilians in such situations.Footnote 240
National military manuals and outputs of expert bodies since the publication of the San Remo Manual
Of those national military manuals that contain dedicated chapters on law of naval warfare issues reviewed by the authors, most do not provide much detail in terms of who and what must be considered for the purposes of proportionality and precautions in attack at sea.
A small number – including the Norwegian Manual,Footnote 241 the Danish ManualFootnote 242 and the US Commander’s Handbook on the Law of Naval Operations (US Commander’s Handbook) – appear to consider that a “vessel-based construct” applies generally to attacks against military objectives at sea. In this regard, precautions are directed primarily at minimizing harm to other vessels and objects in the surrounding area rather than to persons or objects aboard the target vessel itself.Footnote 243 The US Commander’s Handbook provides thatFootnote 244
[b]ecause the primary military objectives at sea are vessels, and the principle of proportionality is applied using a vessel-based construct, absent particular information, naval commanders are not generally required to conduct an individualized proportionality assessment of embarked personnel on the vessel once it has been deemed a lawful military objective.Footnote 245
That said, it is noteworthy that the US Commander’s Handbook states that a more detailed proportionality assessment may be required where the commander has “particular information”.Footnote 246 Both the Handbook and the US Manual also note that precautions must be taken to protect civilian workers on board a vessel which is a military objective, and that such workers must be considered in a proportionality analysis (including where it is a warship).Footnote 247 These latter statements seem to suggest that, in the US view, not everything or everyone on board a vessel that qualifies as a military objective automatically assumes the vessel’s legal status.
The Danish Manual states that those who plan an attack must
take all feasible measures to gather information which will assist in determining whether or not vessels or objects which are not military objectives are present in an area of attack [aligning with the San Remo Manual]. In practice, this entails a requirement to do what is feasible to compile a thorough situational analysis, particularly in coastal or other areas with heavy sea and/or air traffic.Footnote 248
The Danish Manual does not appear to mention civilians or cargo aboard vessels under attack. This could suggest that only other ships and aircraft need to be considered in proportionality assessments, or alternatively, it may reflect a practical focus on the situations most likely to pose most risk to civilians.
In a section of the 2025 Norwegian Manual on “Target Selection in Maritime Operations”, it is noted that while the relevant rules are largely the same as for land operations, their application at sea can present unique challenges. The Manual further asserts that some rules have traditionally differed – and continue to differ – between the two domains.Footnote 249 Concerning attacks against vessels that qualify as lawful targets – whether warships, auxiliary vessels, or enemy merchant ships meeting the criteria for lawful military targetsFootnote 250 – the vessel itself is the target, and there is no tradition of separating crew from vessel when assessing whether the vessel may be attacked, as is the case for land operations. Moreover, the rules on civilians directly participating in hostilities do not apply in operations that do not involve land.Footnote 251 Accordingly, in Norway’s view the status of persons on board the vessel, whether combatants or civilians or other non-combatants, is not relevant for assessing the legality of attacking the vessel.Footnote 252
What remains unclear from the three examples above is whether these States regard the “vessel-based approach” as a matter of law, or merely as a matter of operational practice. In other words, is it a legal principle that persons and objects on board assume the status of the vessel, or is it instead a practical reality – for example, that the presence of civilians on a warship or auxiliary vessel is generally not treated as excessive in a proportionality assessment, and that few, if any, feasible precautions can be taken to avoid harming them?
Other national military manuals, however, appear to suggest a different approach. In the French Manual’s section addressing restrictions on the conduct of hostilities specific to the maritime domain, the emphasis on the use of force is not just vessel-based but also on the protection of persons.Footnote 253 Meanwhile, the New Zealand Manual mandates that
[n]o attack is to be made upon a civilian ship by a New Zealand force without the express authority of [the Chief of Defence Force]. If authority is given, the commander of the New Zealand force is to ensure that:
a. at least one prior warning is given to the vessel,
b. civilians who are not directly participating in hostilities are not deliberately targeted,
c. the attack is conducted in a way to cause the least possible incidental civilian loss of life or injury and the least possible incidental damage to civilian property, and
d. the attack is only conducted if the direct and concrete military advantage outweighs possible incidental civilian loss of life or injury and incidental damage to civilian property.Footnote 254
Many State manuals consider passenger liners, specifically, as vessels exempt from attack “when engaged only in carrying civilian passengers”Footnote 255 and not when “being utilized by the enemy for a military purpose (e.g., transporting troops or military cargo) or refus[ing] to respond to the directions of the intercepting warship or military aircraft”.Footnote 256 Regarding whether passengers on a passenger liner must be included in the proportionality analysis, the limited number of military manuals that address naval warfare are largely silent on the issue. At least one manual, however, appears to suggest that those passengers are relevant considerations:
If a civilian passenger vessel constitutes a military objective and thus is liable to attack, any attack must comply with other applicable rules related to attacks.
In particular, attacks against civilian passenger vessels engaged in passenger service must comply with the requirement that the expected loss of life or injury to civilians, and damage to civilian objects incidental to the attack, must not be excessive in relation to the concrete and direct military advantage expected to be gained.Footnote 257
It is also noteworthy that, in their commentary to Article 12(4) of AP I,Footnote 258 Bothe, Partsch and Solf argue that the principle of proportionality permits the targeting of a warship even if it contains a sickbay, notwithstanding the obligation to respect and protect medical units. This reasoning suggests that those authors did not adopt a strict vessel-based approach as a matter of law – at least not with respect to wounded and sick combatants. If people and objects aboard a vessel automatically assumed the vessel’s legal status, there would be no need to invoke proportionality to justify incidental harm to a medical unit located on a warship.Footnote 259 As the authors explain, the principle of proportionality
is not restricted to the question of the protection of the civilian population for which it has now been codified by Part IV of Protocol I. An obvious example that medical units cannot be exempted by law from suffering collateral damage is the existence of sickbays on men of war. If it were inadmissible to subject medical units to collateral damage, no attempt to sink a warship with a sickbay aboard would be permissible.Footnote 260
The ICRC’s 1987 Commentary on AP I acknowledged that “[a]t sea, civilians on board warships run the risks to which such ships are exposed. If they are on board enemy merchant ships, their fate will depend on the nature of these vessels.”Footnote 261 Relevantly though, the ICRC’s recently updated Commentary on GC II clarifies the organization’s view that the presence of not only civilians but also wounded, sick and/or shipwrecked persons on board vessels is still a relevant consideration for both proportionality and precautions in attack:
Accordingly, the presence of wounded, sick or shipwrecked members of the armed forces in the vicinity of a military objective is to be taken into consideration when carrying out a proportionality assessment prior to an attack. In addition, and on the basis of the same rationale, an attacker must take all feasible precautions in relation not only to civilians but also to wounded, sick and/or shipwrecked members of the armed forces, to protect them from direct attack and collateral damage. It may be difficult for the opponent to know whether an enemy warship also carries wounded and sick persons, unless that fact has been communicated. That difficulty, however, does not release a Party from its obligations to respect the rules on distinction, proportionality and precautions in attacks. Footnote 262
Meanwhile, the Newport Manual adopts the “vessel-based construct” model seemingly reflected in the Danish Manual, the Norwegian Manual and the US Commander’s Handbook:
The law of land warfare focuses on the regulation of the conduct of hostilities and on the protection of victims, civilians, and civilian objects. By contrast, the law of naval warfare is more concerned with the status of objects and platforms, such as ships and aircraft, and the regulation of the exercise of belligerent rights by or against those platforms. Moreover, the principles and rules on targeting at sea will predominantly apply to attacks against objects, that is, platforms such as ships and aircraft, not the individuals who are operating or otherwise on board the platforms.Footnote 263
The Newport Manual authors then appear to subsequently clarify that proportionality applies only to surrounding civilian objects at sea and not to the persons or cargo on the intended targeted platform:
As with other AP I targeting rules, Article 51(5)(b) is not applicable at sea even for States party to AP I. Nonetheless, the rule reflects customary international law that attacks on land and at sea must not be disproportionate. Applying the prohibition of excessive collateral damage at sea often involves different factual considerations than on land. In the case of targeting an enemy military objective at sea, the risk of collateral damage to surrounding civilian objects is unlikely or much lower than in a targeting action on land.
Under the law of naval warfare, the crews or ship’s companies of targetable enemy platforms (such as warships and auxiliaries) need not be subject to any proportionality assessment even where some or all of them are civilians. Nonetheless, naval commanders must still be reasonably satisfied that any attack they launch will not cause excessive collateral damage to civilian objects in the vicinity or injury to civilians external to the enemy warship.Footnote 264
On this basis, the Newport Manual has little difficulty dealing with the question of enemy warships and auxiliaries (even if they have some civilian crew members). However, its consideration of proportionately does not squarely address the question of targeting a merchant vessel in all circumstances and finds that “[i]t is an unsettled issue whether the passengers on board a merchant vessel that is liable to attack must be taken into consideration in a collateral damage assessment”.Footnote 265
Issues requiring further State clarification on precautions and proportionality at sea
As regards attacks directed at or incidentally harming vessels at sea, the foregoing analysis reveals several unresolved questions concerning the interpretation and application of proportionality and precautions in attack:
• What is the correct legal standard governing precautions in naval operations – i.e., “all feasible precautions” or “all reasonable precautions” – and is there a meaningful legal or operational distinction between these formulations?
• Further, which precautionary measures derived from the traditional law of naval warfare or other legal regimes remain relevant to contemporary maritime operations, particularly in relation to merchant vessels?
• Specifically, how do States interpret and apply proportionality in attacks directed against merchant vessels? When are proportionality assessments required, and to what extent are they identical to, analogous with, or distinct from proportionality assessments on land or in other domains?
• Must proportionality assessments consider only civilian vessels and objects in the vicinity of the target, or must they also account for the civilian crew, passengers, other protected persons and cargo aboard the vessel itself?
• Is it legally correct to treat persons – whether civilians or other protected persons such as wounded, sick, shipwrecked or detained combatants – and objects aboard a vessel as automatically assuming the vessel’s status as a matter of law? Or is the so-called vessel-based construct better understood as a matter of operational practice, reflecting practical realities, for example that civilians are rarely present in large numbers on warships or auxiliary vessels, that such vessels often represent high-value military objectives during hostilities, and that in many cases few feasible precautions may be available to mitigate harm to those on board?
Clarifying protections for merchant ships in contemporary naval warfare
Transformation of the maritime domain
Since early naval warfare treaties, AP I and the 1994 San Remo Manual, significant developments have transformed the maritime domain. Today, naval combat reaches far beyond the waves, with operations extending into outer space, cyberspace and the electromagnetic spectrum. Navies can strike across vast distances with stealth and long-range weapons, operating in contested littorals where sea, air, land, cyber, and outer space converge. The threats are real and varied: unmanned maritime systems, aerial drones, precision missiles, swarming fast-attack craft, seabed warfare, naval mines. Naval warfare today is faster, more complex, and far more dangerous – and it is multi-domain by design.
Most naval warfare happens in international waters beyond the sovereignty of any single neutral State, but these spaces are far from empty; they are busy, crowded, alive with activity. Countless actors operate there every day, and most are neutral, completely unconnected to the fight. Moreover, today more than ever, the oceans remain a shared commons and their importance to all of us cannot be overstated.
When conflict erupts at sea, it affects far more than the belligerents. It touches trade, food, energy, communications, migration, leisure and a myriad of other global human activities. Tens of millions of people depend on the sea for food, whether through fishing or aquaculture, feeding millions more around the world. It powers our energy systems, from oil and gas to offshore wind and emerging green technologies. Our communications, too, run through the ocean, with almost all global data travelling via submarine cables. Beneath the waves lie rare metals essential to the technologies we use every day. The seas also support human mobility, both out of necessity and for leisure – some 20 million Americans cruise each year alone.Footnote 266 The ocean sustains life itself, producing roughly half the world’s oxygen,Footnote 267 and of course facilitates global trade.
Few sectors are as exposed to global conflict as civilian shipping. For centuries, merchant vessels have borne the brunt of naval warfare, and civilian crews have often faced the highest risks. Merchant shipping today looks nothing like it did at the end of the Second World War. Global trade is now faster, deeply interconnected, and essential to everyday life in ways that the traditional law of naval warfare simply never imagined.Footnote 268
Today, more than 80% of global trade by volume is transported via sea.Footnote 269 Modern shipping underpins the global economy, driven by innovations in containerization, specialized vessels and digital technologies. At the start of 2025, the global merchant fleet comprised around 112,500 vessels (including cargo- and non-cargo-carrying ships) of at least 100 gross tons registered in over 150 nations.Footnote 270 There were nearly 2 million merchant seafarers,Footnote 271 with several million more individuals working on fishing fleets.Footnote 272
Concurrently, the nature of merchant shipping has evolved considerably: vessels, crews, ownership structures, flags and cargoes are now far more complex and less closely tied to any single nation-State. Civilian seafarers – frequently unconnected to any belligerent party – face increased risks of harm during military operations. In 1945, merchant ships were usually crewed and flagged by the same country, carrying cargo tied to that nation, whereas today, the world’s largest container ship is Swiss-owned, Liberian-flagged, operated by a multinational crew, and can carry more than 24,000 standard 20-foot containers bound for markets across the globe.Footnote 273 Nearly three quarters of the global fleet sail under flags different from the nationality of their owners, making flag alone an unreliable way to distinguish enemy from neutral vessels.Footnote 274
Warfare at sea deeply affects civilians on land. Given international trade’s heavy dependence on maritime routes, even brief disruptions at critical maritime choke points can have profound consequences globally. Accordingly, maintaining open sea lines of communication remains essential to ensure the uninterrupted delivery of food and other essential goods to civilian populations – yet modern means and methods of naval warfare make safe passage harder than ever. Conflict at sea is truly multidimensional: merchant ships can be targeted at long range with increased precision and destructive force, while tactics involving deception, signature management and interference with navigation put civilian mariners at risk, leaving global commerce hanging in the balance.
At the heart of this are ordinary seafarers – the people who keep the world moving yet remain largely invisible. Every missile, drone, naval mine or voyage carrying risky cargo puts their lives at risk. Front lines cut through shipping routes and ports, leading to longer voyages or crews trapped in ports, increased dangers, and the constant strain of navigating war zones – all to sustain the supply chains on which civilians on land depend.
Setting aside the legal and moral debates over the acceptability of such conduct today,Footnote 275 even if economic warfare at sea were to re-emerge on the scale seen in past conflicts, future naval campaigns would be radically different. The sheer scale and configuration of modern container ships would seem to make searches for contraband impossible once the vessels are at sea,Footnote 276 and with long-range weapons, stealth, and unmanned systems, measures to ensure respect for the law – built for another era – must be carefully adapted to uphold the protection that the law offers to merchant ships and seafarers in the reality of the modern maritime domain.
Yet, even as the need to ensure both legal and practical protection for merchant vessels becomes increasingly vital, such considerations must be viewed in the context of long-standing practice. Historically, warring parties have relied on merchant shipping to sustain military operations, primarily for transport, logistical support and at times direct participation in hostilities, but also for the conduct of search and rescue.Footnote 277 These are practices that are all likely to persist in future conflicts at sea, particularly those at scale.Footnote 278
The challenge, therefore, is to ensure the protection of merchant vessels and their cargo, crews and passengers – and the populations that depend upon them – while contending with the harsh realities of modern naval warfare.
The need to integrate carefully humanitarian protections into modern restatements of the international law applicable to armed conflict at sea
Modern restatements of the law applicable to armed conflict at sea have increasingly incorporated the standards, civilian protections, and framework for targeting applicable to land warfare, including the use of “military objective” terminology found in Article 52(2) of AP I. This incorporation persists notwithstanding the interpretive uncertainty surrounding the application of Article 49(3)–(4) of AP I in modern conflicts, and despite the continued recognition of traditional belligerent rights exercisable at sea which are not always easily reconcilable with AP I rules.
As has been demonstrated in this article, the traditional law of naval warfare was focused principally upon capturing enemy and neutral merchant ships. While the traditional law allowed belligerents a qualified right to attack such vessels, the circumstances were narrow and required very direct integration into the opposing belligerent’s warfighting capacity. The right to use force to stop (and possibly ultimately sink) vessels whose conduct fell within qualified forms of resistance to visit and search or breaching a blockade was based upon an escalation of force, hence giving fair warning and opportunity for the merchant ship to submit to capture.
In addition, the requirement in Article 57(4) of AP I to “take all reasonable precautions to avoid losses of civilian lives and damage to civilian objects” in the conduct of military operations at sea has not found very expansive definition in State military manuals. Although it has been suggested the “reasonable precautions” standard in the context of attacks at sea should theoretically be less stringent than the “feasible precautions” standard on land, numerous States, the ICRC and various scholars today assert that the same land-based standard reflected in Article 57 simply applies in all contexts. Even those who consider that a different standard applies have not clarified what such a different standard would mean in practice.
Some questions remain about the operation of proportionality requirements when directing attacks against vessels, and in particular the regard that must be given to non-combatant individuals and civilian objects on board when a vessel qualifies as a lawful target. When it comes to factoring in persons on board a vessel that is to be attacked (perhaps in circumstances where a commander has “particular information” as to the passengers onboard), it would seem that Article 49(4) would have some application, though there is little discussion of this in State military manuals.
The divergent path between the traditional law of naval warfare and land-based IHL is a function of the historical development of the law of naval warfare, but is also said to reflect the realities of fighting at sea and the economic warfare aspect of the law of naval warfare. In line with the development of IHL since the Second World War, the contemporary law of naval warfare ought to reflect a more humanitarian perspective and a lower level of tolerance to civilian harm. However, as has been argued in this paper, this must be done in a clear way which is capable of consistent, practical operation.
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