1. Introduction
Article 2(4) is the centrepiece of the UN Charter’s system of collective security, prohibiting any use of force and, separately, any threat of force. The rationale of this separate prohibition is that threats are corrosive of international peace and security, coercing other states as effectively as the actual use of force. However, international practice has tended to condemn threats of force far less consistently than uses of force, often calibrating any condemnation on the seriousness of the threat and broader policy considerations.Footnote 1 Stances in the literature are ambiguous, with some scholars disapproving of threats for aggravating existing disputes and others suggesting that they may be used as means to prevent or terminate crises.Footnote 2
Threats of force are conventionally divided into two categories, explicit and implicit, which are both prohibited.Footnote 3 Explicit threats typically involve official statements, such as ultimatums, clearly indicating potential military action. Conversely, implicit threats, often arise from military conduct that lends verisimilitude to a future use of force by the acting state.Footnote 4 These can be more intimidating than explicit statements, as actions often speak louder than less credible words. The military build-up by Russia in December 2021 exemplifies an implicit threat of force.Footnote 5 Despite its size, sophistication and unusual nature, Russia denied plans for invasion, leaving states, including Ukraine, uncertain about its intentions.Footnote 6 The Russian representative to the UN asserted that the positioning of military forces within a state’s territory was a sovereign prerogative, not a threatening behaviour - a view shared by China and uncontested by other states.Footnote 7 Without increased international scrutiny, Russia might have coerced Ukraine’s political orientation while evading responsibility.
In international law, implicit threats suffer from a high degree of legal uncertainty. The boundary between an unlawful implicit threat and lawful state conduct is often blurred; intent can be difficult to establish,Footnote 8 the underlying action can frequently be framed as a sovereign right. For instance, in the Nicaragua case, the International Court of Justice (ICJ) stated that ‘in international law there are no rules … whereby the level of armaments of a sovereign State can be limited’ and concluded that the US military and naval manoeuvres did not contravene international law.Footnote 9
This frames the question that this article will be concerned with: How can implicit threats be more securely ascertained in international law? The purpose of the article is to develop a theoretically grounded and methodologically sound standard for ascertaining implicit threats and distinguishing those from permitted conduct of states in international relations. The discussion begins with a summary of the state of the law of threats. It then develops a standard specifically for implicit threats in three steps. First, it provides a general definition of implicit threats, distinct from explicit ones, and exemplifies the elements of this definition to more securely qualify implicit threats. In the second step, it sets out grounds that could justify threats. Thirdly and finally, it identifies the legal consequences of threats and modalities of international reaction to unlawful threats. In concluding, the article proposes an institutional pathway for progressively developing the prohibition on states to make implicit threats of force in their international relations.
2. Threats of force: An incomplete juridical journey
The international prohibition of threats of force, distinct from the use of force, is well established and grounded in Article 2(4) of the UN Charter, which states:
All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.Footnote 10
The Charter prohibition of threats of force has been reiterated and reinforced by the UN General Assembly through the Declaration on Principles of International Law concerning Friendly Relations and Co-operation Among States in Accordance with the Charter of the United Nations (1970) and the Declaration on the Enhancement of the Effectiveness of the Principle of Refraining from the Threat or Use of Force in International Relations (1987). The first posits a ‘duty’ to refrain from the threat and use of force, emphasizing that both constitute ‘a violation of international law and the Charter of the United Nations and shall never be employed as a means of settling international issues’.Footnote 11 The second declaration stipulates that the ‘principle of refraining from the threat and use of force in international relations is universal in character and binding’.Footnote 12 In a more specific context, UN General Assembly resolutions, in line with the ICJ’s Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons, advocate for a multilateral, universal, and binding agreement prohibiting, among others, ‘the use or threat of use of nuclear weapons’.Footnote 13 In addition, Article 8bis of the 1998 Rome Statute of the International Criminal Court (ICC) establishes individual criminal responsibility for acts of aggression, which constitutes an aggravated form of force.Footnote 14 The planning and preparation of such acts, characterized by the Rome Statute as a ‘crime of aggression’, may, in certain circumstances, amount to a threat to commit aggression.Footnote 15
Despite the systemic importance of the separate prohibition of threats of force for the Charter law of collective security, the term is not defined in any of the above instruments. Nor do the records of the San Francisco Conference on the UN Charter or the records of the General Assembly resolutions interpreting the principle of the non-use of force clarify what constitutes a threat.Footnote 16 The ICJ has touched on this part of the Charter in only a handful of judgments, most notably in Corfu Channel and Nicaragua. In both cases, the Court concluded that there was no unlawful threat but offered minimal reasoning to support these conclusions. In its Nuclear Weapons Advisory Opinion, the ICJ set a standard for assessing the legality of threats and provided a few examples in which a threat would be illegal.Footnote 17 In reality, the ICJ was more concerned with the prohibition than any definition of threats of force.Footnote 18
The International Law Commission (ILC) has undertaken a sustained and detailed effort to define threats of force, although it ultimately abandoned the project. The Commission took up the matter in the context of the Code of Offences against Peace and Security of Mankind (later renamed the Code of Crimes against Peace and Security of Mankind) between 1951 to 1995.Footnote 19 In 1985, the ILC Special Rapporteur suggested that a military threat is ‘the intention expressed … by a state to commit an act of aggression’.Footnote 20 The 1989 Commentary to Article 13 of the ILC Draft Code of Offences stipulated that the term ‘threat’ refers to ‘acts undertaken with a view to making a state believe that force will be used against it if certain demands are not met by that state’.Footnote 21 Article 16 of the ILC’s 1991 Draft Code of Crimes defined all threats in the following way, without distinction between implicit and explicit ones:
Threat of aggression consists of declarations, communications, demonstrations of force or any other measures which would give good reason to the Government of a State to believe that aggression is being seriously contemplated against that State.Footnote 22
However, this juridical journey remains incomplete. In 1995, the ILC endorsed the Special Rapporteur’s proposal that ‘the crimes of the threat of aggression and intervention should be left aside for the time being because of their vague and imprecise nature’.Footnote 23 As a result, the final version of the Draft Code (1996) no longer features a provision on the threat of aggression or force.
In parallel, scholars have sought to establish the ordinary meaning of the term threat in Article 2(4) of the UN Charter, pursuant to Article 31(1) of the Vienna Convention on the Law of Treaties (VCLT), without reaching full consensus.Footnote 24 In 1963, Brownlie proffered a widely cited definition, characterizing a threat of force as ‘an express or implied promise by a government of a resort to force conditional on non-acceptance of certain demands of that government’.Footnote 25 Sadurska published an influential article in 1988 on threats of force in modern international law. She conceived of a threat as an act of coercion designed to induce ‘a psychological condition in the target of apprehension’, aiming to achieve ‘a genuine reduction in the range of choices otherwise available to states’.Footnote 26 Stürchler, in one of the few monographs dedicated to the subject, adopted a broad perspective and, in lieu of a single definition, linked threats to the principles of non-intervention, breach of peace, the positive duty of states to negotiate without military pressure, and the use of force itself.Footnote 27 Stürchler also identified two key qualities of threats: imminence and coercion, emphasizing the need for an existing dispute and credibility. He further noted the presence of threats in practices such as militarization and limited military engagements.Footnote 28 Grimal, in another monograph, situated the concept of threat within ‘a twilight zone between diplomacy and war’ and concurred with Brownlie’s and Sadurska’s definitions, additionally noting that a threat can also be indirect and concern more than two states (State A supplying State B to threaten State C).Footnote 29 Grimal explored various dimensions of the issue, including whether preparations for aggression constitute a threat of force, the differential legal and political treatment of threats versus the use of force, the criteria for determining the lawfulness of threats in self-defence, and the legality of non-serious threats. Kleczkowska argued that threats may take the form of physical actions, written communications, or oral statements, which must be credible and perceived as such by the addressee. The latter subjective elements necessitated a case-by-case analysis.Footnote 30 Regarding implicit threats, scholars contend that ‘defining the contours of an implicit threat is an inherently difficult task’,Footnote 31 though they largely converge on the concept of a ‘demonstration of force’, which may occur independently or alongside explicit communication, such as an ultimatum. Despite these difficulties, it is widely recognized that implicit threats in the form of actions may exert a greater influence than mere words, whether spoken or written.Footnote 32
In sum, the ILC interpretation remains the most precise framework to date for identifying a threat of force and serves as an influential benchmark for scholars and commentators, despite referring to a ‘threat of aggression’ rather than simply force. This definition has four key elements: (i) the action; (ii) credibility; (iii) the author of the threat; and (iv) the single category of threats. Each element warrants critical analysis in light of the international debate and the evolving global context since the 1990s.
First, the ILC conceives of the action underpinning a threat in a notably broad sense, because it also covers ‘other measures than demonstrations of force’, hence non-military means may also fall under this definition. However, it is questionable whether this is consistent with the wording of Article 2(4) of the UN Charter. The international debate has now settled that force within the meaning of the provision refers to armed force, rather than economic or other forms of coercion.Footnote 33 Pursuant to the symmetry between uses of force and threats of force, only threats of military (armed) force should be covered. Such symmetry between the two legs of the provision of Article 2(4) of the UN Charter is important for the systematically consistent interpretation mandated by Article 31(1) of the VCLT.
Furthermore, the ILC definition emphasizes credibility. This is clear from the stipulation that there must be a ‘good reason … to believe that aggression is being seriously contemplated’. Similarly, the International Fact-Finding Mission on the Conflict in Georgia of 2008 noted that ‘the emphasis of the practice of states is on credibility. A threat is credible when it appears rational that it may be implemented, when there is a sufficient commitment to run the risk of armed encounter’.Footnote 34 Yet, while it is convincing to demand credibility and understand it as the probability of armed confrontation, this criterion should not remove low-intensity threats from the ambit of Article 2(4) of the UN Charter. Again, maintaining symmetry with the prohibition on the use of force is important.Footnote 35 Also, if credibility of the threat is to be assessed from the perspective of the addressee, this invites questions. Thus, what if a state overinterprets a situation, perceiving it as a threat when it is not? Conversely, if a state deliberately chooses to ignore what is objectively a threat, does that obviate the threat? An inter-subjective corrective is required to address these concerns.
Thirdly, the quoted ILC definition does not specify that a threat must originate from a state.Footnote 36 In contrast, an earlier version had required a ‘threat by the authorities’ of a state.Footnote 37 By leaving open the author of the threat, the definition becomes evolutionary. It is true that the inter-state element remains dominant in the use of force domain,Footnote 38 but the object and purpose of the prohibition to make threats warrant a broader interpretation. Certainly, threats will continue to be primarily issued by states. But the class of potential authors of threats should now encompass non-state actors (NSA), such as insurgencies (e.g. the Polisario Front in Western Sahara), secessionist movements (e.g. the Kosovo Liberation Army (KLA) in Kosovo during the 1990s) and terrorist organizations (e.g. Al Qaeda or Hamas), that are capable of securing arms and can issue credible threats.Footnote 39 Such entities often rely on threats rather than sustained use of force to compel others. For instance, it was the threat of force by a non-state actor that sparked the debate on anticipatory self-defence.Footnote 40 The reverse scenario is also possible – NSA, or putative states, (e.g. Kosovo, Northern Cyprus, Abkhazia, Taiwan, Nagorno Karabakh) could be the addressees of a threat. Whether these entities qualify as states is debatable, but the scope ratione personae of international law has extended to them in certain cases.Footnote 41 In sum, the prohibition ratione personae exceptionally extends beyond states to other entities where they are capable of making credible threats or being the targets of such threats.
Fourthly, and critically for the purposes of this article, the ILC definition covers the general category of threats without distinguishing between implicit and explicit (or express) threats. It mentions declarations and communications, which may be considered explicit threats of force, as well as demonstrations of force and other measures, which may be considered implicit threats of force. But, it lacks elements that clearly distinguish between the two categories and it is arguably not fine-grained enough to adequately capture implicit threats. In the realm of international relations, implicit threats have gained practical significance. This may have to do with the fact that in the case of implicit threats the dividing line between lawful and unlawful conduct is much blurrier. This ambiguity incentivises powerful states to resort to implicit threats to achieve their objectives while maintaining deniability and avoiding international sanctions.
3. Defining implicit threats of force
This section will develop a definition specifically of implicit threats to reduce this ambiguity. It will build on the ILC’s framework, but adapt it to address implicit threats, taking into account the above considerations on action, credibility, and the author of a threat. Section 4 will then provide an exemplifying typology for this general definition.
A starting point for generally defining implicit threats of force is to contrast the terms implicit and explicit. Most dictionaries define explicit as clear and precise and implicit as suggested,Footnote 42 although they acknowledge that these terms may carry different connotations in specific contexts.Footnote 43 A general and intuitive understanding on that basis would be that explicit threats are clear and direct, while implicit threats are inferred or suggested. The follow-on question arises how to determine what makes a threat clear and who determines this clarity.
A possible approach is to define a threat as clear when it is expressed verbally, either orally or in writing, whereas implicit threats are not.Footnote 44 An example could be an ultimatum that clearly articulates the threat of military action should a demand not be met, such the 2023 ECOWAS ultimatum to Niger, which set strict deadlines for the release and reinstatement of deposed President Mohamed Bazoum. The conventional distinction referenced in Section 2 in a similar sense posits that an explicit threat denotes a clearly communicated official statement, while an implicit threat is action. Despite its apparent simplicity, this exclusively formalist approach has shortcomings. In the reality of international relations, such formalism has faded in relevance; formal declarations of war, for instance, have essentially disappeared giving way to less formal and conventional practices.Footnote 45 Therefore, a slight deviation from the formalist approach is needed, allowing for the possibility that a statement can constitute an implicit threat. In state practice, there are examples where opaque or indirect statements were seen as implicit threats of force. For instance, during a debate in the UN Security Council in 1962, Pakistan referred to the official pamphlet issued by India titled ‘Kashmir and the United Nations’, which stated, ‘India is prepared to be patient and tolerant … but it is obvious that there is a limit to patience and tolerance.’ Pakistan recognized these words to be a threat of force.Footnote 46 One could argue that while the words themselves were not outright threatening, they implied a threat. On the other hand, why should explicit threats be limited to verbal expression and not include actions or other forms of non-verbal communication, as it is the case in many domestic laws?Footnote 47 In conclusion, while an articulated pronouncement - or denial - may be indicative of an explicit threat, this is not decisive in every case.
A more substantive approach that has a sound theoretical grounding is called for. Such grounding can be found in legal theory. Legal realism, for instance, emphasizes the importance of considering the broader context of actions, including power dynamics, inferred intentions, and historical precedent.Footnote 48 These factors are essential for understanding implicit threats because they help explain how actions, rather than explicit statements, can communicate underlying intentions and signals of force. Building on legal theory, international relations provide a more recent and specific contribution to apprehension what characterizes an implicit threat. Thus, the constructivist theory in international relations emphasizes the role of social norms, identities, and discourse in international relations.Footnote 49 Implicit threats may be embedded in a narrative or discourse where states rely on shared understandings and historical contexts to communicate their intentions. By contrast, explicit threats disrupt these narratives by challenging the status quo or signalling a significant shift in behaviour.Footnote 50
Insights from strategic studies further shed light on the importance of context in distinguishing implicit and explicit threats. Explicit threats are often integral to a deterrence strategy, where the threatening state seeks to prevent an adversary’s action by clearly outlining the consequences. Implicit threats, on the other hand, operate in a more ambiguous space, where the threatening state relies on the adversary’s inference of potential consequences without openly stating them.Footnote 51 In game theory, explicit threats are akin to moves in a strategic game where intentions and responses are clearly mapped out. Implicit threats, by contrast, introduce uncertainty into the game, forcing the opposing player to guess at the threatening state’s intentions and possible moves.
Arguably the most valuable insights can be gained from general theories of communication, such as Grice’s Cooperative Principle—based on maxims of clarity, relevance, and truthfulness—or Austin’s Speech Act Theory, which suggest that meaning and intent can be inferred from context.Footnote 52 Thus, while a threat that is not verbally communicated might initially be considered implicit, accompanying events on the ground that reveal the coercive intent of the threatening party could render it explicit. The point at which this transition occurs depends on various factors. This focus on the communicative context is further supported by the psychological Theory of Mind, which suggests that individuals recognize that others may have different beliefs, desires, and intentions.Footnote 53 Implicit threats, therefore, are often analysed through social cues and power dynamics, leading to uncertainty, anxiety, and a prolonged period of strategic calculation—responses that align with Uncertainty Reduction Theory.Footnote 54 In contrast, explicit threats are intended to be understood as direct and unambiguous signals, leaving no room for inference and often triggering immediate defensive reactions, as explained by Stress and Coping Theory.Footnote 55 In the present context, a threatening state assumes that the decision-makers of the target state will infer the intention behind its actions regardless of the form in which the threat is conveyed.
Is this communication-based approach helpful for determining international responsibility under international law? This depends on whether it is consistent with the function of the separate prohibition of threats of force, as distinct from its uses, in Article 2(4) of the UN Charter. The function of this prohibition is to prevent one state from compelling another by threats of force, thereby protecting the free decision-making of state. The prohibition hence focuses on communication between two or more states or equivalent actors. Within the ambit of Article 2(4) of the UN Charter, a use of force is action, whereas a threat is communication. The categories of explicit and implicit threats, therefore, represent different types of communication and coercive messaging. They are distinguished by how the acting state’s messaging becomes effective. Thus, explicit threats contain messaging that is straightforward to determine, while implicit threats involve messaging that requires further analysis. For something to qualify as an implicit threat, the acting state’s messaging must have an identifiable addressee and refer to a specific future action. This excludes non-targeted phenomena, unsupported assertions, and ambiguous actions from the ambit of implicit threats, as they remain lawful. While some of these phenomena may eventually lead to a threat, they do not constitute a threat to the addressee with force until they are used to convey a targeted message.Footnote 56
Further, explicit threats differ from implicit threats in the way they present gravity, timing, and credibility of the future miliary action. Gravity refers to the seriousness or severity of the potential consequences, including the scale and impact of the threatened action. Explicit threats often convey the gravity of the potential consequences in a clear and direct manner. The threatening state might specify the extent of the force it intends to use, such as deploying nuclear weapons or launching a full-scale invasion. In the case of implicit threats, the gravity is less clear and must be inferred from actions, context, or the threatening state’s capabilities, positioning of forces, or past conduct. Also, in explicit threats the timing is often clearly communicated, whereas implicit threats often lack a clear timeline, creating uncertainty about when or if the threatened action will occur.Footnote 57
On this basis, the following general definition is proposed: An implicit threat of force is a military message aimed at compelling another actor(s) to act, refrain from acting, or tolerate the acts of others. An implicit threat of force must involve coercive messaging with clearly identifiable issuers and addressees. It must also relate to future action that demonstrates a certain level of gravity, imminence, and credibility. State conduct short of this remains lawful.
4. Determining implicit threats of force
This general definition needs to be exemplified to securely determine instances where an implicit threat of force has occurred. Such a two-step approach was employed in the seminal Resolution 3314 (XXIX) defining aggression, which was later replicated in Article 15 of the 1991 ILC Draft Code of Crimes, and Article 8bis of the ICC Statute. Resolution 3314, in a first step, provides a general definition of aggression and, in its second step, enumerates acts constituting aggression. Appropriately adapted, this two-step methodology can be applied to threats of force. The first step, the development of a general definition of implicit threats, was undertaken above. This section proceeds to the second step by exemplifying the general definition to formulate a workable test. The proposed test consists of four objective elements and one subjective element. The objective elements require: messaging that targets the addressee’s decision-making (Section 4.1.); and reference to a future use of force that is credible, of sufficient gravity, and imminent (Sections 4.2-4.5). Additionally, the subjective element necessitates that the threat be apprehended by the addressee. Each element will now be explored in some depth.
4.1. The messaging
For implicit threats of force to fall under the proviso in Article 2(4) of the UN Charter, they must be directed against the ‘sovereignty, territorial integrity, or political independence’ of a state. However, threats are generally issued to compel decision-making by the target state, rather than undermining ‘sovereignty’ in the abstract. In Nuclear Weapons, the ICJ opined that military duress is illegal when used to acquire territory or achieve specific political or economic objectives.Footnote 58 This reasoning can be extended to include messaging aimed at coercing the addressee’s social, religious, or other forms of decision-making.
4.1.1. Manifestation of military force
The messaging that characterizes an implicit threat must relate to a future use of force. Force in this sense denotes armed force. This is widely accepted for Article 2(4) of the UN Charter, as well as Articles 1(1) and 39 of the UN Charter that refer to aggression.Footnote 59 Hence, a military message is required. In contradistinction, political or economic pressure do not suffice. In this sense, a bellicose right-wing political party winning an election per se does not embody a threat. The dropping of waste on the South Korean part of the border in retaliation for the propaganda leaflets dropped in the North was likewise not a threat. Nor was the organization of a consultative referendum on the annexation of Essequibo to the territory of the Bolivarian Republic of Venezuela tantamount to a threat.Footnote 60 The message of a future use of force may manifest itself through a current display of military force.
Military build-ups are the most intuitive type of such manifestation. A dynamic build-up to support a future military operation against an identifiable target stands out qualitatively and quantitatively.Footnote 61 Factors such as the size and type of personnel and military equipment deployed, their positioning, and the timing of the deployment are relevant in distinguishing a military build-up from static situations, such as long-established military bases or the longstanding stationing of troops.Footnote 62
For example, the UK’s use of a naval presence to secure compliance with its demand for the evacuation of Taba by Turkish troops was regarded as a ‘show of force’,Footnote 63 as was the substantial Iraqi military build-up along the Kuwaiti border in 1994.Footnote 64 More recently, the Russian military build-up in the vicinity of Ukraine in 2022 was perceived as a military threat,Footnote 65 while Belarus characterized NATO’s increased military presence on its western and southern borders as becoming threatening in nature, though not yet rising to the level of a threat.Footnote 66 By contrast, the US Camp Bondsteel, established in 1999, would only pose a threat to Serbia if Serbia sought to regain control over Kosovo. Similarly, the militarization of Greek islands in the eastern Aegean Sea, although contrary to the Lausanne Peace Treaty (1923) and the Paris Peace Treaty (1947), is a sign of defensive rather than aggressive intentions. The temporal context is also significant. In the Corfu Channel case, the ICJ did not find the UK warships’ presence ‘at action stations’ very close to the Albanian coast to be contrary to international law, considering the immediate post-war period.Footnote 67 However, such demonstrations of force are generally held to be illegal in contemporary state practice.
In contrast to build-ups, aerial, land, and naval manoeuvres are typically not linked to a future armed operation. International practice has developed mechanisms to ensure that such manoeuvres convey routine and non-aggressive messages. Instruments such as the Helsinki Final Act and the Vienna Document encourage states to increase transparency in their military actions through measures including: refraining from organizing major military manoeuvres without prior notice; inviting observers; conducting joint inspections; declaring the levels of armament, and allowing verification missions.Footnote 68 For manoeuvres to be a manifestation of force, they must become ‘directed’, meaning they communicate to the addressee that they are in preparations for a future operation. Factors such as frequency, sophistication of the weapons and equipment used, and a lack of communication or transparency can contribute to this perception.Footnote 69 At the far end of the spectrum, military parades merely convey a general message of strength to the domestic and international audience, even if they concurrently employ bellicose rhetoric, like those in Iran or North Korea.Footnote 70
An actual use force can at the same time be a manifestation underpinning the threat of a future military operation. In particular, smaller-scale military incursions into the territory of another state may at the same time constitute a threat of larger-scale aggression. Iraq, Lebanon, and Libya have complained several times to the Security Council using this argument.Footnote 71 In fact, many domestic statutes permit the use of force in self-defence in response to foreign military encroachments.Footnote 72 The US Commander’s Handbook on the Law of Naval Operations sets forth that ‘an aircraft with military markings will be presumed to be conducting a military mission. This is the case both for tactical military aircraft … and unarmed military aircraft capable of being used for intelligence-gathering purposes’.Footnote 73 The Handbook rules operationalize a doctrine of preventive self-defence.Footnote 74 Yet, the mere violation of airspace is better qualified as a threat of force than aggression or a use of force.Footnote 75 The downing of a military or other state aircraft, a fortiori a civil aircraft such as a meteorological balloon covered by Article 3bis of the Chicago Convention, would require a direct and serious threat to national security as well as the compliance with the proportionality requirements of Article 51 of the Draft Articles on State Responsibility (ARSIWA).Footnote 76 Recurring territorial encroachments that are not acted upon may lose their threatening character.Footnote 77 For instance, in 2022, Greece and Turkey accused each other of violating their airspace over 1000 times.
Supplying arms to non-state actors active in another state is a breach of the principle of non-intervention,Footnote 78 but it can also be a use of force. The ICJ in Nicaragua established that arming rebels amounts to a use of force.Footnote 79 Similarly, in Armed Activities on the Territory of the Congo, the ICJ confirmed that a state providing training and military support to private groups that carry out acts of violence within another state’s territory violates the prohibition of the use force.Footnote 80 Hence, messaging that such support will be provided constitutes an implicit threat of force. This principle extends to the provision of arms to states involved in ongoing or probable conflicts, especially when it concerns advanced or nuclear weapons. A pertinent example is the 2023 arms deal between Vladimir Putin and Kim Jong Un.
Finally, emphasis must be placed on the element of attribution, a condition for establishing an internationally wrongful act.Footnote 81 The force manifestation must be attributable to a state. Attribution challenges arise when it involves future actions by non-state actors, either acting independently or in coordination with the threatening state’s forces. The attribution framework articulated in Nicaragua applies here as well. Accordingly, a distinction is to be made between a state’s support for non-state actors and the state’s control over those actors which would render their actions attributable to the state.
4.1.2. Targeted messaging
This messaging must be targeted. Any force manifestation only constitutes a threat if it specifically targets another state or group of states, for only then can it have coercive effect within the communication framework discussed earlier. Targeting is what differentiates an implicit threat from situations that involve a military element but pose merely a general risk to global stability and peace, such as frozen conflict zones or terrorist organizations.Footnote 82 The question then becomes how to determine that there has been a targeted messaging. The Georgia Report offers valuable guidance, noting that militarized acts become threatening
as soon as they are non-routine, suspiciously timed, scaled up, intensified, geographically proximate, staged in the exact mode of a potential military clash, and easily attributable to a foreign policy message, the hostile intent is considered present and the demonstration of force manifest.Footnote 83
Conversely, an inchoate or static fear that may relate to a powerful neighbouring state does not suffice to constitute a threat.
The mere possession of nuclear weapons, for instance, does not normally constitute a threat, as it does not communicate a message specifically targeted at another state. The prevailing opinion among commentators is that nuclear stockpiles and the deterrent policies associated with them are directed ad omnia and hence lack the necessary targeting to be a threat of force.Footnote 84 This view is supported by case law. In Nuclear Weapons, the ICJ did not consider the mere possession of nuclear weapons to violate Article 2(4) of the UN Charter. A similar conclusion was reached more recently in Marshall Islands. In that case, the Marshall Islands had instituted proceedings against the United Kingdom under the compulsory jurisdiction clause, arguing that the UK had failed to meet its disarmament obligations. The ICJ declined jurisdiction due to the lack of a dispute between the parties. In its reasoning, the Court acknowledged the ‘suffering’ experienced by the Marshall Islands between 1946 and 1958 caused by the US nuclear testing programmes, but determined that the current dormant UK nuclear arsenal, by itself, did not constitute grounds for a dispute.Footnote 85 The Court was not persuaded that the UK’s vote against UN General Assembly resolutions on nuclear disarmament or its alleged failure to pursue disarmament negotiations in good faith amounted to sufficient evidence of a dispute.Footnote 86 This rationale can be extended to the substantive law question of whether there was a threat: the UK’s nuclear stockpile, on its own, did not present a threat directed at or targeted towards the Marshall Islands. However, several judges in separate opinions suggested that the very ownership of nuclear weapons can be regarded as a threat to international peace and security.Footnote 87 Finally, actual nuclear tests may cross this line and amount to a targeted threat. For instance, in the wake of the war in Ukraine, North Korea’s continuous nuclear tests compelled Japan to react by enacting the 2022 National Security Strategy, which ‘dramatically transforms Japan’s national security policy after the end of WWII’.Footnote 88 Recent Russian threats to use nuclear weapons provide another example. While the addressee is not clearly defined, it could reasonably be Ukraine and NATO.Footnote 89
It is uncontroversial, furthermore, that a warning by state A given to state B that a state C is contemplating an attack against state B is not a threat by state A, but rather an informational communication.
4.1.3. Associated military conduct
Messaging in the form of manifestations of force should be distinguished from associated military conduct, as the latter, on its own, does not suffice to constitute a threat. Examples of such conduct include increases in military budgets, arms purchases, withdrawal from peace treaties, contracting mercenaries, and providing technical support for military operations. This principle is reflected in the Nicaragua case where the ICJ held that the level of a nation’s armament is a sovereign prerogative and, in the absence of specific international obligations, cannot constitute a threat under Article 2(4) of the UN Charter.Footnote 90 Nevertheless, if there exist reasonable grounds to believe that these arms may be used for aggressive purposes, this could, in exceptional cases, constitute an implicit threat of force. The Nazi Wehrwirtschaft in contravention of the 1919 Versailles Treaty met that test.Footnote 91 In contrast, Poland’s significant increase in military spending in the wake of the Russian aggression in Ukraine serves defensive purposes.
4.2. Credibility
Implicit threats of force must be credible. In this context, credibility refers to the likelihood that the threat will actually be carried out. This likelihood can be concretized through certain factors, including the perceived intent behind the threat, the issuing state’s demonstrated willingness and capacity to follow through, its historical behaviour, and the resources available to support the threatened action.
4.2.1. The existence of a dispute and a related demand
An underlying reason is a necessary condition for a threat to be credible. It is unrealistic to expect any state engaging in military action in the absence of any reason. Following Stürchler’s analysis,
a threat will not be perceived or even recognised without there being, so to speak, a peg of dispute on which to hang the expectation of the use of force. Consequently, a threat that contains no reference to a particular dispute, no issue to create pressure against and no element of coercion usually appears meaningless.Footnote 92
In essence, a dispute between the parties is a prerequisite for a credible threat. Without such a dispute, actions cannot convey a targeted demand to influence a certain course of action. The ILC’s Special Rapporteur agreed that a military threat might emanate from conflict zones - ‘disputes’; or ‘situations’ referred to in Chapters VI and VII of the UN Charter, especially Article 33 (regarding disputes likely to endanger international peace and security) or Article 39 (concerning threats to peace).Footnote 93 It is, thus, essential to first understand what may constitute a relevant dispute. The Guyana/Suriname arbitration case provides a pertinent illustration. There, the Arbitral Tribunal examined a maritime boundary dispute between the two states over contested territorial waters and exclusive economic zones. Tensions escalated when a Canadian oil company, licensed by Guyana, began exploratory drilling in the disputed area. Suriname responded by sending naval vessels and ordered the company to cease the drilling and leave the area immediately. The Tribunal found that Suriname’s alleged ‘law enforcement activities’ constituted a threat of military action in contravention of the United Nations Convention on the Law of the Sea, the UN Charter and general international law.Footnote 94 In other words, the threat was credible because it served to enforce the demand of one party to an existing boundary dispute.
Further, it becomes important to understand what constitutes a dispute that can lend credibility to a threat. The classic definition frames a dispute as arising from two opposing claims regarding law or fact. Recent jurisprudence from the ICJ provides additional clarity, particularly on the procedural requirements for recognizing a dispute over which the Court can exercise jurisdiction.Footnote 95 The Court requires specific enough representations that the other party is aware of and cannot wilfully ignore. While this is a procedural definition, an analogy can be drawn to the substantive law context. Thus, general statements devoid of a specific demand, such as Iran’s call to ‘wipe Israel off the face of the earth’ or the North Korean one to sink Japan and reduce the US to ‘ashes and darkness’, are not disputes that can give credibility to a threat.Footnote 96
4.2.2. The profile of the party accused of a threat: International reputation, current military capacity and the geographical situation
A threat becomes more credible if it is likely that the action will be carried out to enforce the demand. At least three factors related to the actor’s profile contribute to this likelihood. The first factor is the actor’s international reputation, including its record of past military behaviour. If an actor has resorted to explicit or implicit threats of force in the past and followed through on them, it is more likely to do so again.Footnote 97 The second factor is the actor’s capacity to perform military operations. A highly armed state with deployable means is more capable of carrying out a threat, whereas an actor without substantial military resources cannot effectively threaten force.Footnote 98 The geographical proximity between the states is the third relevant factor. Generally, states are more capable of conducting military operations in the vicinity of their territories (or from the territory of an ally) than from a distance, making threats between neighbouring states more credible. Indeed, most wars were waged between neighbouring states.Footnote 99 However, this qualification does not apply to military superpowers capable of projecting force and engaging in operations far from their territories.
Against this general backdrop, the likelihood that the acting party will follow through on a threat—and thus lend it credibility—can be analytically assessed. In the literature, Schelling has developed a model for such analysis,Footnote 100 drawing on principles reminiscent of game theory.Footnote 101 According to Schelling, a party evaluates whether a threat is a more effective means of achieving its objective than offering an incentive, and, if so, how to calibrate the threat’s intensity.Footnote 102 When it becomes evident that the party’s payoff is greater if the threat is carried out, the credibility of the threat is significantly enhanced.
4.3. Gravity
The messaging must also reach a certain level of gravity. The above discussed ILC’s definition contains the qualifier ‘seriously’ to indicate this. While gravity is an essential criterion to exclude de minimis events from constituting an implicit threat, calibrating it precisely remains challenging.Footnote 103 How expansive, intense and specific must a threatening message be to meet the minimum gravity threshold? The minimal threshold for a future use of force that can underpin a threat will be pegged to the threshold for actual uses of force. The law on the use of force provides an upward gradation through semantic markers based on scale and effects. For example, aggression is understood as ‘the most serious and dangerous form of the illegal use of force’.Footnote 104 Further, only the ‘most grave forms of the use of force’ constitute an ‘armed attack’, which justify the recourse to self-defence under Article 51 of the UN Charter.Footnote 105 This gradation makes it easier to ascertain clear instances where force is used.Footnote 106 By contrast, determining the lower threshold for unlawful uses of force is more challenging.Footnote 107 Scholars have questioned whether this gradation is transferable to threatsFootnote 108 ; others consider it relevant in determining consequences and reactions to threats.Footnote 109 It seems, however, convincing that a threat relating to a future aggression or armed attack clears the gravity threshold. On the other hand, a threat related to lower-intensity force is best assessed on a case-by-case basis.
4.4. Imminence
The messaging must pertain to a military action that is sufficiently imminent. Of course, imminence cannot mean a specific time or timeline, for, under the general definition proffered above implicit threats often lack a clear timeline, creating uncertainty about when or if the threatened action will occur. Rather, the criterion of imminence excludes from the category of implicit threats any latent risks that are only realizable at an indeterminate point in the future. This is in line with the object and purpose of Article 2(4) of the UN Charter, as latent situations lack the power to coerce the current decision-making of the addressee.Footnote 110 The ICJ’s advisory opinion in Nuclear Weapons refers to exactly this distinction. There, the Court noted the defensive role of nuclear weapons based on deterrence.Footnote 111 The Court acknowledged that such weapons may be used in the future and that their use could be lawful under certain circumstances. However, without an indication of imminent use against a specific target, these weapons constitute a latent risk rather than an implicit threat of force.Footnote 112 This distinction can be further illustrated by the case of Iran’s nuclear programme, which has long been a concern for the international community. Iran’s ‘inalienable right … to develop research, production and use of nuclear energy for peaceful purposes’ is contested on the presumption of non-peaceful intentions.Footnote 113 As a result, the Joint Comprehensive Plan of Action (JCPA) was negotiated between the UN Security Council - with Germany and the EU High Representative - and Iran. The agreement substantially reduced Iran’s stockpile of low-enriched uranium and established a framework for regular inspections.Footnote 114 The JCPA addresses a latent situation that, while marked by ongoing tension, does not yet constitute an imminent threat.
In extant conflicts, it can be problematic to establish at what point a latent situation becomes an acute threat. For instance, in 2018, Qatar accused Saudi Arabia of a military threat emanating from the leaked letter sent by Saudi King Salman to French President Emmanuel Macron stating that Saudi Arabia was prepared to consider all measures, including military action, in response to Qatar’s purchase of the S-400 defence system from Russia.Footnote 115 On the backdrop of the tensions following the partition of British India,Footnote 116 in 2019, Pakistan’s Permanent Representative to the UN complained to the First Committee (Disarmament and International Security) that tensions between Pakistan and India had escalated due to the Kashmir dispute. Pakistan expressed insecurity over statements suggesting that India’s ‘no first use’ nuclear weapons policy might have shifted toward a preventive stance.Footnote 117 In 2021, US Navy commanders expressed concerns about the bolstering of the Russian and Chinese submarine fleets, describing them as a ‘persistent, proximate threat’. This threat was amplified by the advanced cruise missiles deployed on these submarines, which ‘have the range and accuracy to strike military and civilian targets throughout the US and Canada.’Footnote 118 Similarly, both the US and Israel have identified Iran’s increased production of standard and long-range drones as a significant security concern.Footnote 119 At the East Asia Summit in Jakarta in September 2023, South Korean President Yoon Suk Yeol described North Korea’s nuclear and missile programs as a grave violation of UN Security Council resolutions, posing ‘“an existential threat” to the Indo-Pacific region and a direct threat to regional peace’.Footnote 120 Moreover, in 2010, Russian President Dmitry Medvedev emphasized that Russia viewed the potential NATO membership of Georgia and Ukraine as a threat to its security and that of Europe.Footnote 121 Yet, while military alliances based on collective self-defence, such as NATO, can transfer arsenals, technology, and personnel between member states,Footnote 122 these actions in themselves do not constitute a threat of force.
The immediacy criterion is also relevant in escalating conflicts, where the protagonists react to each other’s actions. Each military action then at the same time contains a threat of escalation in the next round, should the other party react. The question, in these constellations, boils down to immediacy. Is the future, implicitly articulated escalation sufficiently proximate to constitute a threat? The current crisis in the Middle East provides illustration.
4.5. Apprehension of the threat by the affected state and the international community
The final criterion is subjective: the threat must be apprehended as such by the targeted state. This is especially crucial for implicit threats due to the communicative nature of such threats, as discussed earlier. If the addressee does not apprehend another state’s conduct as containing a threat, then that conduct cannot have any motivating effect on it.Footnote 123
The subjective element was stressed by the Chilean representative in the debate preceding the adoption of the Friendly Relations Declaration, who observed that ‘a threat is any action…which tends to produce in the other State a justified fear’.Footnote 124 On a domestic level, penal codes often require a threat to evoke a (justified) feeling of worry in the victim,Footnote 125 with criminal investigations typically requiring the victim’s complaint.Footnote 126 The apprehension criterion itself is familiar to the system of the Charter. It is well-established that a state can only invoke the right to individual and collective self-defence when it perceives itself as a victim of an armed attack.Footnote 127 The ICJ stated this first in Nicaragua and reaffirmed in Oil Platforms,Footnote 128 where the Court ruled that the US could not provide military assistance to neutral states engaged in shipping in the Persian Gulf so long as those did not see themselves as victims and formally requested support.
The apprehension test, however, cannot be entirely subjective. A state may overinterpret the actions of another state as constituting a threat. Hence, the implicit threat must be intersubjectively verifiable. The ILC aptly opined that the government of a state concerned must have a ‘good reason to believe’ that aggression is ’seriously contemplated’.Footnote 129 In demanding seriousness of the threat, the ILC clarified that ‘the threat does not depend on the subjective appraisal by the State which feels threatened, but on objective elements capable of verification by an impartial third party’.Footnote 130 A threat should in this sense be realistic from an objective point of view. This is particularly critical in cases of implicit threats, which, as discussed above, require contextual interpretation. The reaction of third states, usually underpinned by their own or shared intelligence reports, can intersubjectively corroborate the apprehension felt by the threatened state.
The intersubjective verification of a threat comes also into play in the opposite scenario, where the targeted state either does not perceive itself as threatened or decides to ignore the threat and refrains from reacting. A purely subjective view would lead to the conclusion that a disregarded or dismissed threat is no threat at all. The answer to the question whether this is convincing depends on the object and purpose of the prohibition of Article 2(4) of the UN Charter to threaten force. While the provision certainly aims to protect the free decision-making of states, it also seeks to prevent the act of threatening force itself. That purpose excludes any effect on the targeted state. Even if a state wilfully ignores a threat, the threat remains unlawful as long as it is discernible to an impartial observer.
Preferably, this verification role should be institutionalized. The UNSC, with its primary responsibility for maintaining international peace and security, is well-suited to assume this function.Footnote 131 For example, the Security Council received Iranian concerns over alleged Israeli ‘unlawful and insolent threats’ to attack Iran’s nuclear facilities, though it ultimately did not act on them.Footnote 132
Alternatively, a court or tribunal can provide verification.Footnote 133 In the two cases in which the ICJ was concerned with potential threats, it actually did not verify the addressee state’s concerns. It downplayed the Albanian concern about potential aggressive actions by the Royal Navy in the Corfu Strait.Footnote 134 Similarly, the Court did not support Nicaragua’s assertion that it was unable to exercise legal rights in the Exclusive Economic Zone and Continental Shelf due to threats by the Colombian Navy.Footnote 135 In this regard, implicit threats call for a novel assessment of evidence-gathering tools, particularly through the work of intelligence services. Intelligence on the acting party can provide crucial insights into the context of a manifestation of force and whether it is likely to serve as a warning or escalate immediately to military action. Intelligence agencies assess threats from various perspectives and through different methods, such as intercepting secret communications, conducting surveillance through sophisticated programs (e.g., Pegasus), monitoring military movements via satellites, and gathering information from local human sources.Footnote 136 Such intelligence must be robust enough to withstand scrutiny in a court setting. In Questions relating to the Seizure and Detention of Certain Documents and Data, the ICJ recognized that intelligence (spying) can be evaluated against international standards, in that case - the right of states to a fair process.Footnote 137 Timor-Leste’s indication of an in-camera proceeding to present evidence on the merits highlights a procedural option that could be explored further in the future.
5. The (il)legality of implicit threats of force
The previous section has proposed a test for ascertaining implicit threats of force. For such a threat to exist, there must be a targeted message indicating a future, attributable aggressive act, and this message must be credible and apprehended as such by the addressee or the international community.Footnote 138 A threat meeting these qualifications falls under Article 2(4) of the UN Charter. This section addresses the logical next question, namely whether an established implicit threat can be justified.
Within the Charter system, threats are prima facie illegal but can be exceptionally justified just as prima facie unlawful uses of force can be, although the specificity of justification may diverge slightly in each scenario. In Nuclear Weapons, the ICJ held that ‘if the use of force itself in a given case is illegal … the threat to use such force will likewise be illegal’.Footnote 139 It followed through to state that, ‘if it [the threat] is to be lawful, the declared readiness of a State to use force must be a use of force that is in conformity with the Charter’, propounding symmetry in justification.Footnote 140 In light of this, the established exceptions justifying the use to force - authorization by the UN Security Council under Chapter VII of the UN Charter and self-defence against an armed attack under Article 51 of the UN Charter – also justify a state to threaten force.Footnote 141
It remains controversial whether, in addition, a state can be justified to use force by invitation from ‘illegitimate’ or ‘incompetent’ authorities, to protect and rescue its own citizens abroad, or for a humanitarian intervention,Footnote 142 and, by extension, whether these exceptions are valid for issuing threats, if symmetry is to be maintained. On the other hand, scholars that consider threats a suitable means to prevent a conflict from escalating may feel that a threat could be justified if issued for this purpose, a rationale not easily transposable to the use-of-force ambit.Footnote 143 Accepting such additional justificatory grounds for threats, however, risks weakening the Charter system and leading the law of collective security down a slippery slope. Therefore, Security Council authorization and self-defence shall remain the only valid symmetrical justifications for threats of force, whether explicitly or implicitly made.Footnote 144 Still, the particularities of implicit threats throw up distinct questions as to how to apply these two justificatory grounds.
First, the UN Security Council is more likely to authorize a use of force rather than making a threat.Footnote 145 For example, United Nations Security Council Resolution 678 (1990) authorized the use of ‘all necessary means’ to enforce Resolution 660 (1990) and subsequent relevant resolutions.Footnote 146 The US Congress relied on this as the legal basis for military action against Iraq, though this interpretation remains controversial. On 16 October 2002, Congress passed the ‘Authorization for Use of Military Force Against Iraq Resolution of 2002’ (H.J. Res. 114) authorizing the President to use military force to enforce relevant UN Security Council resolutions and defend US national security, which led to the mobilization of US and coalition forces and the invasion of Iraq in March 2003. In contrast, NATO threats issued to the Federal Republic of Yugoslavia (FRY) between 1998-1999 regarding Kosovo remained illegal without explicit authorization to use force.Footnote 147 So, even if the threat was intended to prevent a broader humanitarian crisis, the NATO bombing of the FRY being a breach of international law means that the preceding threat was likewise illegal. However, threats can be legitimized ex post facto by the UN Security Council. By way of example, the US military threat to Haiti to bring back the disposed president Jean-Bertrand Aristide in the 1991 Haitian coup d’état was legalized in 1994 through Security Council Resolution 940, which authorized ‘Member States to form a multinational force under unified command … to facilitate … the return of the legitimately elected President’.Footnote 148
Second, the legality of implicit threats made in self-defence also requires further clarification. Threats can be issued in individual or collective self-defence only in case of an armed attack. A threat may be made to stop an armed attack that is occurring or to revert an attack that has already occurred.Footnote 149 Threats could also be issued to prevent an attack that is imminent. This is to be assessed ex ante, akin to the use of force in anticipatory self-defence.Footnote 150 Threats aimed at countering a reasonably anticipated attack remain lawful even if the attack that actually occurred did not meet the threshold of an armed attack, as the threats may have lowered the scale of the attack.Footnote 151 This is an adaptation of the concept of anticipatory self-defence to the particularities of threats of force, but it safeguards essential symmetry with the law governing the use of force.Footnote 152 Finally, any threat made in response to an armed attack must be proportionate and necessary. As Secretary Webster noted in the Caroline case, ‘threats issued [in self-defence] must still be necessary and proportionate’.Footnote 153 Threats that do not fall under these conditions are unlawful. That implies that a threat received cannot be countered by a threat unless the initial threat implies an armed attack, although it has to be said that the state practice on this question is thin.
6. Consequences of unlawful implicit threats of force
Once an unlawful implicit threat has been established, this is a breach of a primary international obligation, engaging the acting state’s international responsibility under Article 1 of the ARSIWA.Footnote 154 This responsibility entails consequences in the form of secondary obligations for the acting state that the Articles set out.
Accordingly, the acting state must cease making the threat. That would usually require it to remove the military manifestation that underpins the targeted messaging. The acting state could also be required to provide reparation to the victim state for any damage suffered from the implicit threat.Footnote 155 In addition, the concerned state may take countermeasures aimed at stopping the threat. Any countermeasures must be non-forcible; a threat received cannot be countered by a threat issued.
A further question is in what instances a third state can be held complicit and bear consequences according to Article 16 of ARSIWA for another state’s breach of the no-threat rule. It seems to some observers of state practice that the threshold for complicity in threats is substantially lower than for complicity in the use of force. Messaging relating to financial or military support for another state could be a breach of the no-threat rule.Footnote 156 Uganda, for example, perceived alleged Sudanese military support to the Democratic Republic of the Congo as a ‘grave threat’ to its security.Footnote 157 This principle extends to the financing of mercenaries operating on another state’s territory. The 1977 Convention for the Elimination of Mercenarism in Africa stresses that mercenaries pose a ‘grave threat … to the independence, sovereignty, territorial integrity and harmonious development of Member States of the Organization of African Unity’.Footnote 158 The UN Secretary-General and the UNHCR Special Rapporteur on the use of mercenaries have repeatedly condemned the use of mercenaries, foreign fighters and private military and security contractors, underlining their detrimental impact on human rights, international peace and security, and characterized them as a ‘threat’.Footnote 159
Article 41 of the ARSIWA provides for additional consequences for serious breaches of peremptory norms (jus cogens) by third states. This provision obligates third states to refrain from recognizing situations resulting from such breaches and to cooperate in restoring the status quo ante.Footnote 160 Under the ILC draft conclusions on the identification of peremptory norms, an international rule acquires peremptory status if it is accepted by the whole international community of states and does not permit any derogation.Footnote 161 Scholars have questioned whether this is the case for threats of force,Footnote 162 particularly as threats are neither mentioned in the Commentaries to Articles 26 and 40 nor featured in the ILC’s draft Conclusion 23, which enumerates ius cogens norms.Footnote 163 The ILC, however, acknowledged that the list of ius cogens norms in Conclusion 23 is non-exhaustive.Footnote 164 This ambiguity can be addressed by applying the principle of symmetry between threats of force and the use of force, which likewise still has not found a stand-alone position on the list, but has been mentioned throughout the text of the ILC reports on peremptory norms since 1966 as jus cogens. Besides, the use of force could arguably by subsumed under the category of aggression, which is listed as a jus cogens norm in Conclusion 23.Footnote 165 Accordingly, under a proper construction, the enhanced consequences of a breach for peremptory international law apply to force, regardless of whether force is actually used or threatened. This entails for all states a duty of non-recognition of any territorial changes resulting from a threat. Such non-recognition aligns with the Friendly Relations Declaration, which mandates non-recognition as a sanction for threats. It is also consistent with the status of self-determination as a peremptory norm, which can be violated by the implicit threat of force.Footnote 166
Independently of whether the prohibition to threaten force has attained jus cogens status, consequences will still ensue because it is an obligation erga omnes, meaning all states have a legal interest in its observance. Third states can invoke this responsibility of the acting state. Article 48 of ARSIWA provides for the invocation of responsibility by a state other than the injured state. They can also take countermeasures. For instance, in response to the Russian military threat in Ukraine, Germany decided to halt the Nord Stream 2 pipeline project,Footnote 167 while the EU Parliament, in its resolution of 16 December 2021, condemned the large Russian military build-up and suggested that sanctions against Russia should include suspension from the SWIFT payment system.Footnote 168 In Legality of the Wall, the ICJ linked the duty of non-recognition of territorial changes to the erga omnes character of the breached norm, not necessarily its peremptory status.Footnote 169 The Court made this observation with reference to aggression.Footnote 170 This principle applies by analogy to territorial changes resulting from threats. Furthermore, states are obligated to cooperate to prevent and stop an implicit threat that has this aim.Footnote 171
The qualification of the prohibition to threaten force as erga omnes also has a procedural consequence. In its recent jurisprudence, the ICJ has significantly strengthened the judicial enforceability of international obligations erga omnes. In a series of cases starting with Belgium v. Senegal, the Court recognized that any state party may institute compliance proceedings without having to demonstrate standing.Footnote 172 This jurisprudence will extend to Article 2(4) of the UN Charter; hence any state has standing to institute proceedings in cases of implicit threats, whether directed against itself or against another state.
6.1. Practice
While the consequences of implicit threats can be delineated as above, there is a considerable gulf with practice. So far, there have been very limited consequences for an unlawful threat of force in state practice. For instance, the 1999 Kumanovo Agreement to put an end to the war in Kosovo was imposed on Serbia under the implicit threat of further NATO bombardments. Yet the validity of the Kumanovo Agreement did not come under scrutiny under Article 52 of the VCLT, which indicates that a treaty is void if its conclusion has been procured by the threat or use of force.Footnote 173 The same pertains to the continuous threat by China to intervene in Taiwan where China has not faced any sanctions. While Taiwan is not formally a state, an armed attack would be contrary to the Friendly Relations Declaration stipulating ‘the duty to refrain from any forcible action which deprives peoples … of their right to self-determination and freedom and independence’.Footnote 174 Judicial and arbitral bodies have largely remained silent on the consequences of threats. An exception is the Guyana v. Suriname award, where the arbitral tribunal of the Permanent Court of Arbitration found that Surinamese naval actions in the disputed area constituted a threat of force contrary to international law. It did, however, reject Guyana’s request for monetary compensation.Footnote 175
Scarce and largely lenient state and institutional reactions towards implicit threats may stem from the conviction that some level of threatened force as a deterrent may be necessary to maintain global stability.Footnote 176 Schachter has maintained that Article 2(4) of the UN Charter is so rarely invoked against implicit threats due to ‘the subtleties of power relations and the difficulty of demonstrating coercive intent’ as well as ‘the general recognition of and tolerance for disparities of power and of their effect in maintaining the dominant and subordinate relationships between unequal states’.Footnote 177 White and Cryer noted that states often issue a ‘collective sigh of relief that actual force has not been used’, while Roscini remarked that unfulfilled threats are considered unworthy of debate, in comparison to the actual use of force when such threats materialize.Footnote 178 They may also arise from legal indeterminacy. Waxman pointed out that implicit threats receive less scrutiny because of a methodological preference to look into ‘cases’ or post factum patterns, rather than studying less observable factors such as parties’ intentions, perceptions, and signalling.Footnote 179 Kleczkowska, partially on this basis, claimed that the prohibition of threats plays a secondary role in maintaining international peace and security.Footnote 180 Corten, on the other hand, cautions against any asymmetry between the threat of force and the use of force in terms of international reaction, as states usually condemned both. Those not condemned, he labels as ‘precedents’.Footnote 181
6.2. Institutional responses
Against this background of an unsatisfactory state practice in reacting to threats, institutionalized responses to implicit threats ought to be considered. This response is primarily for the UN to decide on, as the guardian of Article 2(4) of the UN Charter. From the perspective of the Charter, the institutional response should address threats as potential precursors of aggression and other uses of force. It should aim to manage the risk of an escalation and to discourage the resort to armed force.Footnote 182 This function engages both the UN Security Council and the UN General Assembly.
The UN Security has primary responsibility. Faced with an implicit threat of force, the UN Security Council should take a proactive approach. It should closely monitor the situation, gathering intelligence and reports from UN member states, observers, and relevant international organizations to understand the nature and extent of the threat. On this basis, the Security Council should closely engage in diplomatic dialogue with the involved parties to de-escalate tensions. This may include direct negotiations or facilitating communication between the states involved.
The Security Council can also adopt resolutions condemning the implicit threat of force, if necessary. Such resolutions can call for restraint, the cessation of provocative actions, and adherence to international law. Such resolutions fall within Chapter VI of the UN Charter. Should, however, these recommendatory measures fail to bring result, the Security Council pursuant to Article 39 may ‘determine the existence of any threat to the peace, breach of the peace, or act of aggression’ and introduce more severe preventive measures provided by Chapter VII of the UN Charter.
Although the threat of force is not explicitly enumerated in Article 3 of Resolution 3314 defining aggression, the list is not exhaustive, and the Security Council, under Article 4, is empowered to determine that the threat of force constitutes an act of aggression. Note that the Security Council has rarely identified a breach of the peace or an act of aggression under Article 39 of the UN Charter. It typically classifies situations as threats to the peace, even in instances where a breach of the peace or an act of aggression appears evident.Footnote 183 Under Article 41 of the UN Charter, the Council could impose sanctions or arms embargoes to deter further escalation, while under Article 42 it could also resort to military actions to ‘restore’ international peace and security.
In practice, the UN Security Council has condemned the threat of force relatively rarely.Footnote 184 However, recent Security Council practice points into this direction, even though it is not conclusive. Thus, the Security Council resolutions on the Iraqi-Kuwait dispute had condemned the deployment of a huge number of Iraqi military forces on the border that preceded the attack. In the context of the Russian-Ukrainian war, at the mentioned UN Security Council meetings on the situation in Ukraine, states grounded their diplomatic language and agendas in the international law of collective security and the management of risks to global stability. Almost all recommended the Normandy format in the context of the implementation of the Minsk agreements.Footnote 185 In addition, almost all states reaffirmed the principles of sovereignty, territorial integrity and good neighbourliness. At the UN Security Council meeting on 31 January 2022, all members strongly suggested the return to the negotiation table. Western states preferred a stronger accent. Almost all expressed unwavering support for Ukrainian territorial integrity, including the Donetsk and Lugansk regions.Footnote 186 Poland and the Baltic states were at the forefront of more decisive rhetoric against Russia.Footnote 187 Other states, to the contrary, appealed for a peaceful diplomatic process. India’s representative to the UN, for instance, stated ‘Quiet and constructive diplomacy is the need of the hour. Any steps that increase tension may best be avoided by all sides in the larger interest of securing international peace and security.’Footnote 188 Similarly, the Brazilian representative underlined that ‘Open references to military actions, unilateral economic sanctions and other measures are developments that should be avoided, in accordance with the Charter of the United Nations.’Footnote 189 The Mexican representative referred to the UN Resolution 3314 defining aggression and said that such would be cause for the Security Council to act,Footnote 190 while the Albanian representative mentioned the Budapest Memorandum, which affirms the territorial integrity of Ukraine and provides that Russia, the UK and the US (signatories) must refrain from the threat or use of force against the territorial integrity or political independence of Ukraine.Footnote 191
The UN General Assembly, as the representative body of the international community, could play a concurrent role in the process in case of a deadlock in the Security Council.Footnote 192 It could demand the immediate cessation of an implicit threat and assurances of non-repetition. In the Ukraine-Russia war, the General Assembly did not engage with the threat preceding the invasion, but it did condemn the attack that realised the threat. During its eleventh emergency special session, the UN General Assembly passed a resolution on Russia’s invasion of Ukraine, which in Point 2 strongly deplored Russian aggression on Ukraine and labelled it as a violation of Article 2(4) of the UN Charter, whereas in Point 3 it demanded that Russia immediately ceases its use of force against Ukraine and refrain from any further unlawful threat or use of force against any Member State.Footnote 193 Russia also lost its seat on the Human Rights Council,Footnote 194 as well as membership of the regional Council of Europe.Footnote 195 This institutional response to the threat realised could be a precedent, backing up the General Assembly’s response to threats in the future.
The international judicial function provides another institutional mechanism to censure unlawful threats, in parallel to its more active stance on uses of force recently. The ICJ has ordered Russia to immediately suspend its military operations under the Genocide Convention.Footnote 196 International criminal courts also have a role to play. In the Russia-Ukraine conflict, the ICC Prosecutor launched the investigation for alleged war crimes,Footnote 197 while Pre-Trial Chamber II of the ICC issued arrest warrants against Vladimir Putin and Maria Lvova-Belova.Footnote 198
7. Completing the juridical journey
Events like Russia’s conflict with Ukraine are a stark reminder of the uncertainty regarding threats of force in general and implicit threats in particular. To strengthen the legal framework, it is desirable that the international community resumes the juridical journey towards a codification of the threat of force that it abandoned after almost half a century of work, while being conscious of the use of force developments that have occurred in the meantime. There is a parallel with iterative progressive development of the law on aggression law in the UN Charter. Aggression was defined in General Assembly Resolution 3314 (XXIX) in 1974. The ILC drafted Articles on State Responsibility adopted in 2001, clarifying that an act of aggression invokes state responsibility under Article 40(1). The 2010 Kampala Amendment to the ICC Statute finally codified that aggression entails individual criminal responsibility.Footnote 199
The next phase in this journey can build on extant key documents. These are the 1970 Friendly Relations Declaration and the 1987 Declaration on the Enhancement of the Effectiveness of the Principle of Refraining from the Threat or Use of Force in International Relations, which are open to further concretization by the General Assembly. The ILC could resume the work paused some 50 years ago and now distinguish between explicit and implicit threats, consider the division of threats into more and less serious categories based on their gravity, and articulate the responsibility of states for this wrongful act, and possibly that of individuals as well. In the ICC Statute, what will make a threat justiciable is its subsumption under another prohibition, such as ‘planning or preparation of an act of aggression’.Footnote 200 This is a matter of judicial interpretation.
8. Conclusions
Threats of force are prohibited by Article 2(4) of the UN Charter yet remain ill-defined and difficult to ascertain in practice. Implicit threats of force have emerged as a particularly problematic category as they can be denied or concealed under established principles of international law while still effectively coercing the victim state. The article has developed a framework to more securely ascertain implicit threats of force. This framework is grounded in seeing threats as coercive communication between states, distinct from action that characterizes any use of force. On this basis, the framework generally defines what constitutes an implicit threat and provides an exemplifying standard; it then identifies possible justifications and finally sets out the legal consequences of unlawful threats. By more clearly regulating implicit threats in this way, international law can assist states in addressing conflicts at an earlier stage, preventing their escalating into actual military confrontation.
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