While there is widespread agreement as to the existence of a jus cogens norm in the jus ad bellum which prohibits the use of force, opinions diverge as to the precise scope of that jus cogens norm. A narrow ban on aggression, Article 2(4) of the Charter, and even the jus ad bellum as a whole have all been suggested as candidates. So long as the jus cogens status of the prohibition is being referred to for purely rhetorical reasons, this uncertainty does not cause too many problems. However, if we want to consider the effects of the presence of a jus cogens norm, such as how it may impact the modification of the jus ad bellum, the scope of that jus cogens norm must be defined with more precision. This article will argue that if there is a jus cogens norm in the jus ad bellum, it must be the customary norm which prohibits non-consensual uses of force that are neither validly authorised under the UN Charter nor lawful exercises of self-defence.
This conclusion as to the scope of the jus cogens norm is not particularly controversial. However, in demonstrating how this conclusion is reached this article aims to do two things. First, it will clarify the method by which jus cogens norms should be identified in practice; a question which has not been addressed in detail in the literature to date. In particular, it will be argued that the conclusion above follows from the necessarily customary nature of jus cogens norms, as well as a correct understanding and application of what it means for a norm to be ‘accepted and recognized as a norm from which no derogation is permitted’. Second, this article will show that the lack of consensus as to the scope of the jus cogens norm prohibiting force results from uncertainty as to the structure of the underlying customary and treaty law norms that comprise the jus ad bellum. Yet once what it means for a norm to ‘permit no derogation’ has been clarified, the scope of the jus cogens norm in the jus ad bellum may be identified without the need to resolve these much more difficult questions of structure. Indeed, the conclusion above as to the scope of the jus cogens norm leaves open multiple possibilities as to how the customary and treaty norms setting out the prohibition and its apparent derogations may be analysed.
I. OBSTACLES TO IDENTIFYING THE JUS COGENS NORM IN THE JUS AD BELLUM
Referring to the extensive literature on the nature of jus cogens norms, Saul rightly observes that ‘[w]hat is striking … is the general absence of detailed exploration of the methodological process that should be undertaken to determine whether or not a norm has jus cogens status’, with scholars instead focusing on justificatory theories for the concept of jus cogens.Footnote 1 Similarly, ‘[t]he most prominent approach to the identification of jus cogens norms that is found in judicial practice is to accord a norm the status without further explanation’.Footnote 2 The same can be said of the practice of States, which typically assert that a particular norm has jus cogens status without setting out any detailed reasoning as to how that particular norm has been identified as jus cogens.Footnote 3 At most, such statements are accompanied by a vague reference to the importance of the norm, or to a previous judicial decision recognising the norm as having jus cogens status.Footnote 4 The International Law Commission (ILC) has also asserted in various outputs that a norm has jus cogens status without providing any detailed reasoning in support, most recently including in the annex to its draft conclusions on peremptory norms ‘a non-exhaustive list of norms previously referred to by the Commission as having peremptory character’.Footnote 5
This article is therefore not concerned with adding to the scholarship justifying the existence of jus cogens norms in international law. Being based on fundamentally different views about the nature of law, it is not clear that the disagreements as to the nature and purpose of jus cogens norms can ever be resolved.Footnote 6 Instead, using the jus ad bellum as an example, this article will set out a methodology for how jus cogens norms can be identified in practice. The analysis here is premised on the view that jus cogens norms are neither a form of natural law nor ‘international public order’ but like all international law norms arise from the consent and practice of States. A norm of international law—including a norm of jus cogens—is identified through the application of secondary rules of the international legal system (in the sense of rules governing law creation and modification, not in the sense of State responsibility).
In contemporary international law, the secondary rules governing the creation and effects of jus cogens norms derive from the Vienna Convention on the Law of Treaties (VCLT). Although there were occasional references to jus cogens in practice before 1969,Footnote 7 it was the inclusion of the concept in the VCLT that solidified its existence in positive international law.Footnote 8 As a matter of treaty law the VCLT is of course binding only on its parties, but Article 53 is now widely accepted as providing the definition and criteria for identification of jus cogens norms beyond the VCLT and these may now be considered to be part of customary law.Footnote 9 The language of Article 53 was adopted virtually unchanged by the ILC in 2019 as both the definition and criteria for identification of jus cogens norms in its draft conclusions on peremptory norms.Footnote 10 Customary law thus provides two cumulative criteria for the identification of a norm as jus cogens:
(a) it is a norm of general international law; and
(b) it is accepted and recognized by the international community of States as a whole as a norm from which no derogation is permittedFootnote 11
The obstacles to identifying the precise scope of the jus cogens norm in the jus ad bellum make it a useful lens through which the identification of jus cogens norms via the application of these two criteria can be analysed. First, there is an evidential difficulty: while there is widespread acceptance that there is a jus cogens norm in the jus ad bellum that prohibits force, a variety of different jus ad bellum norms have been recognised as jus cogens—from the prohibition of aggression;Footnote 12 to the prohibition in Article 2(4) of the Charter; to ‘the law of the Charter concerning the prohibition of the use of force’Footnote 13 which potentially includes not only Article 2(4) but the provisions of Chapter VII allowing for the authorisation of force; to the ‘general international law on unilateral use of force as a whole’, apparently including those norms regulating self-defence.Footnote 14 In some cases the norm identified as jus cogens is a norm of treaty law, while in others it is unclear if the norm referred to is a norm of custom, treaty, both, or neither. For example, a norm in the jus ad bellum was recognised as jus cogens in the memorials of Iran in the case concerning Oil Platforms (the ‘obligation imposed on all Members under Article 2(4) of the Charter’),Footnote 15 Spain in Fisheries Jurisdiction (‘la norme impérative qui interdit l'usage et la menace du recours à la force’),Footnote 16 Nicaragua in Military and Paramilitary Activities in and against Nicaragua (‘Article 2(4) of the United Nations Charter … has come to be recognized as jus cogens’),Footnote 17, Yugoslavia in the Legality of the Use of Force cases (‘the obligation not to resort to the use of force against another State’),Footnote 18 and in oral submissions by Canada in Fisheries Jurisdiction (‘the Charter's prohibition of the use of force – Article 2, paragraph 4 – is a peremptory norm’).Footnote 19 Part of the problem seems to be that often the jus cogens status of the jus ad bellum norm is not doing any work: the references to its jus cogens status seem to be purely rhetorical, to emphasise the importance of the norm. As a result, it is possible to get away with a certain amount of vagueness as to the source or scope of that jus cogens norm. Thus, while there is widespread agreement that a jus cogens norm prohibiting the use of force exists, once one goes beyond the most superficial analysis it can be seen that the precise contours of that norm are unclear.
This uncertainty, it is submitted, flows from the complex and contested nature of the jus ad bellum framework itself. It is difficult to identify the contours of the jus cogens prohibition on force in part because it is difficult to identify the contours of the prohibition on force in customary and treaty law. Other widely recognised jus cogens norms are relatively straightforward prohibitions of a particular conduct. While there may be uncertainty at the boundaries as to what behaviour qualifies as torture or genocide, the structure of the prohibited norm is clear and once behaviour comes within its scope it is prohibited absolutely, with no exceptions. Moreover, since neither the Convention Against Torture nor the Convention on the Prevention and Punishment of the Crime of Genocide actually contains a norm prohibiting torture or genocide—rather they contain ancillary obligations to prevent, punish and criminalise that conduct—it is clear that the prohibitive norms being recognised as jus cogens must be the general customary norms prohibiting torture and genocide.Footnote 20 By contrast, even before we begin to consider the existence of a jus cogens norm, the underlying structure of the jus ad bellum is already uncertain: are the customary and Charter prohibitions of force identical and if not in what ways do they diverge? Is consent part of the definition of unlawful force or is it somehow operating as a defence?
Further confusion is created by the fact that the structures of norms that appear to be subject to exceptions, like the prohibition on force, can be analysed in different ways, which in turn may impact the identification of any jus cogens norm. The term ‘exception’ may be used to refer both to conduct that is simply outside the scope of a particular norm, or it may refer to a separate norm which operates alongside the primary norm to modify its application and, for example, render conduct that would otherwise be unlawful under the primary norm lawful.Footnote 21 (For clarity, ‘exception’ will here be used only to refer to this second meaning, of an exception as a separate norm; conduct that is outside the scope of a norm is not referred to as an exception.) Thus, does force used in self-defence fall outside the scope of the customary prohibition, or is it a prima facie violation of that norm that is justified by the exception provided by a separate customary right of self-defence? Does the UN Charter act as lex specialis to the customary prohibition on force, thereby providing an exception to that customary norm, or does force authorised under Chapter VII somehow fall outside the scope of that prohibition? In any case, how are these apparent derogations to the prohibition—self-defence, collective security, and consent—to be reconciled with the existence of a non-derogable norm prohibiting force?
As will be shown below, the acceptance that a jus cogens norm prohibiting force exists in the jus ad bellum can help answer some of these structural questions, or at least limit some of the possible options for how the prohibition and apparent exceptions are analysed. However, for now the point is that, even were the international community as a whole explicitly and unambiguously to state that the ‘prohibition on the use of force’ is jus cogens, we could not straightforwardly identify the scope of that jus cogens norm. A more fine-grained approach is needed.
II. A NORM OF GENERAL INTERNATIONAL LAW
The language of Article 53 makes clear that jus cogens norms do not constitute a separate source of international law; they arise from an existing norm of ‘general international law’ which is then recognised as having jus cogens status.Footnote 22 In practice, there are no examples of a jus cogens norm being recognised without or before the norm has an independent existence as a norm of positive international law. For example, although it is difficult to identify precise dates for the emergence of rules of custom or jus cogens, it is clear that the rules prohibiting genocideFootnote 23 and tortureFootnote 24 emerged as customary norms before or at least simultaneously with their recognition as jus cogens norms. Therefore, to identify a jus cogens norm in the jus ad bellum it is first necessary to identify an existing norm of general international law on which it could plausibly be based.
Yet none of the sources of international law can easily be reconciled with the characteristics to be displayed by jus cogens norms. As Danilenko observes, the international community has recognised the existence of jus cogens, but without taking account of the fact that ‘in international law there is a glaring gap between the requirements of the idea of jus cogens and the possibilities of the existing law-making processes’.Footnote 25 It will be argued here that, while in principle a universally ratified treaty could constitute a ‘general norm of international law’ which could be recognised as having jus cogens status, in practice all jus cogens norms currently in existence must be norms of customary law.
General principles of law, the source listed in Article 38(1)(c) of the ICJ Statute, do not seem promising as a source of jus cogens norms, limited as they are to legal principles common to domestic legal systems, and which do not contain equivalents of, for example, the prohibition on (inter-State) force.Footnote 26 Some scholars take the view that ‘general international law’ could include multilateral treaties.Footnote 27 However, treaty-based jus cogens is difficult to reconcile with the universality of jus cogens norms.Footnote 28 For a State that is not party to the relevant treaty, it is unclear what would be the source of the legally binding norm which has jus cogens status. Jus cogens status makes a norm that is already legally binding non-derogable, it is not a source of law in itself. Yet if jus cogens norms are universal, either the jus cogens norm must somehow itself constitute a source of law for that non-party State, a view precluded by the language of Article 53 and typically only held by those who take a natural law approach to jus cogens, or the treaty norm is somehow binding on a third party as a result of its jus cogens status, contrary to the pacta tertiis principle and the principle of consent which underpins treaty law.
This highlights the more fundamental problem with jus cogens treaty norms. Jus cogens norms are now accepted as having certain features that limit the role of State consent: for example, a State will be bound by a jus cogens norm even where it has not accepted and recognised the norm as having that status, and the persistent objector rule does not apply to jus cogens norms.Footnote 29 For jus cogens norms then to be drawn from that source of international law which is unquestionably rooted in the consent of each party to the creation of each obligation by which they are boundFootnote 30 seems difficult to sustain. The consensual nature of treaty norms shapes virtually every aspect of how treaties are created, interpreted and modified. A jus cogens treaty norm which somehow bound a third party—or prevented a party from availing of a denunciation or amendment provision to which they consented at the time they became a party to the treaty, without such a change being effected by consensual means of treaty amendment or interpretation—would so distort the essential nature of treaty norms as consensually agreed obligations that it is difficult to see how such a norm could still sensibly be called a treaty norm. As Judge Dugard has observed, ‘the concept [of jus cogens] is not to be used as an instrument to overthrow accepted doctrines of international law’, such as the principle of consent to jurisdiction—it seems this should be the case a fortiori for the foundational principle of consent to treaty obligations.Footnote 31
It is possible to imagine a universal treaty whose provisions were drafted (or interpreted) so as to create the effects of jus cogens and which could potentially constitute a treaty-based norm of jus cogens. In the same way as Article 103 of the UN Charter provides that its norms will prevail over conflicting treaty norms, a treaty could provide for conflicting treaties to be treated as invalid, which would be given effect among its parties. Similarly, subsequent practice or agreement of the parties could reinterpret the treaty so that the denunciation clauses were considered obsolete and any amendment clauses could be reinterpreted to meet the Article 53 test for identification of jus cogens. In such a case one could perhaps interpret this as acceptance and recognition that the treaty norm no longer permitted any derogation and—since for a universal treaty this would necessarily embrace the international community as a whole—had become jus cogens. The universality and non-consensual characteristics of jus cogens norms could thus potentially be created through ordinary operations of treaty law which respect the principle of consent of the parties. However, for now this remains purely hypothetical. Until such a treaty emerges, jus cogens treaty norms remain either impossible or, even if possible in principle, not currently existing in practice. Even the UN Charter falls short of universal ratificationFootnote 32 and multilateral treaties rarely even attract ratifications from ‘the international community of States as a whole’.Footnote 33
In the 1966 commentary to its draft articles on the law of treaties, the ILC stated that ‘a modification of a rule of jus cogens would today most probably be effected through a general multilateral treaty’.Footnote 34 However, this does not necessarily mean that the treaty norm itself would be jus cogens. Rather, the widespread agreement on the importance of the norm that would be required for the conclusion of such a treaty would likely lead to the rapid emergence of a parallel customary norm, accompanied by the acceptance and recognition of the international community as a whole required to give it jus cogens status. This new customary jus cogens norm would then replace the old customary jus cogens norm. In practice, therefore, the inability of treaty norms alone to act as the source of jus cogens norms may be of little impact, as any treaty norm which enjoys the widespread support that would be necessary for identification as a jus cogens norm will likely also come to exist as a norm of customary international law. Indeed, this seems the best explanation of the effect of Article 2(4) of the UN Charter on custom after 1945.
In its 2019 draft conclusions on jus cogens the ILC stated that ‘[c]ustomary international law is the most common basis for peremptory norms of general international law (jus cogens)’ but that ‘[t]reaty provisions and general principles of law may also serve as bases for peremptory norms of general international law (jus cogens)’.Footnote 35 However, the commentary to the conclusion reveals a greater scepticism about treaty norms acting as a source of jus cogens norms, suggesting that where this does occur it is only through the impact that a treaty may have on customary law, as described above:
The role of treaties as an exceptional basis for peremptory norms of general international law (jus cogens) may be understood as a consequence of the relationship between treaty rules and customary international law as described by the International Court of Justice in [the] North Sea Continental Shelf cases. In that case, the Court observed that a treaty rule can codify (or be declaratory of) an existing general rule of international law, or the conclusion of a treaty rule can help crystallize an emerging general rule of international law, or that a treaty rule can, after adoption, come to reflect a general rule on the basis of subsequent practice.Footnote 36
To the extent that this does represent a change in the ILC's position since 1966, this may simply reflect the fact that treaty law has in practice not served as a source of jus cogens norms.Footnote 37 It is difficult to think of any suggested jus cogens norms which exist only in treaty form and cannot also be said to exist in customary law.
The prevailing view holds that ‘general international law’ in Article 53 refers to general customary international law.Footnote 38 It is true that a customary jus cogens norm would also not behave in exactly the same way as a regular customary norm, in terms of its modification and the inapplicability of the lex specialis doctrine, for example. However, the non-consensual effects of jus cogens norms are easier to reconcile with custom as a source, where it is already a more diluted consent to the customary process generally (rather than each customary norm) that is involved.Footnote 39 Whether or not they consent to the particular norm or participate in the practice that underlies it, States in practice will be presumed to be bound by a valid customary norm. Only in the eventuality that they can prove they meet the stringent requirements to be a persistent objector can a State claim the norm is not opposable to them, but effective claims of this kind are rare.Footnote 40 For a customary norm to be a jus cogens norm—which removes the possibility to persistently object to or contract out of the norm—is thus more a difference in degree than in kind. By participating in the international legal system as States, States consent to the possibility that they may be bound by particular jus cogens customary norms to which they have not consented and from which they will not be able to derogate.Footnote 41
Returning to the jus ad bellum, we can therefore conclude as a first step that if there is a norm prohibiting force which has jus cogens status it must be based on the customary prohibition on force, not Article 2(4) of the UN Charter. While the Charter's membership includes the overwhelming majority of States it is not universal, and as a result none of the treaty norms contained in the Charter can form the source for a jus cogens norm. By 1970 at the latest, however, the discussions in the Sixth Committee regarding the draft Friendly Relations Declaration and the text of the Declaration as adopted demonstrated that the prohibition on the use of force was considered a free-standing norm of customary law, independent of the Charter, that bound all States regardless of whether they are UN members.Footnote 42 The prohibition's customary status was later confirmed by the ICJ in Military and Paramilitary Activities in and against Nicaragua.Footnote 43 It is this customary norm prohibiting force that must be the source of any jus cogens norm in the jus ad bellum.
III. ACCEPTED AND RECOGNISED BY THE INTERNATIONAL COMMUNITY AS A WHOLE AS A NORM FROM WHICH NO DEROGATION IS PERMITTED
The second criterion for the identification of jus cogens norms is sometimes described as an enhanced form of opinio juris. Footnote 44 This is perhaps a useful analogy to the extent that both involve the acceptance by States that a norm has a particular legal effect—binding as law in the case of ordinary custom, impermissibility of derogation for jus cogens—and that this acceptance also constitutes that norm as custom or jus cogens respectively. However, describing the second criterion as a form of opinio juris also risks confusion. Regardless of the source of the general norm of international law which forms the basis for the substantive jus cogens norm, the secondary norms that regulate the identification and effects of jus cogens norms must be norms of customary international law.Footnote 45 The same is true of treaty law: the secondary norms that govern the identification, interpretation and effects of treaty norms are norms of customary law (albeit codified in the VCLT for those States that are party). However, identification of a new jus cogens norm does not involve the creation of a new norm (or norms) of customary law which create its jus cogens effects.Footnote 46 Each jus cogens norm is not accompanied by its own bundle of customary norms which, for example, void any treaties conflicting with that particular norm. Rather, the identification of a new jus cogens norm involves the application of secondary customary norms governing the creation of jus cogens norms. Those customary norms provide that a norm of general international law is identified as jus cogens when it is ‘accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted’ and create certain effects for norms so identified. Acceptance and recognition are thus not a form of opinio juris required for the establishment of a new norm of custom, but simply an element that fulfils the requirements of those secondary norms, in order for them to identify a norm as jus cogens.
The acceptance and recognition of a norm as jus cogens by the ‘international community of States as a whole’ need not be universal: a ‘very large majority’ of States will suffice and no one State could have a veto.Footnote 47 This tells us who needs to do the accepting and recognising; however, the more complicated question is what it is that needs to be accepted and recognised. The most common approach is to interpret ‘accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted’ as, in effect, ‘accepted and recognized by the international community of States as a whole as a jus cogens norm’.Footnote 48 This is exemplified by the ILC's recent draft conclusions on jus cogens. While draft conclusion 4 (‘Criteria for the identification of a peremptory norm of general international law (jus cogens)’) simply repeats the language of Article 53, in the commentary the second criterion is described as follows: ‘the norm must be shown to be accepted and recognized by the international community of States as a whole as having a peremptory character’.Footnote 49
It is precisely this interpretation of the second criterion for identification of jus cogens norms that has led to criticism of the Article 53 definition as circular or tautologous.Footnote 50 Jus cogens norms are those accepted and recognised as jus cogens norms.Footnote 51 No information is given as to which norms, with which qualities, should be accepted or recognised as having those effects. One might expect draft conclusion 8, ‘Evidence of acceptance and recognition’, to break down the second criterion and elucidate what is meant by a derogation, or what it means for a norm to permit no derogation—after all, this is what needs to be accepted and recognised for it to be identified as a jus cogens norm. However, the views that need to be held are simply described as acceptance and recognition ‘that the norm in question is one from which no derogation is permitted’, ‘of the non-derogability of such a norm’ or ‘that a norm has a peremptory character’.Footnote 52 There is no analysis of what it means for a norm to permit no derogation or how this manifests in practice: it is simply a synonym for ‘is jus cogens’.
One consequence of this approach is that in practice it limits evidence of acceptance and recognition to what States say they accept as non-derogable: to statements that a norm ‘is jus cogens’, ‘is a peremptory norm’ or ‘is a norm from which no derogation is permitted’. We cannot look directly at which norms States are actually accepting and recognising as permitting no derogation as it is unclear what this entails. Here the analogy with opinio juris for custom is helpful: while opinio juris for custom can of course be found in explicit statements by States about the content of customary law, it can also be inferred from State conduct that treats the alleged customary norm as legally binding. Yet if ‘permits no derogation’ is simply understood to mean ‘is jus cogens’, with no independent content of its own, then we cannot know what conduct to look for from which we could infer acceptance and recognition that a norm permits no derogation.
The limitations of relying on statements that a norm ‘is jus cogens’ are well illustrated by the jus ad bellum where, as discussed above, the statements of States leave it unclear which precise norm is accepted and recognised as jus cogens by the international community as a whole. States have frequently recognised the existence of a jus cogens norm in the jus ad bellum. As set out in detail by Corten, during discussions in the Sixth Committee leading to the adoption of the Friendly Relations Declaration numerous States explicitly referred to the prohibition on the use of force as a jus cogens norm, while many others used more general language that nevertheless implies recognition of jus cogens status.Footnote 53 The discussions leading to the ILC's draft Articles on the Law of Treaties show that the principle set out in Article 2(4) was accepted as jus cogens.Footnote 54 The prohibition on the use of force was also recognised as jus cogens at the 1969 Vienna Conference, when it was the most frequently cited example of a norm of jus cogens.Footnote 55 Discussions leading to the adoption of the Declaration on the Enhancement of the Effectiveness of the Principle of Refraining from the Threat or Use of Force in International Relations,Footnote 56 also demonstrated agreement among States as to the jus cogens nature of a norm in the jus ad bellum.Footnote 57 Corten shows convincingly that it is not merely the prohibition on aggression that States recognise as jus cogens, but the ‘prohibition on force’.Footnote 58 However, even so, the contours of the norm being recognised as non-derogable remain unclear—does ‘force’ refer only to non-consensual force? Is force in self-defence excluded from the prohibition's scope?
So, while statements are a valid way of identifying acceptance and recognition of a norm as jus cogens, in this case considering statements by States explicitly accepting and recognising a jus ad bellum norm ‘as jus cogens’ does not really advance us very far beyond our starting point. From the statements it is at least clear that support for the existence of a jus cogens norm that prohibits force is very widespread and that we can proceed on the assumption that a jus cogens norm prohibiting force does indeed exist in the jus ad bellum. Having analysed the first criterion for identification of jus cogens norms, we also know that any references to Article 2(4) of the Charter as jus cogens must be understood as referring to the parallel customary norm that prohibits the use of force, not the treaty norm itself. However, to identify precisely the scope of the customary norm in the jus ad bellum that is jus cogens—the prohibition on force, the prohibition plus the exceptions, or the prohibition on non-consensual force only etc—a more fine-grained approach is necessary, which looks at the extent to which a norm is accepted and recognised as a norm ‘from which no derogation is permitted’.
A. No Derogation Is Possible
The first step must be to clarify what is meant by ‘derogation’ in this context. Despite forming the ‘keystone’ of the Article 53 definition of jus cogens,Footnote 59 ‘derogation’ is the subject of terminological confusion,Footnote 60 with different writers understanding the term differently.Footnote 61
To say a norm is non-derogable must mean more than that it is legally binding, which is a characteristic of all customary norms. A derogation therefore cannot be the same as a breach of the customary norm. As Tomuschat writes:
Sometimes, one encounters an understanding to the effect that only rules of jus cogens are really endowed with juridically compulsory effect … such an understanding of jus cogens rests on a grave misperception. Each and every rule of customary law is binding, and jus cogens as a higher-ranking group of legal rules comes into play only when the validity of a treaty is to be evaluated or when it has to be determined whether, by other means, a legal position can be created or maintained that is incompatible with such peremptory rules.Footnote 62
Derogation is thus not concerned with factual conduct that is not in compliance with an obligation of a party that is bound by a norm,Footnote 63 but rather with legal or normative acts.Footnote 64 As normative acts, derogations can only be made by actors with the power to create new international legal norms, whether individually, bilaterally or collectively.
Unsurprisingly given their focus, in the drafting of the VCLT and the ILC work that preceded it the only kind of ‘derogation’ from jus cogens contemplated was a bilateral or multilateral treaty by which two or more States attempt to ‘contract out’ of the conflicting jus cogens norm. Since jus cogens norms permit no derogation, Articles 53 and 64 VCLT provide that any such treaties will be void. This understanding of derogation corresponds to the origins of the term ‘jus cogens’ in Roman law, which divided all legal rules into jus cogens and jus dispositivum. The division represented the limits of contractual freedom of parties. Jus dispositivum rules applied to the extent that parties had not departed from them in giving a different content to their contract inter se, whereas jus cogens rules excluded any such freedom of contract.Footnote 65
Some writers continue to take the view that ‘the main legal effect of peremptory norms is consequently to sterilise the operation of the lex specialis principle’Footnote 66 and so protect the norm which has that status from fragmentation or bilateralisation.Footnote 67 Regional custom that conflicts with a jus cogens norm would also constitute a derogation on this view, as it purports to create a conflicting special legal regime among a more restricted set of parties.Footnote 68 However, even before the conclusion of the VCLT it was suggested that this might be just one application of a broader concept of derogation,Footnote 69 and today derogation is not understood in this narrow sense,Footnote 70 as can be seen in the closing phrase of the quotation from Tomuschat above. Since the conclusion of the VCLT, and in particular since the completion of the ILC's work on State responsibility in 2001, it is clear that the concept of derogation has come to be understood as including, but also going far beyond, the conclusion of conflicting treaties or emergence of special customary regimes. In particular, Article 26 of the ILC's Articles on the Responsibility of States for Internationally Wrongful Acts provides that the invocation of circumstances precluding wrongfulness is excluded in relation to breaches of all jus cogens norms.
Today it seems that to say a jus cogens norm permits no derogation is understood rather as meaning that ‘[i]rrespective of the situation, there will never be a legally valid reason to depart from a rule of jus cogens’,Footnote 71 with ‘departing’ understood as not performing the conduct (positive or negative) required by the customary norm which has jus cogens status. There may be many legally valid reasons to depart from a regular customary norm: conduct may fall within an exception created by another customary norm; a treaty may act as lex specialis, excluding the customary norm from the relations of the parties; or the law of State responsibility may provide that although there is a breach of that customary norm it is not wrongful. It makes no difference whether the derogating norm purports to make the conduct lawful all things considered, justified, or merely excused. All these examples would constitute derogations from the jus cogens norm. What matters is whether the norm is purporting to provide a legally valid reason for ‘any act … that is not in conformity with an obligation arising under a peremptory norm of general international law (jus cogens)’.Footnote 72 For example, where the jus cogens norm is a prohibition, from the court's reasoning in Jurisdictional Immunities of the State it seems that a derogating norm will be one that creates a ‘direct conflict’ with the jus cogens norm in relation to ‘whether or not the conduct in respect of which the proceedings are brought [that is, conduct accepted to be in breach of norms assumed to have jus cogens status] was lawful or unlawful’.Footnote 73
It is important to recall that it is not the act contrary to the jus cogens norm that constitutes the derogation— for example, the act that constitutes the countermeasure. As factual conduct that constitutes a breach of the jus cogens norm. The derogation would rather be the norm of general customary law that provides that the wrongfulness of that breach is precluded because it constitutes, for example, a countermeasure: that norm is purporting to provide a legally valid reason to depart from the conduct required by the jus cogens norm. As the generality of the customary norms of State responsibility demonstrates, derogations are not limited to conflicting lex specialis.Footnote 74 Of course, where the norm that is being breached is an ordinary customary norm, there is no problem in the customary law of State responsibility providing a legally valid reason to depart from it in certain circumstances by, for example, taking countermeasures. Customary and treaty norms constantly provide legally valid reasons to depart from the conduct prescribed by the other by creating exceptions or acting as lex specialis. It is this combination and interaction of different norms that determine the constellation of legal obligations applicable to a particular State. It is only for jus cogens norms that such derogations are not permitted.
While the language of Article 53 provides that no derogation is ‘permitted’ from a jus cogens norm, the second criterion is better expressed as providing that no derogation is ‘possible’. Attempting to derogate from a jus cogens norm is not prohibited and does not of itself lead to responsibility for the States concerned. Rather, a purported derogation to a jus cogens norm is invalid— it cannot operate in the manner intended. Such a purported derogation may lead to State responsibility—for example, if a State has acted contrary to their obligations under a customary jus cogens norm expecting to be able to rely on a treaty excluding its application—but this results from their breach of the customary jus cogens norm, not the fact of derogation itself.
Even as the meaning of derogation has expanded, it is important to note that ‘the scope of jus cogens is not unlimited’.Footnote 75 The ICJ's recent jurisprudence has placed clear limits on the concept and confirmed that jus cogens ‘is not a trump card’ and jus cogens norms do not take precedence over all other norms.Footnote 76 In particular, the fact that a rule would ‘hinder the enforcement’ of the jus cogens norm is not enough for a rule to be disapplied.Footnote 77 Even where a dispute relates to compliance with a jus cogens norm, that ‘cannot of itself provide a basis for the jurisdiction of the Court’ where no basis for jurisdiction otherwise exists.Footnote 78 In Immunities, the ICJ clarified that:
A jus cogens rule is one from which no derogation is permitted but the rules which determine the scope and extent of jurisdiction and when that jurisdiction may be exercised do not derogate from those substantive rules which possess jus cogens status, nor is there anything inherent in the concept of jus cogens which would require their modification or would displace their application.Footnote 79
It is, of course, possible that a procedural norm, such as a norm conferring jurisdiction on a tribunal in relation to a particular norm, could itself have jus cogens status so that a norm purporting to provide a legal reason to depart from that procedural rule would constitute a derogation. However, otherwise ‘the application of a procedural rule does not amount to derogation from substantive rules of jus cogens’.Footnote 80
B. Accepted and Recognised
With an understanding of what it means for no derogation from a norm to be possible, we are now able to move beyond the statements of States and also use their conduct to infer which norm in the jus ad bellum is accepted and recognised by the international community as a whole as not possible to derogate from. In the same way that States can demonstrate opinio juris for custom by condemning breaches of that norm as unlawful or invoking the responsibility of another State, a State can also demonstrate its acceptance that a norm is non-derogable by treating as void purported derogations from that norm by other States, such as a bilateral treaty conflicting with a jus cogens norm. It may also be possible—in principle—to infer that a State accepts that derogation from a norm is not possible based on a State's (or States’) own behaviour in not attempting to derogate from the norm. Although, as will be discussed below, this may prove more difficult in practice. Derogations are normative acts that purport to create a legally valid reason to depart from the jus cogens norm, that is, to perform the conduct prohibited by the customary norm in the case of a jus cogens prohibition.Footnote 81 Thus, it could be possible for a State to demonstrate its acceptance and recognition that a norm permits no derogation through its behaviour as a legislator of international law: by not attempting to create norms that purport to provide legal reasons to depart from the jus cogens norm. This would obviously include not attempting to conclude treaties that conflict with the jus cogens norm as well as groups of States refraining from creating regional custom that conflicts with the jus cogens norm of general custom.
In addition, since a conflicting lex specialis is not the only sense in which derogation is now understood, another general customary norm—such as the rules on circumstances precluding wrongfulness—would derogate from a jus cogens norm if it purported to create a legally valid reason to depart from the conduct it requires. As a result, a norm of general customary law (A) which provides an exception or defence in relation to another general customary norm (B) will derogate from that norm. Where they have created an exception through the process of customary law States collectively demonstrate their acceptance that derogation from customary norm B is possible—there can be and are legally valid reasons to depart from the conduct it requires. Conversely, where States have created a customary norm C but not created any derogations from that norm, this could demonstrate their acceptance and recognition that derogation from that norm is not possible. As Byers writes: ‘[o]pinio juris, or something like opinio juris, appears to be at the root of the non-derogable character of jus cogens rules because States, quite simply, do not believe that it is possible to contract out of jus cogens rules’;Footnote 82 to which we may add that States simply do not accept it is possible to create a customary norm derogating from that jus cogens norm. They ‘regard those rules as being so important to the international society of States, and to how that society has come to define itself, that they can conceive of no exceptions to them.’Footnote 83
As a result, part of the difficulty in identifying the jus cogens norm in the jus ad bellum has been that States have explicitly accepted the prohibition on the use of force as jus cogens through their statements, while simultaneously accepting the existence of apparent derogations from that prohibition in the form of the two so-called ‘exceptions’ and the possibility that a State may validly consent to a use of force on its territory.Footnote 84 All of these examples appear to provide a legally valid reason for one State to use force against another. If such derogations from the prohibition on force are unquestionably possible, so the argument goes, then it cannot be a norm of jus cogens.Footnote 85
To avoid this conclusion that the prohibition on force cannot be jus cogens, writers sometimes go to the other extreme and suggest that it is ‘not simply the prohibition of the use of force that is peremptory, but it is also all of the rules on the use of force under international law, including the rules governing self-defence and the rules on forcible action as authorized by the Security Council.’Footnote 86 On this view, identification of a norm as jus cogens would be like the touch of King Midas that causes jus cogens status to spread through every branch of the underlying customary norm and every other customary norm connected to that norm, making it non-derogable in every extremity. Yet, not only would such arguments result in a formulation of the jus cogens norm so lengthy and complicated as to be virtually unworkable,Footnote 87 but by deducing the scope of the jus cogens norm rather than inducing it from the acceptance and recognition of States, this approach would abandon the Article 53 criteria: a norm is only jus cogens if it is accepted and recognised as non-derogable.Footnote 88
In any case, it is not necessary to resort to such convoluted solutions to avoid the conclusion that the prohibition on force cannot be jus cogens. The problem with the argument that a general norm subject to apparent derogations cannot be non-derogable is that it assumes that the jus cogens norm must be coextensive with the general customary norm that underlies it; or put another way, that the underlying customary norm must be non-derogable in its entirety. On this view, the fact that any of the conduct prohibited by the general customary norm may—for example—be consented to means the norm cannot therefore have jus cogens status. It takes an ‘all or nothing’ approach to jus cogens status: the possibility of derogation in relation to some conduct that comes within the scope of that customary norm means the norm as a whole is not non-derogable and so not jus cogens. However, there is no necessary shape that the jus cogens prohibition on force must conform to, and it need not be the same as the general customary norm on which it is based. All the possibility of consent tells us is that to the extent force may be consented to it is not prohibited by a norm with jus cogens status. The possibility of consent or other derogations does not mean that there can be no jus cogens prohibition on force; only that it must be narrower than one may initially think. A customary norm may be jus cogens in part—that is, to the extent that derogation is not possible—while other aspects of the customary norm will remain susceptible to derogation.Footnote 89 All that matters is that the norm to be given jus cogens status forms a coherent and intelligible norm in itself (eg non-consensual inter-State force is prohibited), and the fact that it forms part of, or falls within the scope of, a larger non-jus cogens customary norm (eg inter-State force is prohibited) is no obstacle. There can be a core, narrow jus cogens norm, surrounded by a penumbra of derogable customary law. In such cases, identifying the scope of the jus cogens norm becomes a question of identifying where the boundary between derogable and non-derogable customary law lies.
Most customary norms are subject to multiple derogations, both in the form of treaty law (for example, a treaty that acts as lex specialis) and also customary law (for example, another customary norm exists creating an exception to the norm). Customary norms which are not subject to any derogations will be rare, but it is precisely this failure to legislate exceptions and other derogations into existence through the ordinary customary or treaty law processes that may, in principle, provide evidence of the acceptance and recognition of States that such derogations from the norm are not possible.
However, there are difficulties in using the absence of derogations alone to identify the extent to which a norm is accepted and recognised as non-derogable. When evaluating whether a customary prohibition exists, a State's omission to perform a particular conduct is ambiguous because it may reflect an acceptance (opinio juris) that the conduct is prohibited by customary law, but equally it may result from the State's choice not to exercise a customary permission to perform the conduct.Footnote 90 In the same way, failure to create a derogation to a norm may reflect acceptance and recognition that derogation is not possible to this extent, but equally it may simply be that States have no interests in creating such a norm, and so a supporting practice has not developed. As a result, we cannot infer solely from the omission to create a derogation that a State considers the norm to be non-derogable to this extent. A further difficulty is presented by the higher quantitative threshold, which requires acceptance and recognition that the norm is non-derogable by the ‘international community as a whole’. Even if we can be sure that a State or States accept that a customary derogation from a norm is not possible, all that is required to prevent such a derogating customary norm being established is that those States are sufficient in number to prevent the emergence of a widespread and representative practice and opinio juris in support of the derogating norm. Without additional evidence in the form of statements, this will not be enough to establish the acceptance and recognition of the international community as a whole (a very large majority of States) that such a derogation is not possible.
Yet, the opposite deduction—using the existence of derogations to identify the extent to which derogation from the norm is possible—does not raise these evidential problems. Where there is a general customary norm that statements from the international community as a whole have identified as having jus cogens status, the precise scope of the jus cogens norm may thus be identified by starting from the general customary norm which is said to provide the source for the alleged jus cogens norm and taking account of all apparent derogations to that general norm in order to identify the precise contours of the norm that is accepted and recognised as non-derogable. Where there is—for example—an established customary derogation from a general norm it is clear that the widespread and representative practice and opinio juris required for the formation of that customary norm means it is impossible that the international community as a whole (a very large majority of States) holds the opposite view, that it is not possible to derogate from the norm to this extent. The scope of a jus cogens norm may thus be identified negatively, by looking at the structure of the general norm on which the jus cogens norm is said to be based and the circumstances in which there is or is not a legally valid reason to depart from the underlying customary norm.
This approach therefore deals with the ‘exceptions’ to the prohibition on force not by including them within the jus cogens norm, as in the expansive approach above, but by excluding them from its scope. The existence of apparent derogations to the prohibition on force is not an obstacle to its identification as jus cogens, but the key to that process. Starting with the customary prohibition on force as the underlying norm of general international law which has been explicitly recognised by States as jus cogens through their statements, we must then take account of all derogations to that general norm. To the extent that the norm is subject to derogation, it cannot be accepted and recognised as non-derogable by the international community as a whole, allowing us to identify, by a process of elimination, that part of the general norm from which derogation is accepted and recognised as not possible.
This approach may appear at odds with how we typically think of jus cogens norms operating: first a norm is identified as jus cogens, and from that status certain effects are deduced, such as the inability to derogate. This approach would turn this on its head. Yet, ‘[t]he “peremptory” character of a legal norm in general international law is a conclusion, not a premise’Footnote 91 and if we accept that the elements from Article 53 correctly reflect the customary criteria for identifying a norm as jus cogens then it is the acceptance and recognition of non-derogability alone that can justify that conclusion.
The approach suggested above may also appear to render the concept of jus cogens rather empty. What is the point of jus cogens status if any derogations can simply be explained away as part of the definition of the scope of the norm? However, this is a necessary consequence of any approach to identification of jus cogens that accepts that the customary criteria for identification derived from Article 53 VCLT make acceptance and recognition that no derogation is permitted the key to identification of jus cogens norms. Unless one adopts a content-based criterion for identification of jus cogens norms which sets out, for example, that jus cogens norms must protect certain fundamental values—the approach rejected by States at the Vienna conferenceFootnote 92—the international community of States as a whole, provided it acts collectively, is free to allow any derogations to any norms they please and identify the content of the non-derogable norm in such a way that it imposes no meaningful restriction on the freedom of action of States. There is no necessary content for the norms that States recognise as non-derogable; indeed, there is no obligation for States to identify any jus cogens norms at all. The norms recognised as jus cogens may tend to be norms that States consider to be of fundamental importance or that carry particular moral weight, but this is not necessary for a norm to acquire jus cogens status.
Instead, the value of a norm's jus cogens status lies in its impact on modification of the non-derogable norm once identified. Just as in many domestic systems the only protection from abrogation for entrenched rights is a more onerous process of change, whatever the jus cogens norm is, no new derogation can be created until the jus cogens norm is validly modified through the acceptance and recognition of the new non-derogable norm by the international community of States as a whole. Until that occurs, individual States, or even groups of States that fall short of a ‘very large majority’ will neither be able to derogate from nor modify the contours of an existing jus cogens norm, for example by creating new derogations or altering its scope through the ordinary customary process.Footnote 93
III. THE STRUCTURE OF THE JUS AD BELLUM
Applying the methodology above to the jus ad bellum, we can conclude that the jus cogens norm in the jus ad bellum must be the customary norm which prohibits non-consensual force that does not fall within either of the two established so-called ‘exceptions’: authorisation under the UN Charter and self-defence. Starting from the general customary prohibition on force, an apparent derogation from that norm exists in the form of authorisation by the Security Council, acting under Chapter VII of the UN Charter. If such uses of force were prohibited by a jus cogens norm, the Charter would be purporting to provide a legally valid reason to depart from the jus cogens norm and, one would conclude, must be void through the operation of either the customary rule codified in Article 53 VCLT or, depending on the time at which the prohibition on force attained jus cogens status, the customary rule codified in Article 64 VCLT. Yet the UN Charter not only purports to provide a legally valid reason for using force in its text (as interpreted through subsequent practice)—authorisation under Chapter VII—but these provisions have been applied on multiple occasions without the validity of the Charter in whole or in part ever being called into question. The universal acceptance, including by the 193 UN member States, that it is possible for such a treaty to be valid means that the opposite view— that a norm prohibiting the use of force when authorised under the Charter is non-derogable—cannot be accepted and recognised by the international community as a whole. Such uses of force must fall outside the scope of the jus cogens prohibition on force. Moreover, they help define the scope of that jus cogens norm, by demonstrating the limits of the norm that can be accepted and recognised as non-derogable.
Turning to self-defence, it is not a treaty that constitutes the apparent derogation from the customary prohibition but the customary right of self-defence.Footnote 94 If force in self-defence were prohibited by the jus cogens prohibition on force, a customary right of self-defence would be a norm purporting to provide a legally valid reason to depart from that jus cogens norm. Either the customary norm of self-defence would be invalid or, more likely, like the right of self-help it would have disappeared from customary law as a prohibition on force in self-defence came to be identified as jus cogens.Footnote 95 Yet since the customary right of self-defence unquestionably exists,Footnote 96 we must conclude that the jus cogens prohibition on the use of force does not extend to a prohibition on force used in self-defence. The universal acceptance that the right of self-defence exists in customary law means the prohibition on the use of force cannot be accepted and recognised by the international community as a whole as non-derogable to the extent it includes force used in self-defence within its scope. Such uses of force must be outside the scope of the jus cogens prohibition.
Importantly, these conclusions are without prejudice to the analysis of the underlying structure of the customary and treaty norms in the jus ad bellum, which proves to be a much more complex question than the scope of the jus cogens norm. The definition of the jus cogens norm given above is compatible with more than one analysis of the structure of the jus ad bellum. For example, given that a State may consent to the use of force by another State and this will render that use of force lawful,Footnote 97 consensual force cannot be prohibited by a jus cogens norm. Of course, this does not tell us how consent to the use of force operates: consensual force may simply not constitute prohibited force for the purposes of the underlying customary prohibition on force,Footnote 98 so that in this regard the scope of the customary and jus cogens prohibitions are identical. Alternatively, both consensual and non-consensual uses of force may be prohibited by the customary norm, so consensual force is prima facie prohibited by the customary prohibition but consent acts as a circumstance precluding wrongfulness or defence for breaches of that prohibition.Footnote 99 Either way, with regard to the scope of the jus cogens norm their position is the same: it cannot prohibit consensual force. The general customary norm may be jus cogens in part or to a certain extent: just because conduct falls outside the scope of the jus cogens part of that prohibition does not mean it is not prohibited by the derogable customary norm. Put another way, if a particular type of conduct is regarded as being lawful ‘all things considered’ it cannot be prohibited by a jus cogens norm; but this does not tell us—nor do we need to know in order to identify the scope of the jus cogens norm—anything more about why that conduct is lawful ‘all things considered’. It could be perfectly legal ab initio, or prima facie unlawful but justified or excused.
Turning to the so-called ‘exceptions’ to the prohibition on the use of force, this conclusion as to the scope of the jus cogens norm also leaves open different possibilities as to the underlying structure of the UN collective security framework in customary and treaty law —and indeed whether it is truly an exception to the customary prohibition on force. Force authorised under the Charter may be prohibited by the customary prohibition on force, with the Charter acting as lex specialis among UN members. In this case, the Charter derogates from the customary prohibition on force. Alternatively, force authorised under the Charter may simply fall outside the scope of the prohibition on force which, for example, prohibits unilateral force only.Footnote 100 In this case, the Charter does not derogate from the customary prohibition, nor is it a true exception, as such uses of force are simply not prohibited by that customary norm, even prima facie. Identifying the correct structure of the ‘exception’ lies beyond the scope of this article; the point here is simply that this does not need to be resolved in order for us to conclude that the scope of the jus cogens norm in the jus ad bellum must not extend to a prohibition on force authorised under the Charter. Either the Charter provides a legally valid reason to perform conduct prohibited by the customary prohibition on force, in which case that norm cannot be non-derogable to that extent; or such uses of force were never prohibited by the customary prohibition on force in the first place, in which case they could never fall within the scope of the jus cogens prohibition, which is based on that general norm.
Similarly, the conclusion above does not resolve the question of the structure of the right of self-defence as an apparent ‘exception’ to the customary prohibition on force, much less its relationship with Articles 51 and 2(4) of the Charter. Force used in self-defence may simply not qualify as prohibited force under customary law and so fall outside the scope of both the customary and jus cogens prohibitions.Footnote 101 On the other hand, force used in self-defence may be a prima facie breach of the customary prohibition on force which is then made lawful by the operation of a separate customary norm that creates a right of self-defence. In the latter case, self-defence would be a true exception to the customary prohibition which would be jus cogens in part—to the extent that force is permitted in self-defence it is not non-derogable. In either case, the scope of the jus cogens prohibition on force will not extend to force used in self-defence.
In this way, the derogations from the customary prohibition on force identify which part of that norm is non-derogable. Once the meaning of derogation is clarified, it follows that any force that is regarded as lawful ‘all things considered’ cannot be prohibited by a jus cogens norm. Depending on the structure of the underlying norms, either the derogation defines the limits of the underlying customary prohibition on force itself, or it demarcates the boundary between that part of the customary prohibition that is derogable and that which is non-derogable. In either case, the apparent derogations are used to identify the scope of the jus cogens norm. However, what this reveals is that identifying the scope of the jus cogens norm in the jus ad bellum is relatively straightforward when compared with the more complex question of the structure of the underlying customary and treaty norms that make up the jus ad bellum.
Many authors have reached a similar conclusion as to the scope of the jus cogens prohibition on force while attempting to square the circle of a jus cogens norm that is subject to apparent derogations. For example, for Orakhelashvili and Kolb the so-called exceptions to the prohibition on the use of force go to the substantive scope of application of the rule and are ‘included within the content of the [jus cogens] norm itself’.Footnote 102 However, few have considered how this interface between the non-derogable norm and the norms regulating uses of force that fall outside its scope operates. This raises the difficult question of the level of abstraction with which that boundary is defined, and in turn how the content of the jus cogens norm is defined.
That the derogations to the general customary norm are used to identify the scope of the jus cogens norm does not necessarily mean that the definition of the jus cogens norm must mirror in every detail the contours of those derogating norms. If that were the case, and if the extent to which force may lawfully be used in self-defence determines the limits of the jus cogens norm, then the requirements of necessity and proportionality, the definitions of armed attack and imminence, indeed every element of the right of self-defence, must be reflected in the definition of the jus cogens norm.Footnote 103 At its boundary with self-defence the jus cogens prohibition would be like a negative image of the customary right. The same would also apply to any other derogation which is being used to identify the scope of the non-derogable norm, such as authorisation of force under the Charter or consent.
Such an approach risks absurd consequences, as the definition of the jus cogens norm becomes so long and complex as to be unworkable.Footnote 104 Moreover, this does not really correspond with how we think about legal norms in practice. It seems implausible that the acceptance and recognition of States that a norm is non-derogable could extend to every detailed element of the customary prohibition on which it is based. What is more likely is that the jus cogens norm is defined at a higher level of abstraction and prohibits force that is not a lawful exercise of self-defence, however that is defined by the customary law of self-defence at a particular time. In this way, the scope of the jus cogens norm may be ambulatory: it tracks changes in the customary law of self-defence as it evolves. What matters is the conclusion as to lawfulness that is reached through the application of the body of law that regulates self-defence—or the law that governs the validity of consent to force, or the collective security provisions of the Charter.
This conclusion has interesting implications for the modification of jus cogens norms. Article 53 VCLT provides that a jus cogens norm ‘can be modified only by a subsequent norm of general international law having the same character’. While jus cogens norms are not immutable, they may only be modified through their replacement by another jus cogens norm in accordance with the process for the identification of jus cogens norms. That is, the two criteria must be fulfilled, including the acceptance and recognition of the new contours of the jus cogens norm by a very large majority of States. However, if the jus cogens norm only prohibits uses of force which are concluded to be unlawful through application of the customary right of self-defence, this suggests that there can be a certain amount of change to the customary norms comprising the right of self-defence without this amounting to a modification of the jus cogens norm, with the more demanding requirements that this entails.
An important constraint is placed on this flexibility by the scope of the jus cogens norm. If the change to customary law were so significant that it goes beyond modification of the content of self-defence and modifies the jus cogens norm itself that provides the outer boundaries of that ambulatory right, then the requirements for modification of a jus cogens norm would need to be fulfilled, not merely those of customary international law. For example, an attempt to transform self-defence into a broader doctrine of self-help or self-preservation. Thus, it is essential to identify with precision the scope of the norm accepted and recognised as jus cogens, as this will determine how much and what kinds of change may be tolerated before it amounts to a modification of the jus cogens norm.
Drawing these kinds of fine distinctions and determining the level of abstraction at which the jus cogens norm is identified and its limits expressed will require a detailed analysis of the acceptance and recognition of States, and to attempt to do so for the jus cogens norm in the jus ad bellum would require a much lengthier analysis than is possible here. In practice, the content of the jus cogens norm may most easily be determined by examining how modifications to that norm have occurred in practice, as we can see if we turn to the other so-called ‘exception’ to the prohibition on force. Since 1945, the original interpretation of the text of Article 27(3) of the UN Charter has been modified to allow force to be authorised by Security Council decisions adopted with the abstention of a permanent member. This modification of the collective security system resulted from the interpretation of the Charter through subsequent practice of its members, as codified in Article 31(3)(b) VCLT: that is, the presidential rulings and positions of Council members combined with the acquiescence of other parties to the Charter.Footnote 105 Although the practice was likely generally accepted by UN members long before it was confirmed by the Court's 1971 advisory opinion, even by 1971 only 132 States were parties to the Charter—far from the ‘very large majority’, even of those States in existence at that time, which would be required to modify the jus cogens prohibition on force. This suggests that the limitation on the scope of the jus cogens norm constituted by collective security exists at a level of abstraction above the detailed procedural requirements for the functioning of the Security Council as set out in the text of the Charter as drafted. That is, the jus cogens norm must have prohibited ‘force not validly authorized by the Security Council’ or perhaps ‘force not validly authorized under the Charter’, whatever that happens to be at the time, not ‘force not authorized by a decision of the Security Council made by an affirmative vote of nine members including the concurring votes of the permanent members’.Footnote 106 That specific change to the Security Council decision-making process must not have entailed a modification of the jus cogens norm.
In this way, an ‘ambulatory’ approach to defining the scope of the jus cogens norm can help account for the ‘notoriously flexible’ nature of the jus ad bellum, despite the presence of a jus cogens norm at its heart.Footnote 107 Not only that, but this may explain how the prohibition on the use of force can be identified as a jus cogens norm while key elements of the jus ad bellum remain uncertain and contested by States.Footnote 108 Green's 2011 observation that the existence and scope of the alleged rights of anticipatory or pre-emptive self-defence are hotly debated by States is no less true today. It would indeed be difficult to frame a definition of a jus cogens norm that accounted for this uncertainty. However, if the definition of the jus cogens norm is only concerned with the conclusion as to the lawfulness of a use of force under the customary law of self-defence, not with the elements that establish that lawfulness, then this problem does not arise. The debate can continue at the level of custom while the jus cogens prohibition remains unchanged.Footnote 109
IV. CONCLUSION
This article has argued that the jus cogens norm in the jus ad bellum is the customary norm which prohibits non-consensual force that does not fall within either of the two apparent exceptions: authorisation under the UN Charter and self-defence. In doing so, it has been argued that in the absence of any universal treaties, all existing jus cogens norms must be norms of customary international law. For a norm to derogate from another norm means it purports to provide a legally valid reason to depart from that norm. Where a general customary norm is alleged to be jus cogens but its contours are unclear, the scope of the non-derogable norm may be identified by using any established derogations to that norm to infer to what extent the norm is accepted and recognised by the international community as a whole as non-derogable. Finally, the conclusion above as to the scope of the jus cogens norm in the jus ad bellum leaves open a range of explanations of the structure of the underlying customary and treaty law norms. The flexibility and uncertainty of the content of the apparent exceptions to the jus cogens prohibition on force suggest that they could be ambulatory—they track changes in customary and treaty law without the jus cogens norm itself being modified.
