A. The Prohibition of Crimes Against Humanity

The prohibition of crimes against humanity has not been explicitly recognised by the ICJ as a peremptory norm. In the 2012 Jurisdictional Immunities case, however, the ICJ appeared implicitly to accept the peremptory status of the prohibition of both crimes against humanity and war crimes. In the course of rejecting the argument that ‘no rule which is not of the status of jus cogens may be applied if to do so would hinder the enforcement of a jus cogens rule’, the Court referred to its earlier judgment in the Arrest Warrant case, and said that in that case:

In the Arrest Warrant case, the Congolese Minister for Foreign Affairs was suspected of having committed ‘war crimes or crimes against humanity’—the Court did not in its judgment distinguish between the status of those two crimes. In upholding his immunity from criminal prosecution, the Court did not expressly address the question of whether those crimes were prohibited by jus cogens norms. It did, however, reference the statement of Lord Millet in the Pinochet case that there could be no immunity for crimes protected by jus cogens norms,Footnote ³⁴ and found that customary international law did not recognise such an exception to the immunity of government ministers. While it would be a stretch to interpret Arrest Warrant itself as endorsement of the peremptory status of crimes against humanity and war crimes, when the judgment is read together with the Court's later reference in Jurisdictional Immunities to rules ‘undoubtedly possess[ing] the character of jus cogens’, it does appear that the ICJ has accepted that both crimes against humanity and war crimes are prohibited by jus cogens norms.

A number of other international courts and tribunals have regarded the prohibition of crimes against humanity as a jus cogens norm. The International Criminal Court (ICC) has said that ‘it is generally agreed that the interdiction of crimes against humanity enjoys the status of jus cogens’,Footnote ³⁵ and the International Criminal Tribunal for the former Yugoslavia (ICTY) has said similarly that the norms ‘prohibiting war crimes, crimes against humanity and genocide’ are ‘peremptory norms of international law or jus cogens’.Footnote ³⁶ The Special Court of Sierra Leone has also seemingly accepted the jus cogens status of crimes against humanity: in the context of a discussion about crimes against humanity and universal jurisdiction, the Court said that ‘a state cannot sweep [crimes against humanity] into oblivion and forgetfulness … [because] the obligation to protect human dignity is a peremptory norm’.Footnote ³⁷

The peremptory status of the prohibition of crimes against humanity has also been accepted by the inter-American human rights system, as well as by several domestic courts. In 2006 the Inter-American Court of Human Rights held that the ‘prohibition to commit crimes against humanity is a jus cogens rule’,Footnote ³⁸ and that statement has been subsequently relied upon by the Inter-American Commission on Human Rights.Footnote ³⁹ The Argentinian and Chilean Supreme Courts have both described the prohibition of crimes against humanity as a jus cogens rule,Footnote ⁴⁰ and the Court of Bosnia and Herzegovina has described the jus cogens status of ‘criminal liability for crimes against humanity’ as ‘established’.Footnote ⁴¹ Domestic courts in the US, Greece, Spain and the Republic of Korea have all seemingly accepted, for the most part without critique or analysis, that the prohibition of crimes against humanity is a peremptory norm,Footnote ⁴² and the Italian Supreme Court has accepted the jus cogens nature of ‘norms for the protection of fundamental human rights’.Footnote ⁴³

The ILC's Draft Articles on Crimes Against Humanity describe the prohibition of crimes against humanity as a peremptory norm of international law in its preambular paragraph. Special Rapporteur on Peremptory Norms Dire Tladi has observed that when written responses were received from States on the Draft Articles, ‘only one State, France, questioned the inclusion of the preambular paragraph’, and even in that case France was not questioning the correctness of the preambular paragraph, but merely its ‘appropriateness given that the subject of jus cogens was being considered in a different topic’.Footnote ⁴⁴

The most authoritative modern definition of crimes against humanity is that provided in the Rome Statute of the International Criminal Court (Rome Statute). Article 7 of the Rome Statute identifies a series of criminal acts that, ‘when committed as a part of a widespread or systematic attack directed against any civilian population, with knowledge of the attack’, constitute a crime against humanity. The acts include murder, extermination, imprisonment in violation of fundamental rules of international law, torture, rape and other forms of sexual violence, and enforced disappearance.Footnote ⁴⁵ It has been suggested that it is unclear whether ‘all of the “underlying crimes” encompassed within this definition of crimes against humanity are indeed protected by jus cogens’;Footnote ⁴⁶ however, such suggestion seems to miss the point that it is the widespread and systematic nature of the ‘attack’ that gives the criminal acts their character as crimes against humanity. It bears noting that in the jurisprudence referred to above, when the prohibition of crimes against humanity is described as a jus cogens norm, there is typically no distinction drawn between the specific underlying criminal acts. Thus, if the prohibition of crimes against humanity has been authoritatively recognised as a jus cogens norm—as the preceding analysis suggests—it ought not to matter whether the widespread and systematic attack is comprised of acts of murder, torture, deportation, rape or other form of sexual violence, or any combination thereof.

B. The ‘Basic Rules of International Humanitarian Law’

The ILC's 2019 Draft Conclusions include ‘the basic rules of international humanitarian law’ as part of the ‘non-exhaustive list’ of norms the ILC has previously referred to as peremptory.Footnote ⁴⁷ In support, the ILC references two outputs of the Study Group on Fragmentation of International Law, and its own commentary to the ARSIWA.Footnote ⁴⁸ The outputs of the Study Group referred to merely reference the commentary to the ARSIWA, and note that the ‘basic rules of international humanitarian law’ are among the most ‘frequently cited’ candidates for consideration as jus cogens norms.Footnote ⁴⁹ The commentary to the ARSIWA references the ICJ's Nuclear Weapons Advisory Opinion, in which the Court described the ‘fundamental rules’ of international humanitarian law as ‘intrangressible principles of international customary law’, and asserts that in light of that, it would ‘seem justified to treat these [rules] as peremptory’.Footnote ⁵⁰

In Nuclear Weapons, the ICJ in fact explicitly declined to express a view on the peremptory status of the rules of international humanitarian law.Footnote ⁵¹ Thus, some scholars have rightly observed that it cannot be assumed that by ‘intrangressible’, the Court meant peremptory.Footnote ⁵² However, as Special Rapporteur Dire Tladi has observed, ‘it is not clear what else the term can mean in that context. It surely could not mean rules that may not be violated … since, by definition all rules, including rules of a jus disposivitum character, would be of that nature.’Footnote ⁵³ Accepting that by ‘intransgressible’, the Court had to have meant more than merely inviolable, but acknowledging also that the Court explicitly opted not to use the word ‘peremptory’, it is asserted here that what the Court must have meant was non-derogable. It is important to recall that non-derogable in this context means not only that a rule cannot be violated, but that States cannot absolve themselves of obligations flowing from the rule. As Katie Johnston explains, ‘a derogation is … not concerned with factual conduct that is not in compliance with an obligation of a party that is bound by a norm, but rather with legal or normative acts’.Footnote ⁵⁴ The reason that ‘intransgressible’ must have meant ‘non-derogable’, is that there is simply no other relevant descriptor of the status of rules of customary international law between binding (inviolable), and peremptory. Thus, in describing the fundamental rules of international humanitarian law as ‘intransgressible principles of customary international law’, the ICJ was effectively describing those rules as rules of customary international law that could not be derogated from—in other words, peremptory norms, according to the criteria prescribed by the VCLT. This approach aligns with the view expressed by the ICTY, that ‘most customary rules of international humanitarian law’ are peremptory, ‘ie of a non-derogable and overriding character’.Footnote ⁵⁵

In its commentaries to the 2019 Draft Conclusions, the ILC does not elaborate which of the ‘basic rules of international humanitarian law’ it is referring to, in its non-exhaustive list of peremptory norms. Thus, several States have critiqued the inclusion of this norm in the list, on the basis that it is insufficiently precise.Footnote ⁵⁶ While this criticism seems justified, it bears noting that the question of which rules of international humanitarian law constitute the ‘basic rules’—or to use the ICJ's term, the ‘fundamental rules’—is in fact not particularly controversial. In Nuclear Weapons, the ICJ defined the ‘cardinal principles contained in the texts constituting the fabric of humanitarian law’—that is, those it proceeded to describe as intransgressible—as follows:

The ICTY has also recognised the prohibition of deliberate attacks on civilians or civilian objects as a ‘universally recognised principle’, together with the principle that ‘reasonable care must be taken in attacking military objectives so that civilians are not needlessly injured’.Footnote ⁵⁸ It notes that this latter principle has ‘always been applied in conjunction with the principle of proportionality, whereby any incidental (and unintentional) damage to civilians must not be out of proportion to the direct military advantage gained by the military attack’.Footnote ⁵⁹ The ICTY has recognised further the customary (and uncontested) nature of the prohibition of attacks that do not discriminate between civilians and combatants.Footnote ⁶⁰

The General Assembly has similarly affirmed, as ‘basic principles for the protection of civilians in armed conflicts’: the requirement that distinction be made between combatants and civilians; the requirement that precautions be taken to avoid injury, loss or damage to civilians; the prohibition of attacks targeting civilians or civilian objects; and the requirement that civilians should not be the object of reprisals, forcible transfers or other assaults on their dignity.Footnote ⁶¹

The International Committee of the Red Cross asserts that the principle of distinction, the prohibition of indiscriminate attacks, the requirements of proportionality and precautions in attack, and the prohibition of weapons causing superfluous injury or unnecessary suffering, are all rules of customary international law applicable in both international and non-international armed conflict.Footnote ⁶² While these references relate generally to the customary, not necessarily peremptory, nature of the principles described, the point here is to illustrate that the ‘basic’ or ‘fundamental’ rules of international humanitarian law are well established and for the most part uncontroversial.

In addition to the ICJ's somewhat masked recognition of the peremptory nature of the ‘fundamental rules’ of international humanitarian law, there is some judicial support for the peremptory nature of war crimes. As discussed above in relation to crimes against humanity, in its Jurisdictional Immunities Advisory Opinion, the ICJ seemed implicitly to accept the peremptory nature of the norms prohibiting both war crimes and crimes against humanity.Footnote ⁶³ The ICTY, the US District Court of the Eastern District of New York and the Greek Supreme Court have all explicitly described the prohibition of war crimes as a rule of jus cogens,Footnote ⁶⁴ and the US Appeals Court, the Argentinian Supreme Court and the Italian Supreme Court have done so implicitly.Footnote ⁶⁵ Numerous scholars also include war crimes in their list of peremptory norms.Footnote ⁶⁶

This judicial and scholarly recognition of the prohibition of ‘war crimes’ as a peremptory norm, rather than the ‘basic rules of international humanitarian law’, warrants examination. It is pertinent to recall here that international humanitarian law and international criminal law serve different purposes. As Adil Haque describes, international humanitarian law ‘guide[s] the prospective conduct of military commanders and soldiers on the battlefield’, whereas international criminal law, ‘guide[s] the retrospective evaluation of past offences by courts’.Footnote ⁶⁷ Put otherwise, international criminal law translates principles of international humanitarian law into specific crimes for which individuals can be tried and punished. The Rome Statue, specifically, translates principles of international humanitarian law into crimes for which individuals can be tried and punished at the ICC.

Prior to the Rome Statute, earlier international criminal tribunals defined war crimes simply as violations (or serious violations, in the case of the ICTY) of humanitarian law.Footnote ⁶⁸ Conversely, the Rome Statute's definition of war crimes does not fully reflect the basic rules of international humanitarian law, as enshrined in customary international law, on which those crimes are based. The definition of war crimes in the Rome Statute does not, for example, encompass the reckless or negligent failure to distinguish between civilians and combatants; nor the use of indiscriminate means or methods of warfare; nor the pursuit of military advantage at unnecessary cost to civilians; nor the incidental killing of civilians that is not ‘clearly’ excessive relative to the military advantage sought.Footnote ⁶⁹ In short, as Haque has shown, the Rome Statute ‘fails to fully enforce four core principles of humanitarian law designed to protect civilians: distinction, discrimination, necessity, and proportionality’.Footnote ⁷⁰

The Rome Statute is generally regarded as providing the most authoritative definition of war crimes. Because of the gap between war crimes as described in the Rome Statute, and the principles of customary international humanitarian law on which those crimes are based, the tendency of jurists and scholars to include ‘war crimes’ in their list of peremptory norms, instead of the fundamental rules of international humanitarian law, is problematic—at least as concerns the implications for State responsibility. It also regresses from the ICJ's recognition in its Nuclear Weapons Advisory Opinion of the ‘fundamental rules of humanitarian law’ as ‘intransgressible’ (non-derogable) rules of customary international law.

The primary source of peremptory norms of international law is customary international law, not the treaty rules that reflect—sometimes only partially—those customary norms.Footnote ⁷¹ This is so particularly in the case of the Rome Statute, which was negotiated and agreed for the specific purpose of designating the crimes within the jurisdiction of the ICC. What this means for the task of identifying peremptory norms in the field of international humanitarian law is that the point of reference must be customary international humanitarian law, and in particular the ‘fundamental rules’ recognised by the ICJ as ‘intransgressible’, rather than the Rome Statute.

It bears noting that in the ICJ's Jurisdictional Immunities Advisory Opinion, in which the Court seemingly accepted the peremptory nature of war crimes and crimes against humanity, the question with which the Court was concerned was one of individual criminal responsibility. It might be supposed that if the Court had been called upon to consider the substantive principles of international humanitarian law—and perhaps, whether a State could absolve itself of obligations arising from those principles—the Court might have referred to the peremptory nature of the fundamental rules of international humanitarian law, rather than the peremptory nature of the crimes deriving from those rules.

As it stands, what the ICJ appears to have done—reading together Nuclear Weapons, Arrest Warrant and Jurisdictional Immunities—is to have recognised the peremptory nature of the fundamental rules of international humanitarian law (in particular the principle of distinction and the prohibition of unnecessary suffering), and additionally, recognised the peremptory nature of the rules that have translated those principles, albeit imperfectly, into crimes attracting individual criminal responsibility.

With regard to the obligation to cooperate to end serious breaches of peremptory norms, what this means is that States are obliged to cooperate to end serious breaches of the ‘fundamental rules’ of international humanitarian law, as defined by customary international law and interpreted by international courts and tribunals,Footnote ⁷² and more specifically, to cooperate also to bring an end to serious breaches of the prohibition of war crimes (that is, all war crimes, there being no such thing as a non-serious war crime).

A. Status of the Obligation to Cooperate

As noted above, in its commentary to the 2019 Draft Conclusions, the ILC asserts that the obligation to cooperate to end serious breaches of peremptory norms of international law is ‘now recognised under international law’.Footnote ⁷³ In support, the ILC cites the ICJ's Wall and Chagos Advisory Opinions, a decision of the Inter-American Court of Human Rights, a decision of the UK House of Lords, and its own Draft Articles on the Protection of Persons in the Event of Disasters.Footnote ⁷⁴

The ICJ's Wall (2004) and Chagos (2019) Advisory Opinions do not explicitly affirm that customary international law recognises an obligation to cooperate to end serious breaches of peremptory norms. In its Wall Advisory Opinion, the ICJ found that Israel's construction of a wall in the Occupied Palestinian Territories violated Israel's obligation erga omnes to respect the Palestinian right of self-determination; and that as such, all States were under an obligation ‘to see to it that any impediment, resulting from the construction of the wall, to the exercise of the Palestinian people of its right to self-determination is brought to an end’.Footnote ⁷⁵ Similarly, in its Chagos Advisory Opinion, the ICJ affirmed that the obligation to respect the right to self-determination was an obligation erga omnes, and proceeded to determine that States ‘must cooperate with the United Nations to complete the decolonization of Mauritius’.Footnote ⁷⁶ This element of the Chagos Advisory Opinion has been rightly criticised as seemingly conflating obligations erga omnes with the obligations associated with a serious breach of a peremptory norm.Footnote ⁷⁷

In fact, in international law it does not automatically follow that just because an obligation has erga omnes status, all States have an obligation to put an end to a breach of that obligation—as they would for a serious breach of a peremptory norm. The ICJ did not in either its Wall or Chagos Advisory Opinions make any reference to peremptory norms, let alone to the specific consequences that flow from the identification of a norm as peremptory.Footnote ⁷⁸ The Wall and Chagos Opinions are important insofar as they support the general notion that third States have a positive duty to cooperate to bring an end to breaches of at least some obligations erga omnes, irrespective of any treaty obligation owed by the concerned State. It seems a stretch, however, to regard the Opinions as affirming the existence in customary international law of an obligation to cooperate to end serious breaches of peremptory norms.Footnote ⁷⁹

The assertion that customary international law recognises an obligation to cooperate to end serious breaches of peremptory norms is controversial. As several scholars have observed, State practice in support of such an obligation is not plentiful, and it is difficult to find evidence of opinio juris to the effect that States regard themselves as bound by customary international law to cooperate to bring an end to serious breaches of peremptory norms.Footnote ⁸⁰ Such concern was echoed by several States in their written submissions on the Draft Conclusions, provided to the ILC in 2021 ahead of its 73rd Session. The US, for example, argued that there was ‘no basis to assert that there is a binding obligation on non-breaching States to address the wrongful act, as there is neither a relevant rule of customary international law nor express agreement of States to accept such an obligation’.Footnote ⁸¹ Israel asserted that ‘the particular consequences referred to in Draft Conclusion 19 [including the obligation to cooperate to bring an end to serious breaches of peremptory norms] do not reflect existing international law’;Footnote ⁸² and the UK, similarly, ‘encouraged the Commission to acknowledge the unsettled status’ of the obligations flowing from serious breaches of peremptory norms.Footnote ⁸³ Other States, however, expressed support for the ILC's assertion regarding the customary status of the obligation to cooperate;Footnote ⁸⁴ while others refrained from expressing a view either for or against the customary status of the obligation, but requested the ILC to provide further evidence of State practice and opinio juris in support of such status.Footnote ⁸⁵

In addition to the concerns expressed by some States regarding the ILC's assessment of the obligation to cooperate to end serious breaches of peremptory norms, it is pertinent to note that several States have expressed concern regarding the pace at which the Draft Conclusions have taken shape, the lack of consultation with States, the insufficiency of State practice underpinning the ILC's conclusions, and the extent to which the Draft Conclusions push the boundaries of the ILC's mandate to codify and progressively develop international law. At the 2019 meeting of the General Assembly's Sixth Committee at which the Draft Conclusions were debated, Slovakia's representative said that the ILC had ‘boldly proceeded to the adoption of the whole set of draft conclusions on first reading’ and that a ‘rushed outcome, with scant regard for the divergent views of States, was unlikely to lead to success’.Footnote ⁸⁶ The UK, in its written submission to the ILC on the Draft Conclusions, recalled that it had ‘consistently urged the Commission to progress cautiously and to take full account of the lack of practice when proposing draft conclusions’.Footnote ⁸⁷ Spain, similarly, cautioned that ‘the effort of systemic construction will only be robust and have normative authority if it enjoys the necessary consensus of the international community of States’, and it described the Draft Conclusions as a ‘starting point for a constructive dialogue’.Footnote ⁸⁸

ILC pronouncements are not, in themselves, sources of law. As such, an assessment of the extent to which the asserted obligation to cooperate in fact reflects customary international law will be informed by further comments received from States on the Draft Conclusions, and—over time—by the way in which that obligation is accepted by States as well as by courts and tribunals. In the meantime, it bears noting that while ILC pronouncements are not sources of law, they are persuasive ‘subsidiary means for the determination of rules of law’.Footnote ⁸⁹ In 2008, Justice Rosalyn Higgins remarked upon ‘the high percentage [of ICJ judgments] in which reference has been made to the work of the ILC’;Footnote ⁹⁰ and since then the Court has continued to refer to the ILC's work in determining the customary status of rules.Footnote ⁹¹ As of 31 January 2020, the ICJ had expressly relied upon the ILC's work in 23 decisions and advisory opinions.Footnote ⁹² In doing so, it typically has not engaged in its own analysis of the State practice and opinio juris on which the ILC's work relies.Footnote ⁹³

Danae Azaria suggests that the ILC's pronouncements may be regarded as an ‘offer of interpretation’ to States, and that ‘whenever States fail to engage with the ILC's interpretive offer, international courts and tribunals are likely to rely on the ILC's interpretive pronouncements as a subsidiary means for determining rules of law’.Footnote ⁹⁴ As such an offer, the ARSIWA have been particularly persuasive. By early 2019 the ARSIWA had been referenced in 249 decisions of international tribunals,Footnote ⁹⁵ and as David Caron has observed, many courts and tribunals now do ‘not assess [the ARSIWA], but instead … accept them as a given’.Footnote ⁹⁶ As of early 2016, Article 41 of the ARSIWA, on the obligation to cooperate, had been referenced at least 23 times in decisions and advisory opinions of the ICJ—more than almost any other Article in the ARSIWA.Footnote ⁹⁷ In short, while the status of the asserted obligation to cooperate to end serious breaches of peremptory norms is to some extent undermined by the concerns expressed by States both at the 2019 meeting of the General Assembly's Sixth Committee, and subsequently in written comments to the ILC, it is—conversely—bolstered by the long-standing reliance by courts and tribunals on the ILC's pronouncements in general, and the ARSIWA in particular.

This article does not interrogate further the assertion that the obligation to cooperate to end serious breaches of peremptory norms has attained customary status. Whether or not one accepts such assertion, it is the case that Draft Conclusion 19 (the obligation to cooperate) may be relied upon by States as a guide—an ‘interpretive offer’, to use Azaria's term—regarding the conduct of their international relations. As such, this article proceeds on the basis that, regardless of the precise legal status of the obligation to cooperate, there is value—particularly for States who aspire to ‘best practice’ in their international relations—in better understanding what that obligation requires. In particular, there is value in better understanding how States might fulfil the obligation by leveraging their membership of international organisations.

B. The Nature of the Obligation to Cooperate

The ILC's commentary to Article 41(1) of the ARSIWA describes the obligation to cooperate to end serious breaches of peremptory norms as a ‘positive duty’ that calls for ‘a joint and coordinated effort by all States to counteract the effects of these breaches’.Footnote ⁹⁸ Neither the ARSIWA nor the 2019 Draft Conclusions suggest what measures States should take to fulfil that obligation.

Both the ICJ and the ILC, in their respective analyses of the obligation to bring an end to breaches of certain norms of international law—self-determination, in the case of the ICJ, and peremptory norms, in the case of the ILC—have emphasised the role of the UN. In its Wall Advisory Opinion, the ICJ, having found that States were under an obligation to ‘see to it’ that any impediment to the Palestinian right to self-determination was brought to an end, said that ‘the United Nations, and especially the General Assembly and the Security Council, should consider what further action is required to bring to an end the illegal situation’.Footnote ⁹⁹ Similarly in its Chagos Advisory Opinion, as noted above, the Court referred to the obligation of States to ‘cooperate with the United Nations’ to ensure the decolonisation of Mauritius.Footnote ¹⁰⁰

The ILC's commentary to Article 41(1) of the ARSIWA notes that cooperation ‘could be organised in the framework of a competent international organisation, including the United Nations’.Footnote ¹⁰¹ The commentary to the Draft Conclusions notes similarly that ‘the collective system of the United Nations is the preferred framework for cooperative action’.Footnote ¹⁰² It notes further that when faced with a serious breach of a peremptory norm, ‘where an international organisation has discretion to act, the obligation to cooperate imposes a duty on the members of that international organisation to act with a view to the organisation exercising that discretion in a manner to bring to an end the breach of a peremptory norm of international law’.Footnote ¹⁰³

In the absence of more precise guidance from either the ILC or the ICJ regarding what is required by the obligation to cooperate, reference may be made to the comments of the ICJ in the Bosnian Genocide Case regarding the nature of the obligation to prevent genocide, and to the well-established notion in international law of ‘due diligence’, upon which the Court in that case expressly relied. In relation to the task of ‘determining the specific scope of the duty to prevent in the Genocide Convention’, the Court said that ‘in this area the notion of “due diligence”, which calls for an assessment in concreto, is of critical importance’.Footnote ¹⁰⁴

Across various areas of international law—international environmental law, international investment law, the law of the sea and international human rights law, among others—the due diligence standard has been used to provide substance to State obligations that are ‘somewhat indeterminate or unclear’.Footnote ¹⁰⁵ In the Bosnian Genocide Case, as Larissa van den Herik and Emma Irving observe, the due diligence standard was utilised to ‘substantiate a provision [the obligation to prevent genocide] that had previously been labelled as normatively empty’.Footnote ¹⁰⁶ In a similar vein, the ‘notion of due diligence’ can substantiate the obligation to cooperate to end serious breaches of peremptory norms.

In 2016, the Study Group on Due Diligence of the International Law Association (ILA) described the due diligence standard as:

Due diligence standards differ depending on the primary obligation to which they attach. The Seabed Disputes Chamber of the International Tribunal of the Law of the Sea (ITLOS), in its Seabed Mining Advisory Opinion, observed that ‘the content of “due diligence” obligations may not easily be described in precise terms’;Footnote ¹⁰⁸ and the ILA Study Group observed similarly that ‘there is not one single standard of due diligence that applies to all primary norms’.Footnote ¹⁰⁹ For purposes of understanding the obligation to cooperate to end serious breaches of peremptory norms, specifically crimes against humanity and the basic rules of international humanitarian law, the ICJ's application of the due diligence standard in the Bosnian Genocide Case is particularly pertinent.

In that case the Court invoked the notion of due diligence to support its interpretation of the obligation as one of ‘conduct and not result, in the sense that a State cannot be under an obligation to succeed, … in preventing the commission of genocide: the obligation is rather to employ all means reasonably available to them, so as to prevent genocide so far as possible’.Footnote ¹¹⁰ This correlates with the ‘reasonableness’ test, common to due diligence standards across the various areas of international law in which the standard has been employed. The ILA Study Group described ‘reasonableness’ as the ‘overarching standard’, and as a ‘golden thread in determining which measures States should take in a duly diligent manner’.Footnote ¹¹¹ In the Bosnian Genocide Case, the ICJ applied a version of this standard when it said that if a State ‘has available to it means likely to have a deterrent effect on those suspected of preparing genocide, or reasonably suspected of harbouring specific intent …, it is under a duty to make such use of these means as the circumstances permit’.Footnote ¹¹² Drawing on this analysis, one might suppose that as applied to the obligation to cooperate to end serious breaches of peremptory norms, duly diligent States would be expected to use all available means, so far as circumstances permit, that are likely to deter the ongoing commission of those breaches.

Due diligence standards have typically required a connection between the State whose responsibility is in question and the perpetrator of the wrongful conduct. The ILC Study Group described this requirement as being about a State's control over territory where the wrongs are occurring, or influence over non-State actors engaging in the wrongful conduct.Footnote ¹¹³ In the Bosnian Genocide Case the ICJ accepted that a State's due diligence obligation to prevent genocide was not territorially limited, but it did nevertheless premise the obligation on a State's ‘capacity to influence’ the genocidal actors.Footnote ¹¹⁴ As noted above, however, due diligence standards differ according to the legal obligation to which they attach. For purposes of the obligation to cooperate to end serious breaches of peremptory norms, neither the commentary to the ARSIWA nor the commentary to the Draft Conclusions premise the obligation on a relationship between third States and the perpetrator(s). Indeed, in the commentary to the Draft Conclusions, the ILC's reliance on the ICJ's Wall and Chagos Advisory Opinions—and the reference in both of those Opinions to the duty of ‘all States’—suggests that the obligation to cooperate is not contingent upon a relationship between third States and the perpetrator(s).Footnote ¹¹⁵

Drawing on the analysis of the ICJ in the Bosnian Genocide Case, together with other jurisprudence and academic commentary on due diligence, one can suppose that States with particular relationships with the perpetrator(s) of serious breaches of peremptory norms might have heightened obligations; but there will also be steps that all States should take, by way of due diligence, irrespective of any relationship with the perpetrator(s). Bearing in mind the comments of both the ICJ and the ILC preferencing cooperation through the UN, an example of such a step would be support for an appropriate UN resolution, aimed at bringing a serious breach of a peremptory norm to an end.

In recent years, some scholars have asserted that the obligation to cooperate to end serious breaches of peremptory norms requires members of the Security Council to not block resolutions which ‘attempt to end or alleviate the violation of a peremptory norm’.Footnote ¹¹⁶ Such arguments run up against the fact that a basic characteristic of due diligence obligations is that they do not prescribe the specific conduct that States must take. As van den Herik and Irving observe in relation to the obligation to prevent genocide and crimes against humanity, the obligation ‘does not contain a clear-cut positive obligation to resort to a specific type of action’; rather, ‘at the core of the obligation … lies an obligation to make informed decisions about the most appropriate course of conduct, coupled with an obligation to explain those decisions and subsequent conduct’.Footnote ¹¹⁷ On the other hand, however, it is clear that States must use all available means. In relation to the obligation to cooperate, and recalling again the ILC's and the ICJ's preference for cooperation through the UN, it is difficult to see how a member of the Security Council could block a resolution aimed at ending a serious breach of a peremptory norm—imposing an arms embargo or targeted sanctions, for example—without falling afoul of its obligation to cooperate. At the very least, the non-cooperating State would be required to provide adequate justification for its preferred course of action.Footnote ¹¹⁸

While it is undoubtedly members of the Security Council that have the greatest capacity to address serious breaches of peremptory norms, the inevitability of competing national interests means that collective action through the Council is not always possible. The unique powers and responsibility of the Security Council do not obviate the obligations of other States to cooperate to end serious breaches of peremptory norms, using all means available to them, including their membership of other international organisations. If the Council will not take appropriate measures, the General Assembly provides an appropriate forum through which States can fulfil such obligation. And as for members of the Security Council, duly diligent members of the General Assembly should normally be expected to support resolutions aimed at ending serious breaches of peremptory norms, unless they can provide good reason for not doing so. Such an analysis is supported by the statement of the ILC in its commentary to the Draft Conclusions, that when faced with a serious breach of a peremptory norm, member States of an international organisation are under a duty to ‘act with a view to the organisation exercising [its] discretion in a manner’ to bring an end to the breach.Footnote ¹¹⁹

In the academic commentary on due diligence, there are three additional insights that shed light on the nature of the obligation to cooperate to end serious breaches of peremptory norms. The first relates to the importance of solidarity. As Nina Jørgensen has observed, some measures taken in response to an infringement of fundamental interests of the international community ‘may have minimal effect if … evaded or ignored by other States’. Similarly, efforts by States to pass a General Assembly resolution aimed at ending serious breaches of peremptory norms will be ineffective without the support of a two-thirds majority. Thus, ‘it would seem that where the effectiveness of measures rests on universality’—or, in this case, a majority—the obligation to cooperate means that ‘States may not claim neutrality’.Footnote ¹²⁰ On this point it is pertinent to recall also the comments of the ICJ in the Bosnian Genocide Case that it is ‘irrelevant whether the State whose responsibility is in issue claims, or even proves, that even if it had employed all means reasonably at its disposal, they would not have sufficed to prevent the commission of genocide’, because ‘the possibility remains that the combined effort of several States, each complying with its obligation to prevent, might have achieved the result … which the efforts of only one State were insufficient to produce’.Footnote ¹²¹ It is, indeed, the power of the ‘combined efforts of all States’ that is the essence of the obligation to cooperate.

The second insight from the commentary on due diligence relates to the requirement of knowledge, and the relevance of digital technologies in this regard. The ILA Study Group said that ‘States can usually only be expected to act in accordance with a due diligence obligation to prevent harm if the State has knowledge of the situation which requires action.’Footnote ¹²² Similarly in the Bosnian Genocide Case, the Court said that a State's duty to prevent genocide arises ‘at the instant that the State learns of, or should normally have learned of, the existence of a serious risk that genocide will be committed’.Footnote ¹²³ In both jurisprudence and academic commentary, it has been recognised that the question of what a State ought to have known must be assessed in the light of available technology—and as such, that standards may be expected to change over time. In 2011, the ITLOS Seabed Disputes Chamber recognised that ‘measures considered sufficiently diligent at a certain moment may become not diligent enough in light, for instance, of new scientific or technological knowledge’.Footnote ¹²⁴ In the context of crimes against humanity or violations of the basic rules of international humanitarian law, as digital technologies become more advanced and readily accessible, and social media brings previously out-of-reach situations into the public domain, it will likely become increasingly implausible for States to claim that they did not know about the wrongful conduct in question.Footnote ¹²⁵

The third insight from the commentary on due diligence that is pertinent to the obligation to cooperate is that due diligence standards may be expected to vary according to the ‘importance of the interest requiring protection’.Footnote ¹²⁶ The ILA Study Group on Due Diligence explained in its 2016 report that ‘what is considered to be reasonable in exercising due diligence to prevent genocide (a violation of a norm of jus cogens) will obviously be more demanding than that which is expected for the prevention of harm to property or financial interests’.Footnote ¹²⁷ Thus, one would expect that the level of due diligence required in relation to the obligation to cooperate to end serious breaches of peremptory norms should be of the highest order.

To conclude on the nature of the obligation to cooperate: while a member State could not be held responsible for a failure on the part of either the Security Council or the General Assembly to take action aimed at ending a serious breach of a peremptory norm, a State could feasibly be held responsible for failing to vote in favour of a resolution aimed at putting an end to the breach—on the basis that such a resolution is one of the means available to States that might have a deterrent effect on the perpetrator(s). Extrapolating from the approach taken by the ICJ in the Bosnian Genocide Case, and drawing on the well-established due diligence standard, it should be irrelevant for a State to argue that even if it had tried to get an appropriate resolution through the Council or the Assembly, the resolution would never have passed, or that even if it had passed, it would not have brought the serious breach in question to an end; because the ‘possibility remains’ that if a sufficient number of States had banded together in support of an appropriate UN resolution, that combined action may have been sufficient to deter the perpetrators of the serious breach of the peremptory norm.

A. Condemning and Legally Characterising the Crimes in Question

As a first step in responding to credible reports of crimes against humanity and/or violations of international humanitarian law, the General Assembly may critique the conduct in question, and explicitly describe it as likely constituting a crime against humanity and/or a violation of the fundamental rules of international humanitarian law, or a war crime. Such a statement would not in itself have any binding legal effect, but it would attest to the existence of international consensus—or majority view—regarding the characterisation of the conduct in question, and that characterisation could have legal consequences.

The ICJ has accepted that the General Assembly may pass ‘resolutions which make determinations or have operative design’. The Court has described such resolutions as ‘not a finding of facts, but the formulation of a legal situation’.Footnote ¹³³ One of the ways in which the Assembly has acted upon this competence is by characterising State conduct as being in breach of particular rules of international law, including the fundamental rules of international humanitarian law, and the norms prohibiting crimes against humanity and war crimes. For example, in the context of decolonisation in the 1960s, the Assembly ‘condemn[ed] the policies of racial discrimination and segregation [in Southern Rhodesia]…, which constitute a crime against humanity’,Footnote ¹³⁴ and similarly condemned the policies of the Portuguese Government in relation to the Portuguese Territories ‘as a crime against humanity’.Footnote ¹³⁵ In relation to apartheid South Africa in the 1980s, the Assembly ‘reaffirm[ed] that apartheid is a crime against humanity’,Footnote ¹³⁶ and in relation to Israel, also in the 1980s, the Assembly ‘declar[ed] that Israel's grave breaches of [the Fourth Geneva Convention] are war crimes’.Footnote ¹³⁷ In relation to Bosnia and Herzegovina, the Assembly recognised that the practice of rape in the context of armed conflict ‘constitutes a war crime’, and in certain circumstances, ‘a crime against humanity and an act of genocide’.Footnote ¹³⁸ Following the Rwandan genocide, the Assembly expressed its concern regarding reports that ‘genocide and systematic, widespread and flagrant violations of international humanitarian law and crimes against humanity have been committed’.Footnote ¹³⁹ Similarly following the alleged genocide against the Rohingya in Myanmar in 2017, the Assembly expressed concern at the findings of the IIFFM that there was ‘sufficient information to warrant investigation and prosecution … for genocide …, [and] that crimes against humanity and war crimes have been committed’.Footnote ¹⁴⁰

General Assembly resolutions of this nature may be relied upon by States or international organisations as a basis for their own actions, or may be accorded evidentiary value by international courts or tribunals. To provide just one example: as noted above, following the alleged genocide against the Rohingya in Myanmar in 2017, the Assembly passed Resolution 73/264 (2018), reiterating a number of the findings of the IIFFM that were suggestive of genocide, and expressing concern at the IIFFM's finding that there was sufficient information to warrant investigation and prosecution for genocide.Footnote ¹⁴¹ That resolution enhanced the feasibility of a State party to the Genocide Convention instituting proceedings before the ICJ, alleging a breach of the Genocide Convention—as indeed Gambia did in 2019.Footnote ¹⁴² When the ICJ responded to Gambia's application by ordering provisional measures to prevent genocide, it relied heavily on the Assembly's resolutions.Footnote ¹⁴³

In relation to the Syrian conflict, the General Assembly has already condemned the ‘systematic, widespread and gross violations of international human rights law and violations of international humanitarian law’, including inter alia the ‘indiscriminate and disproportionate attacks against the civilian population and against civilian infrastructure’, the ‘deliberate targeting [of] civilians or civilian objects’ and the ‘starvation of civilians as a method of warfare and the use of chemical weapons’. The Assembly has recognised, moreover, that some of the violations of international humanitarian and human rights law committed in Syria constitute war crimes or crimes against humanity.Footnote ¹⁴⁴ In relation to Myanmar, as noted above, the Assembly has expressed concern at the findings of the IIFFM that crimes against humanity and war crimes have been committed, and that ‘the military has consistently failed to respect international human rights law and international humanitarian law’.Footnote ¹⁴⁵ In response to the violence that has been ongoing since the 2021 military coup in Myanmar, as a first step towards fulfilling their obligation to cooperate to end what appear to be serious breaches of peremptory norms, States could propose an Assembly resolution expressing concern regarding the reports of the Special Rapporteur on human rights in Myanmar that the attacks on protesters amount to crimes against humanity. Moreover, in relation to both Syria and Myanmar, the Assembly could go one step further and characterise the crimes in question as serious breaches of peremptory norms, thus implicitly evoking—albeit without any binding legal effect—the responsibilities of States to cooperate to bring those serious breaches to an end.

B. Making Recommendations to and Requesting Reports from the Security Council

A second way in which the General Assembly may be used by States to fulfil their obligation to cooperate to end serious breaches of peremptory norms is by making recommendations to or requesting reports from the Security Council.

The UN Charter empowers the General Assembly to make recommendations to the Security Council on any matter within the scope of the Charter, including—explicitly—recommendations relating to international peace and security.Footnote ¹⁴⁶ The Assembly has made recommendations to the Security Council on several occasions in the past. Its recommendations have ranged from general requests that the Council adopt appropriate measures or ‘concrete recommendations for action’,Footnote ¹⁴⁷ to more specific recommendations regarding exactly what measures the Council should impose. Among other things, the Assembly has recommended that the Council: impose mandatory oil and arms embargoes;Footnote ¹⁴⁸ impose ‘comprehensive and mandatory sanctions’;Footnote ¹⁴⁹ take steps relating to emergency airdrops of humanitarian aid and safe areas for civilians;Footnote ¹⁵⁰ establish an ad hoc international criminal tribunal;Footnote ¹⁵¹ and refer situations to the ICC.Footnote ¹⁵² The Council is not obliged to act on the Assembly's recommendations, but history suggests that in some circumstances, Assembly recommendations may contribute to a build-up of momentum that can ultimately pressure the Council to act.Footnote ¹⁵³ As such, in situations in which States are aware of a serious breach of a peremptory norm, and the Council is not adequately responding, utilising the Assembly to make recommendations to the Council is an important step that States can take in fulfilment of their obligation to cooperate to bring that breach to an end. In relation to both Syria and Myanmar, for example, the Assembly could recommend that the Council impose a mandatory arms embargo, impose mandatory targeted sanctions against those credibly suspected of having committed crimes against humanity and/or war crimes, and refer suspected perpetrators to the ICC.

The UN Charter also empowers the General Assembly to ‘receive and consider … special reports from the Security Council’, with such reports to include ‘an account of the measures that the Security Council has decided upon or taken to maintain international peace and security’.Footnote ¹⁵⁴ The Charter does not state explicitly that the Assembly may request such reports; it is presumed, however, that the Assembly's competence to ‘receive and consider’ special reports also entails a competence to request them.Footnote ¹⁵⁵

In situations in which States have knowledge of a serious breach of a peremptory norm of international law, and the Security Council is not responding effectively, a request by the Assembly for a report from the Council regarding its handling of the matter could raise the public profile of a crisis and, in turn, increase pressure on the Council to act. In relation to the serious breaches of peremptory norms in Syria and Myanmar, such requests could, for example, explicitly reference—respectively—the reports of the Commission of Inquiry on Syria, and the statement by the Special Rapporteur on human rights in Myanmar regarding the likely occurrence of crimes against humanity, and could request an account of measures that the Council will be taking in response to those reports, in fulfilment of its responsibility for the maintenance of international peace and security.

C. Recommending Sanctions

A third—and more substantive—way in which States may use the General Assembly as a means through which to fulfil their obligations in response to serious breaches of peremptory norms is to recommend to States that they impose sanctions targeting the suspected perpetrators. As noted above, pursuant to its recommendatory powers in the UN Charter, the Assembly is competent to make recommendations on any matter within the scope of the Charter, and specifically relating to international peace and security and human rights. The Assembly's recommendatory powers on matters of international peace and security are limited by Article 11(2) of the UN Charter, which provides that the Assembly shall refer any question ‘on which action is necessary’ to the Security Council; however, the ICJ has affirmed that this restriction applies only to ‘coercive or enforcement action’.Footnote ¹⁵⁶ It is broadly accepted that the Assembly is competent to recommend measures not involving the use of military force.Footnote ¹⁵⁷

The General Assembly has previously recommended to States that they impose a range of diplomatic, economic and other sanctions. The most robust of the Assembly's sanctions recommendations were made in the 1960s–80s, in response to South African aggression and apartheid, and in support of the independence struggles in the Portuguese Territories and Southern Rhodesia. The Assembly's recommendations to States in these contexts included that they adopt ‘comprehensive and mandatory sanctions’, prevent the sale and supply of arms, sever diplomatic relations, boycott trade, and more generally adopt legislative, administrative and other measures in order to isolate the concerned regimes ‘politically, economically, militarily and culturally’.Footnote ¹⁵⁸ Other contexts in which the Assembly has recommended sanctions and/or arms embargoes include the Korean War,Footnote ¹⁵⁹ the Congolese civil war,Footnote ¹⁶⁰ Israeli aggression against Iraq,Footnote ¹⁶¹ and Israel's occupation of and aggression against the Palestinian territories.Footnote ¹⁶² Most of the Assembly's sanctions recommendations have been made without the Security Council having imposed mandatory sanctions. Thus, the Assembly's recommendations have been for States to act autonomously—insofar as that term is used to mean without Council authorisationFootnote ¹⁶³—in imposing the recommended measures.

The effectiveness of sanctions as a tool to combat human rights violations is contested. In general, research shows that sanctions can in some circumstances influence State behaviour, but that success is limited. One oft-cited study of 174 sanctions regimes from the First World War through to 2000 found sanctions to be ‘at least partially successful’ in 34 per cent of cases.Footnote ¹⁶⁴ A more recent analysis focusing only on multilateral sanctions found that sanctions were more effective at ‘constraining’ a target (depriving a target of access to resources needed to engage in a proscribed activity) or ‘signalling’ (publicly asserting the target's deviation from an international norm) than they were at coercing a change of behaviour. That study found that 27 per cent of the sanctions episodes examined were effective at ‘constraining’, 27 per cent were effective at ‘signalling’, and just 10 per cent were effective at coercing behavioural change.Footnote ¹⁶⁵

The fact that sanctions cannot in themselves be counted upon to end serious breaches of peremptory norms, however, does not mean that sanctions should not be utilised as part of a package of measures aimed at bringing such breaches to an end. Targeted sanctions have been consistently recommended by human rights bodies as a response both to the post-coup violence in Myanmar, and war crimes and crimes against humanity in Syria.Footnote ¹⁶⁶ In relation to Myanmar, the General Assembly's resolution in June 2021 recommending that States ‘prevent the flow of arms into Myanmar’ was a significant step forward, and an indication of what the Assembly can do, when the political will is there. The Assembly could go further and recommend that States discontinue any cooperation with the Myanmar military, discontinue any financial assistance to government bodies seen as legitimising the military junta, prohibit transactions with enterprises owned or controlled by the military, and impose asset freezes and travel bans against individuals allegedly responsible for crimes against humanity. In relation to Syria, in fulfilment of the obligation to cooperate to end serious breaches of peremptory norms, States could similarly work towards an Assembly resolution recommending to States that they impose targeted sanctions such as asset freezes and travel bans against individuals allegedly responsible for crimes against humanity and war crimes.

D. Denying Legitimacy to the Responsible Regime

A fourth way in which States may utilise the General Assembly as a forum through which to fulfil their obligations in relation to serious breaches of peremptory norms is to deny legitimacy to an allegedly responsible regime. Such a course will likely only be considered constructive in very particular circumstances, namely, when the regime in question came to power by force, and there is an alternate authority with a legitimate claim to be the government—a scenario arguably playing out now in Myanmar.

There are two ways in which the General Assembly may deny legitimacy to a regime: it may reject the credentials of the delegates put forward to represent their States at the Assembly itself; and/or it may pass a resolution describing the regime in question as illegitimate, and calling upon States not to accord that regime diplomatic recognition.

On the first point, the General Assembly's procedural rules state that member States should submit their representatives’ credentials to the Secretary-General ahead of General Assembly sessions, and that such credentials should ‘be issued either by the Head of State or Government or by the Minister for Foreign Affairs’.Footnote ¹⁶⁷ The Assembly's Credentials Committee verifies that the administrative requirements are satisfied, and on that basis makes a recommendation to the Assembly. In what is typically a procedural exercise, the Assembly then approves or rejects the recommended credentials. In the rare event that competing authorities claim at once to represent a member State, the Assembly has previously determined that its decisions will be made ‘in light of the Purposes and Principles of the Charter and the circumstances of each case’.Footnote ¹⁶⁸ In the case of Myanmar in 2021, competing credentials claims were submitted by representatives of both the military junta, and the civilian government in exile. As this article goes to press, the Credentials Committee has just recommended to the Assembly that it defer its decision on Myanmar's representation.Footnote ¹⁶⁹ While this course of action will rightly deny the junta the privilege of representing Myanmar at the Assembly, in the context of the alleged crimes against humanity perpetrated by Myanmar's military junta following the 2021 coup, it should be open to the Assembly to not only defer its decision on Myanmar's representation, but to go further and outrightly reject the credentials of the junta's representatives.Footnote ¹⁷⁰

Some scholars assert that even in the absence of competing claims by rival authorities both claiming to represent their State, the General Assembly may still make a substantive enquiry into the ‘representativeness’ of the proposed delegate(s). In a 2008 opinion on the feasibility of challenging the credentials of Myanmar's then-ruling military junta, for example, nine eminent legal scholars opined that ‘where a situation arises from internal or external repression … the Credentials Committee may consider … factors such as the legitimacy of the entity issuing the credentials, the means by which it achieved and retains power, and its human rights record’.Footnote ¹⁷¹ The opinion concluded that, even in the absence of competing claims, it was open to the Credentials Committee to recommend to the Assembly that the credentials of the junta's representatives be rejected, on the basis of—among other things—the junta's violation of the ‘peremptory norms of international human rights law’.Footnote ¹⁷²

Relatedly, in the case of Myanmar, the Assembly could pass a resolution describing the military junta as illegitimate, and calling upon States not to accord it diplomatic recognition. Such a resolution would not be unprecedented. In the 1960s, the Assembly passed a series of resolutions on what was then Southern Rhodesia, calling upon States to end relations with the ‘illegal racist minority regime’.Footnote ¹⁷³ Similarly in relation to apartheid South Africa in the 1970s, the Assembly passed a resolution proclaiming that South Africa's ‘racist regime’ was illegitimate and had no right to represent the South African people.Footnote ¹⁷⁴

Many States have a policy of not formally recognising new governments when they come to power; rather, they recognise ‘States not governments’, and imply their acceptance (or not) of new governments through governmental acts such as public statements, the conduct of diplomatic relations and ministerial contact.Footnote ¹⁷⁵ At the time of writing, many States have refrained from conveying their acceptance of Myanmar's military junta. Conversely, however, the junta's representatives were accorded the right to represent Myanmar at the Human Rights Council in March 2021 (twice), and at the meeting of the UN Economic and Social Commission for Asia and the Pacific in April.Footnote ¹⁷⁶ A General Assembly resolution describing the junta as illegitimate could reduce the likelihood of States as well as international and regional organisations engaging in further conduct that boosts the junta's legitimacy. As with the other measures described above, such a resolution could not be expected to immediately prompt the junta to curb its violent suppression of protests; indeed, the military has said explicitly that it has ‘to learn to walk with only few friends’.Footnote ¹⁷⁷ But if those few remaining friends were to come together, pursuant to an Assembly recommendation, and take steps to isolate Myanmar's military junta economically, politically and diplomatically, there is a possibility that such isolation, combined with other pressures, could suffice to influence the military to end its serious breaches of peremptory norms of international law.