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Max Planck Encyclopedia of Public International Law [MPEPIL]

Ius cogens

Dire Tladi

From: Oxford Public International Law (http://opil.ouplaw.com). (c) Oxford University Press, 2023. All Rights Reserved.date: 22 July 2024

Subject(s):
Sources of international law — Erga omnes obligations — Peremptory norms / ius cogens

Published under the auspices of the Max Planck Institute for Comparative Public Law and International Law under the direction of Professor Anne Peters (2021–) and Professor Rüdiger Wolfrum (2004–2020). 

Editor’s Note: A different entry under the title ‘Ius cogens’, written in January 2009 by Jochen Abr. Frowein, appeared as part of the online Max Planck Encyclopedia of Public International Law until the publication of this entry in 2024. This new entry has been published at the same web address (URL) as the original entry. The original entry was also published in the 2012 print edition of the Encyclopedia (R Wolfrum (ed) The Max Planck Encyclopedia of Public International Law (OUP Oxford 2012) vol VI, 443–46).

A.  Introduction

Due to its potentially far-reaching effects, ius cogens has become one of the most important concepts in international law. Yet, at the same time, it remains a concept that is shrouded in mystery. Its precise content, including which rules qualify as ius cogens, as well as consequences attaching to it, have remained contested for decades.

Ius cogens is, in many ways, an enigma and for that reason is often described in magical or miraculous terms. It purports to put in place a hierarchical system which challenges the horizontality and thus the basic structure of international law in several mutually reinforcing ways. First, the implication that the making of international law is not necessarily tied to State consent and the will of States goes against a foundational conception of international law. Second, and flowing from the first point, the idea that there are some rules from which States cannot validly contract out fits uncomfortably in the horizontal structure of international law. Third, and perhaps most striking, the idea that an obligation, otherwise validly established, could be deemed void or ineffective on account of ius cogens appears inimical to international law.

Yet for all its potential destabilizing effect on international law, ius cogens has remained largely unimpactful on the legal system, with the result that it has been described as a car stuck in its garage. This is not to say that it has not been invoked or mentioned because it most certainly has. But, for all its mentions and invocation, it has hardly been directly applied. There are many explanations that can be put forward for this. One explanation may be the fear that applying ius cogens may open up Pandora’s box, leading to the unsettling of international law. Or it may be that there is just too much uncertainty concerning its contours, in terms of both identification and consequences. Another possible explanation may be that, in a legal system with so many other tools, it has not been necessary to resort to what may be regarded as its most powerful tool. Perhaps, because ius cogens is an exceptional concept, it should be expected that its application will be exceptionally rare.

The International Law Commission (ILC) (the ‘Commission’) has, over the years, been instrumental in the development of ius cogens. In its recent Draft Conclusions on Identification and Legal Consequences of Peremptory Norms of General International Law (jus cogens) ([2022]; ‘ILC Draft Conclusions’), the Commission shed further light on the salient aspects of the concept, and, perhaps, helped give it one more nudge out of the garage.

B.  Nature and Historical Development

1.  The Legal Nature of ius cogens

Rules of international law are generally ius dispositivum. This means that States are generally free to contract out of the rules and to establish particular rules as between themselves without affecting the general legal regime. The concept of ius cogens presupposes the existence of a certain class of norms that, because of their character, are not susceptible to derogation and may thus not be contracted out, not even by agreement by States.

There are two main competing theories that explain the nature of ius cogens. At one end of the spectrum is the positivist philosophy (Legal Positivism), which posits that ius cogens in international law is merely an expression of the consensual theory of law and, as such, that the rules of ius cogens are arrived at in ways similar to other rules of international law, namely through the agreement and conduct of States. At the other end of the spectrum, ius cogens is seen as an expression of natural law (Natural Law and Justice), whose norms exist independent of the will of States and are based on particular immutable rules.

Writings on ius cogens in international law, particularly early on, were based overwhelmingly on natural law philosophy, the basic idea of natural law being that beyond positive law—law made through the agreement and practice of States—there is a higher set of norms against which positive law is to be judged. While there are various strands of natural law, at its core natural law espouses morality as its source and therefore the source of ius cogens. This understanding of ius cogens was the dominant approach until the adoption of the Vienna Convention on the Law of Treaties (1969) (‘VCLT’), which opened up the possibility for a more positivist approach to ius cogens. Article 53 VCLT provides that a ius cogens norm is a norm accepted and recognized by the international community of States as a whole as one from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character. The reference to ‘acceptance and recognition’ by the ‘international community of States as a whole’ suggests a more positivist leaning (emphasis added). There are of course other theories to explain the nature of ius cogens, but for the most part these other theories are variations of the positivist/natural law debate.

There is support for both approaches in the jurisprudence of international courts and tribunals, including individual opinions of judges. The Inter-American Court of Human Rights (IACtHR) seemed to adopt a natural law approach when, for example, it juxtaposed ‘the voluntarist conception of international law’ with ‘the ideal of construction of an international community with greater cohesion … in the light of Law and in search of Justice’, with the latter reflecting a move ‘from jus dispositivum to jus cogens’ (Constantine et al v Trinidad and Tobago [2001] para. 38). Yet that same court has, in other decisions, seemingly based its determination of the ius cogens status of a rule, in part, on ‘international agreement’ (Osorio Rivera and Family Members v Peru [2013] para. 112).

The first report of the Special Rapporteur on ius cogens (UN ILC Special Rapporteur D Tladi ‘First Report on jus cogens’ [2016]) suggested that the positivist and natural law approaches to ius cogens can co-exist. There, it is suggested that the binding and peremptory force of ius cogens is best understood as an interaction between natural law (non-consensualism) and positivism (consensualism), with natural law providing the underlying reason for the peremptory character of the norm, while positivism provides that manifestation of the binding force in an objective fashion (para. 59).

2.  Historical Evolution

10  The first appearance of the term ius cogens is in the Roman Emperor Justinian’s Digest. However, there it was used in a different context (see D.39.5.29pr [Digest of Justinian]). Nonetheless, Roman Law did recognize something akin to ius cogens. In several passages in the Digest, there appears the observation that ‘Ius publicum privatorum pactis mutari non potest’, ‘private pacts cannot derogate from public law’, with ius publicum denoting all those rules from which individuals may not depart by separate agreements (D.2.14.38; see also D.11.7.20). The Codex also states ‘Pacta, quae contra leges constitutionesque vel contra bonos mores fiunt, nullam vim habere indubitati iuris est’, which means ‘agreements contrary to laws or constitutions, or contrary to good morals, have no force’ (Codex 2.3.6).

11  In the context of international law, the idea that there are some rules of international law that apply independent of the will of States can be traced back to writers such as Hugo Grotius, Emer de Vattel, and Christian Wolff (Natural Law and Justice). Grotius’s famous De Jure Belli ac Pacis contains many references to immutable law (H Grotius De Jure Belli ac Pacis Libri Tres [orig 1646] FW Kelsey (tr) [Clarendon Press Oxford 1925]). As described above, much of this early work is based on natural law as the source of the immutable, which cannot be changed by States through agreement or practice. Yet, in the 19th century, with the rise of the positivist law approach to international law, sovereignty and the will of States became the dominant theory to understanding international law and its binding force. However, in the era of the dominance of positivism, international lawyers did not generally subscribe to complete freedom of the will of States in international law. Several international lawyers of the time wrote about the limits to the will of States but, by and large, sought to explain this limit by positive law, including existing treaties. The prohibition of slavery and the slave trade provided the most common example of rules restricting the freedom of States in international lawmaking.

12  The period after World War I saw a resurgence of the idea of non-derogable rules, including, for example, Art. 20 Covenant of the League of Nations (1919) (League of Nations), which purported to abrogate previously concluded agreements inconsistent with the Covenant and prohibited the conclusion of agreements inconsistent with it (History of International Law, World War I to World War II). While Art. 20, being a treaty rule applicable to parties to the treaty, was by no means ius cogens, it had an influence on the development of the concept. In this period, ius cogens was also invoked in judicial and arbitral decisions. For example, in a separate opinion in the Oscar Chinn Case (1934), Judge Schücking famously invoked ius cogens when determining that a treaty provision was invalid on account of inconsistency with the General Act of Berlin. Ius cogens was also explicitly invoked under the French-Mexican Claims Commission (Mexican Claims Commissions), in the Pablo Nájera (of the Lebanon) Case (1928), in which the Tribunal interpreted Art. 18 Covenant of the League of Nations as a rule having ‘le caractère d’une règle de droit à laquelle il n’est pas libre aux Etats, membres de la Société des Nations, de déroger par des stipulations particulières, entre eux (jus cogens)’ (at 470) (‘the character of a rule of law from which States, members of the League of Nations, are not free, as between themselves, to derogate from by particular rules’ [translation by the author]).

13  The most significant period for the development of the concept of ius cogens came in the period after World War II (History of International Law, since World War II). Not only did this period, in response to World War II violations, lead to the affirmation of norms that are generally accepted today as ius cogens, such as the prohibition on the use of force (Use of Force, Prohibition of), the prohibition of genocide and the prohibition of crimes against humanity, but it was also in this period that the ILC introduced into the mainstream of international law the concept of ius cogens.

14  The first introduction of the notion of the invalidity of treaties on account of inconsistency with the content of a rule of international law appeared in the Fourth Report on the Law of Treaties topic, namely in Sir Hersch Lauterpacht’s First Report (ILC Special Rapporteur H Lauterpacht ‘Report on the Law of Treaties’ [1953]). Lauterpacht proposed that a treaty would be void ‘if its performance involves an act which is illegal under international law’ (Art. 15). For Lauterpacht the test for illegality ‘is not inconsistency with customary international law pure and simple’, but rather ‘inconsistency with such overriding principles of international law which may be regarded as constituting principles of international public policy (ordre international public)’ (Comment to Art. 15 at 155). However, it was in the third report of Sir Gerald Fitzmaurice—the eighth report on the law of treaties overall—that the term ‘ius cogens’ first appeared (ILC Special Rapporteur G Fitzmaurice ‘Third Report on the Law of Treaties’ [1958]). Fitzmaurice proposed that for a treaty to be valid ‘it should be in conformity with or not contravene, or that its execution should not involve an infraction of those principles and rules of international law which are in the nature of jus cogens’ (Art. 16 at 26). In general, the text recognized that States may always, inter se, depart from ius dispositivum rules of international law by means of an inter se agreement, but that this right only applied if the general rule in question was not one in the nature of ius cogens. Following on from Fitzmaurice, Sir Humphrey Waldock, the last Special Rapporteur on the topic, proposed that a treaty ‘is contrary to international law and void if its object or its execution involves the infringement of a general rule or principle of international law having the character of jus cogens’ (ILC Special Rapporteur H Waldock ‘Second Report on the Law of Treaties’ [1963] Art. 13 (1) at 52). The final text adopted by the Commission on the basis of these proposals read as follows:

A treaty is void if it conflicts with a peremptory norm of general international law from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character (Art. 50 ILC ‘Draft Articles on the Law of Treaties’ [1966]).

15  While at the time there was very little support of the idea of ius cogens in practice—it was a concept with little pedigree outside of scholarship—the idea that a treaty is void if it is inconsistent with fundamental rules of international law was generally welcomed within and beyond the Commission. Even France, which is held up as an example of a long-standing opponent of ius cogens, did not actually oppose the concept but pointed to dangers in its application. France, for example, declared at the Vienna Conference that

[t]he substance of jus cogens was what represented the undeniable expression of the universal conscience, the common denominator of what men of all nationalities regarded as sacrosanct, namely, respect for and protection of the rights of the human person (UN Conference on the Law of Treaties ‘Official Records: Summary Records of the Plenary Meetings and of the Meetings of the Committee of the Whole [First Session]’ [1968] para. 32 at 309).

France was rather concerned not with the concept but with the possibility of unilateral invocation of ius cogens and the consequences that that might have for the stability of treaty relations.

16  At the Vienna Conference, after extensive deliberations, Art. 53 VCLT, which ushered in ius cogens from the periphery of international law scholarship to the centre of international law, was adopted. It provided:

A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. For the purposes of the present Convention, a peremptory norm of general international law is a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.

17  In addition to Art. 53, the VCLT also provides for the emergence of ius cogens norms subsequent to the adoption of a treaty (ius cogens superveniens), and provides that in such instances a treaty in conflict with the ius cogens norm will become invalid (Art. 64 VCLT). To account for the French concerns of unilateral invocation, the VCLT provides for compulsory jurisdiction of the International Court of Justice (ICJ) in the event of the invocation of ius cogens to invalidate a treaty provision (Art. 66 (a) VCLT; Compulsory Jurisdiction: Inter-State Litigation).

18  But the ILC’s work on the law of treaties, and the subsequent inclusion of Art. 53 and other related provisions, did not mark the end of the evolution of ius cogens. As already noted, in 2022, the Commission adopted the Conclusions on Peremptory Norms of General International Law. Between the Commission’s first pronouncement on ius cogens in 1966 and its most recent pronouncement in 2022, the ILC addressed issues pertaining to ius cogens in several of its works which further contributed to its development. This is the case, for example, with the 2001 Articles on Responsibility of States for Internationally Wrongful Acts (‘ARSIWA’) (State Responsibility), the Fragmentation Study (ILC ‘Fragmentation of International Law: Difficulties Arising from the Diversification and Expansion of International Law’ [2006]; Fragmentation of International Law), and the Guiding Principles Applicable to Unilateral Declarations of States Capable of Creating Legal Obligations (2006) (Unilateral Acts of States in International Law). Along the way, there were also important decisions of international courts and tribunals impacting on the development of ius cogens. Examples from the ICJ include the Barcelona Traction Case, where the Court established the notion of obligations erga omnes in international law, a notion that has been instrumental in the development of the notion of ius cogens (Obligations erga omnes).

C.  Identification of ius cogens

1.  General

19  As described above, the period after World War II saw significant developments in the evolution of the concept of ius cogens. Yet, despite these developments, ius cogens has remained stuck in the proverbial garage. One reason for this has been the indeterminacy of the concept—the inability to identify, with a degree of objectivity, which norms qualify as ius cogens and which do not. The result has been that which norms are ius cogens has been in the eye of the beholder and has been based on the subject and policy preferences of the beholder. The ILC Draft Conclusions of 2022 seek to offer objective criteria for the identification of ius cogens and, from this perspective, a common language for assessment of the ius cogens status of individual norms of international law.

20  The first question to answer in trying to establish the criteria for ius cogens is the basis for the criteria. For most commentators, the starting point for identifying the criteria for ius cogens must be the definition in Art. 53 VCLT. This is not affected by the fact that Art. 53 itself qualifies the definition as being ‘[f]or the purposes of [the Vienna] Convention’. The fact that Art. 53 ‘defines’ ius cogens in terms of consequences has also not affected the general view that it is a definition. It is clear that courts and tribunals, both domestic and international, take, as their starting point, Art. 53 as the definition of ius cogens in general terms and not only for the purposes of the law of treaties. The ICJ, in the advisory opinion on the Legality of the Threat or Use of Nuclear Weapons (1996) (Nuclear Weapons Advisory Opinions), referred to Art. 53 as defining ius cogens in the context of ‘principles and rules of humanitarian law’ (para. 83). Similarly, in Prosecutor v Furundžija (1998), the Trial Chamber of the International Criminal Tribunal for the former Yugoslavia (International Criminal Tribunal for the Former Yugoslavia [ICTY]), when describing the prohibition of torture as ius cogens, referred to Art. 53. More to the point, during the ILC’s consideration of the topic, States consistently accepted Art. 53 VCLT as the definition of ius cogens.

21  Accepting that Art. 53 reflects customary international law does not fully answer the question concerning the definition, and therefore the identification, of ius cogens. In particular, even if Art. 53 constitutes customary international law applicable also beyond treaty contexts, Art. 53 may be seen as circular and thus incapable of providing the elements for ius cogens. Part of the reason for the circularity of ius cogens is that Art. 53 attempts to mediate between the consensualist and non-consensualist approaches to ius cogens. Second, the elements in Art. 53 are capable of being understood in different ways, such that it is difficult to distil from those elements criteria to be satisfied in identifying ius cogens. In this context, when formulating what became Art. 53, the ILC in 1966 observed that ‘there [was] no simple criterion by which’ to identify norms of ius cogens (ILC ‘Draft Articles on the Law of Treaties’ Commentary to Art. 50 at 247).

22  Yet, on the basis of the definition of ius cogens in Art. 53, the Commission produced a set of criteria for the identification of ius cogens. The first of these is that the norm in question must be a norm of general international law. The second criterion, referred to in the commentary to Draft Conclusion 4 as a composite criterion, is that the norm in question must be accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can only be modified by a subsequent norm having the same character (ILC Draft Conclusions at 29).

2.  Norm of General International Law

23  The first requirement, that the norm must be one of general international law, distinguishes ius cogens norms from particular rules of international law applicable only between a group of States. Such norms, even if endowed with some level of non-derogability, do not constitute ius cogens. Thus, a non-derogable provision in a treaty between a group of States does not constitute a norm of ius cogens. While there is no generally accepted definition of general international law, in the context of ius cogens, norms of general international law refer to those norms of international law that ‘have equal force for all members of the international community’ (North Sea Continental Shelf Cases [Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands] [1969] at para. 63; North Sea Continental Shelf Cases; General International Law [Principles, Rules and, Standards]).

24  Customary international law is the source of international law (Sources of International Law) most associated with ius cogens. The view that the underlying basis of many norms of ius cogens is customary international law is reflected in scholarly writings and the jurisprudence of international courts and tribunals as well as State practice. Indeed, it is not uncommon to see ius cogens being described as ‘super custom’ or as a ‘higher form of custom’. A good example of the recognition of the strong relation between customary international law and ius cogens in the jurisprudence of the ICJ is Questions relating to the Obligation to Prosecute or Extradite (Belgium v Senegal) (2012), where the Court recognized the prohibition of torture as ‘part of customary international law’ that ‘has become a peremptory norm (jus cogens)’ (para. 99; Questions relating to the Obligation to Prosecute or Extradite [Belgium v Senegal]).

25  While the role of customary international law as a basis of ius cogens is generally unproblematic, the role of other sources remains contested. General principles of law within the meaning of Art. 38 (1) (c) Statute of the International Court of Justice, for example, ‘have equal force for all the members of the international community’ and thus should be regarded as norms of general international law (North Sea Continental Shelf Cases para. 63). There is, however, virtually no practice of ius cogens norms being based on a general principle of law. Treaty rules, on the other hand, are not understood generally as having a general scope of application and therefore cannot be seen as norms of general international law. Yet, it has been pointed out that one of the most recognized norms of ius cogens, the prohibition on the use of force, is, in fact, a treaty norm contained in Art. 2 (4) Charter of the United Nations (1945) (United Nations Charter; ‘UN Charter’). However, while it is the case that Art. 2 (4) reflects a ius cogens norm, it is also true that the rule in Art. 2 (4) is a codification of a rule of customary international law (Military and Paramilitary Activities in and against Nicaragua [Nicaragua v United States of America] [1986] para. 190; Military and Paramilitary Activities in and against Nicaragua Case [Nicaragua v United States of America]), and it is that customary international law rule that became ius cogens. Thus, while there may well be treaty rules that apply equally to all States, such treaty rules would also constitute customary international law.

26  The ILC Draft Conclusions capture the debate on the role of other sources in an interesting way. The second paragraph of Draft Conclusion 5 treats customary international law and treaty law in equal terms, providing that ‘treaty provisions and general principles of law may also serve as bases’ of ius cogens (emphasis added). This provision accepts the possibility of treaty provisions and general principles providing a basis for ius cogens but does so tentatively. Explaining the choice of words, the commentary states that ‘while there is little practice to support’ these two sources as bases of ius cogens, the possibility ‘cannot be a priori excluded’ (Commentary to Draft Conclusion 5 para. 7 at 33). Moreover, with respect to treaty provisions, the commentary recalls that ‘[t]he role of treaties as a basis for’ ius 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 North Sea Continental Shelf cases’, implying that it is the underlying customary international law rule that is the basis for ius cogens (ibid at 34).

3.  Acceptance and Recognition

27  The second criterion, that of acceptance and recognition, is the element that gives a norm its peremptory status and is thus the main criterion for peremptoriness. This criterion is sometimes referred to as opinio iuris cogentis. It is a composite criterion in the sense that, while it is a single criterion, it is composed of several elements. The essence of the criterion is ‘acceptance and recognition’. The other elements describe what must be accepted and recognized, namely that the norm in question may not be derogated from and, second, by whom the acceptance and recognition must be done, namely the international community of States as a whole. Put differently, it has to be shown that the international community of States as a whole accepts and recognizes that the norm in question cannot be derogated from.

28  Perhaps the most contentious element of this composite requirement is the question ‘who’ must accept and recognize the peremptory character of a norm. The ILC, mainly on the basis of the VCLT, took the view that it is the international community of States. This means, in the first place, that States are the source for opinio iuris cogentis. But, in the second place, the phrases ‘international community’ and ‘as a whole’ suggest a communitarianism that goes beyond quantifying the views of States. The assessment requires a very large majority of States to ‘accept and recognize’ the peremptory character of a norm but more than that, this majority should be representative of regions, legal systems, and cultures. The assessment of whether the international community accepts and recognizes the ius cogens character of a norm is context- and fact-specific and will depend on various factors. For example, concrete evidence of acceptance and recognition by a very large majority of States may well not meet the threshold if there are also a significant number of States that have persistently denied the peremptory character of a norm. At the same time, evidence of acceptance and recognition of the peremptory character of a norm by relatively fewer States may well be sufficient if that peremptory character has faced little or no resistance depending on repeated and persistent assertions of its peremptory character.

29  Although it is the attitude of States that matters for the purposes of identifying the peremptory character of norms, the attitudes of other actors are not irrelevant. The most obvious examples of entities whose views can contribute to the identification of the peremptory character of norms are international courts and tribunals, for example the ICJ, and expert bodies (United Nations Committees and Subsidiary Bodies, System of; and Human Rights, Treaty Bodies), for example the ILC. The value of the pronouncements of these bodies, however, is dependent in large part on how they are received by States, and for that reason they provide context and offer subsidiary evidence of ‘acceptance and recognition’. Other non-State actors, such as civil society entities, may also have a role in generating necessary support and creating circumstances conducive to the acceptance and recognition of a norm as ius cogens.

4.  Underlying Values

30  As described above, the modern understanding of ius cogens may be seen as reflecting an interaction between consensualist and non-consensualist approaches to international law. The set of criteria for ius cogens described in Section C.3 above arguably represents the more consensualist aspects. The non-consensualist aspect is represented by the values underlying ius cogens. The view that ius cogens reflects and protects fundamental values of the international community is ubiquitous in the practice of States, international jurisprudence, and scholarly writings, and this view was captured in Conclusion 2 ILC Draft Conclusions.

31  Without referring to ius cogens, the ICJ in the advisory opinion on Reservations to the Convention on the Prevention and Punishment of the Crime of Genocide (1951), described the underlying rationale of the prohibition of genocide, a crime which is undoubtedly a ius cogens norm, as being to address a denial ‘which is contrary to moral law and to the spirit and aims of the United Nations’ (at 23; Genocide Convention, Reservations [Advisory Opinion]). In later genocide-related cases, the Court referred to its advisory opinion and made a more explicit connection between the prohibition of genocide as a norm of ius cogens and fundamental values (eg Application of the Convention on the Prevention and Punishment of the Crime of Genocide Case [Bosnia and Herzegovina v Serbia and Montenegro] [2007] para. 16 and Application of the Convention on the Prevention and Punishment of the Crime of Genocide Case [Croatia v Serbia] [2015] para. 87).

32  The notion that ius cogens reflects fundamental values of the international community was the subject of some criticism from some members of the Commission as well as some States. The main criticism against the reference to fundamental values was that their precise relationship with the definition and criterion of ius cogens was unclear. There are at least three ways to see the role of values in the identification of ius cogens norms. The first possibility is that these values are merely descriptive and offer no contribution to the determination of whether this or that norm is ius cogens. The second possibility is that these values are an integral part of the assessment and thus that any assessment of the peremptory character of a norm must account for values. The third approach is to see these values as underlying ius cogens norms, and thus being an integral part of the norm, without necessarily being part of the identification criteria. This third approach means that one can, and in principle should, seek to identify the peremptory character of a norm without recourse to fundamental values, but that this does not mean that such values are irrelevant. In the context of identification, this third approach would mean that the extent to which the relevant values underlie, are reflected in, or protect a particular norm may thus provide confirmatory or even supplementary evidence of the ius cogens status of that norm.

33  The values in question are the fundamental values of the international community or community interests (Community Interest). These fundamental values are not static and will change and shift with time. Nonetheless, it is possible to identify examples of what would qualify as fundamental values at the current time. These would include the sanctity of life, freedom, dignity, and equality.

D.  Consequences of ius cogens

1.  General

34  The identification of ius cogens, described above, is so contested because, in part, its consequences are potentially so far-reaching. In fact, the consequences of ius cogens themselves are contested in a way that mirrors the consensualism/non-consensualism debate that has characterized the discourse on identification. Thus, on one side of the debate there are those that take the view that the consequences of ius cogens are, because of their underlying values and the higher objectives they seek to achieve, limitless. From this perspective, the consequences are said to include the inapplicability of immunity (State Immunity) in respect of ius cogens prohibitions (or reduction of the scope of such immunities), the establishment of ipso iure jurisdiction in relation to disputes, and even the suspension of the right of permanent members of the Security Council (United Nations, Security Council) to veto in relation to situations involving ius cogens. The narrower view would, for example, limit the consequences of ius cogens to the invalidation of conflicting treaty provisions in the mould of Arts 53 and 64 VCLT and any other consequences defined through the normal process of law-making, ie customary international law and treaty law. Between these two alternatives, there is a continuum of alternatives.

35  While there is some debate about the consequences of ius cogens, there are some consequences that are generally accepted. There are particular consequences of ius cogens for sources of international law and consequences related to State responsibility.

2.  Consequences for Sources of International Law

36  The basic consequence of ius cogens is contained in Art. 53 VCLT and provides that a treaty is void (Treaties, Validity) if, at the time of its conclusion, it conflicts with a norm of ius cogens. Article 64 provides that if a new norm of ius cogens emerges, ‘any existing treaty which is in conflict with that norm becomes void and terminates’. While this basic rule has rarely been applied in practice, ie there aren’t any examples of a treaty being declared invalid on account of ius cogens, its place in the body of international law is hardly in doubt. Indeed, there are many examples of States proclaiming that the validity of a treaty is dependent on its consistency with ius cogens. For example, the United States has expressed the view that a treaty between the Soviet Union and Afghanistan was void for being in breach of ius cogens, while the United Nations General Assembly also seemed to condition the validity of the Camp David Accords (1978) on their consistency with self-determination—a norm widely acknowledged as having ius cogens status (UNGA Res 33/28A ‘Question of Palestine’ [1978] para. 4; UNGA Res 34/65B ‘Question of Palestine’ [1979]; UNGA Res 36/51 [1981] and UNGA Res 39/42 [1984]).

37  This basic rule of invalidity is supplemented by other rules in the VCLT. The first of these is the question of severability, ie whether in the case of invalidity of a treaty on account of conflict with ius cogens the whole treaty becomes invalid or whether the inconsistent provisions are severable so that the rest of the treaty retains its force of law. The VCLT adopts a bifurcated approach. In the case of an Art. 53 situation—where a treaty is at the time of its conclusion in conflict—no severability is possible and the whole treaty is void ab initio. In the case of ius cogens superveniens (Art. 64), severability is possible if certain conditions are met (Art. 44 (3) VCLT).

38  There are questions about whether the bifurcation makes sense and whether, in the interest of the stability of treaty relations it would not be better to simply apply the rule applicable to ius cogens superveniens to all cases of conflict of ius cogens. Under the construction, severance of the offending provisions would be possible if the conditions were met. In the course of the ILC’s consideration of the question, the main reason put forward for the bifurcated approach was fidelity to the VCLT. In addition, it might also be argued that the bifurcated approach serves some kind of punitive purpose, ie that non-severability in the case of treaties concluded in the face of inconsistency with ius cogens was a punitive measure which could not apply in the case of ius cogens superveniens. There are, however, important arguments militating against bifurcation. The first of these is that the application of severability as a general rule would better promote the stability of treaties. A further more practical reason that had been put forward for a general rule of severability was that ‘distinguishing between pre-existing jus cogens and jus cogens superveniens added yet another layer of evidentiary’ complexity to the issues in that ‘a State claiming a conflict between a treaty provision and jus cogens’ would be required to identify the ‘specific moment in time [that] the jus cogens norm had come into existence’ (Comments of Mr. Reinisch, UN Doc A/CN.4/SR.3567 at 6). Notwithstanding these three reasons, when addressing the question, the Commission chose to stick to the language of the VCLT.

39  While the VCLT addresses (some) aspects of the consequences of ius cogens on treaties, there is no treaty instrument that addresses the main consequences of ius cogens for other sources of international law. However, the consequences of ius cogens from the law of treaties are easily transposable to other sources of international law. Applied to customary international law, this would mean, firstly, that a practice, even if carried out with the requisite opinio iuris sive necessitatis, would not establish a rule of customary international law if that rule would conflict with a peremptory norm. Furthermore, a rule of customary international law would cease to exist if a norm of ius cogens in conflict with that rule of customary international law subsequently emerged. Similarly, a principle which would otherwise qualify as a general principle of law within the meaning of Art. 38 (1) (c) ICJ Statute would not become such a general principle if it conflicted with a ius cogens norm. By the same token, a decision of an international organization which would otherwise create binding obligations for States would not create binding obligations if it were in conflict with a ius cogens norm.

40  While the application of the rule of invalidity to other sources of international law is based on the VCLT, there is also sufficient authority in support of the application of the rule. For customary international law the ICJ in Jurisdictional Immunities of the State (Germany v Italy: Greece Intervening) (2012) accepted that the ius cogens norms prevailed over rules of customary international law. The language of ‘prevailing’ is not the same as ‘invalidating’, but when seen in the context of the VCLT, it is reasonable to accept that the effect is invalidation (at para. 92). In this context, the Kenyan High Court in the Kenya Section of the International Commission of Jurists v Attorney General case (2011) stated that ius cogens norms ‘render void’ other customary international law rules.

41  The application of this rule to the UN Security Council raises peculiar issues because decisions of the Security Council themselves enjoy priority over other rules of international law under Art. 103 UN Charter. Yet it is generally accepted in the academic literature that decisions of the Security Council, as with the decisions taken by other organs of the United Nations (UN) and other international organizations, are subject to ius cogens. The position has also been reflected in judicial decisions as well as in many statements by States. The position is perhaps best captured by Judge Lauterpacht in Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Provisional Measures) (Order) (1993), who, having stated that the priority accorded to decisions of the Security Council ‘cannot—as a matter of simple hierarchy of norms—extend to a conflict between a Security Council resolution and jus cogens’, went on to explain the logic as follows:

Indeed, one only has to state the opposite proposition thus—that a Security Council resolution may even require participation in genocide—for its unacceptability to be apparent (Separate Opinion of Judge ad hoc Lauterpacht para. 100).

42  As with the invalidation of treaties, the question might arise whether it is possible to sever the offending part of the rule. This consideration applies more to decisions of international organizations, where the severability of a provision is more likely to be at issue. There is no authority one way or the other, but policy considerations, and in particular the preservation of international relations, would seem to militate in favour of severability whenever possible. For customary international law and unilateral acts, the Commission adopted, in its Draft Conclusions, the VCLT bifurcation, ie that severability is provided for only in the case of ius cogens superveniens. For decisions of international organizations, the Commission provided for severability generally without a distinction based on whether the decision was adopted before or after the emergence of the ius cogens norm.

3.  Consequences Relevant for State Responsibility

43  In addition to the consequences of invalidity of rules of international law that are inconsistent with ius cogens, there are also certain consequences connected with the rules on State responsibility that flow from the peremptory character of norms of international law. These have mainly been developed through the work of the ILC and the jurisprudence of the ICJ but have also generally been accepted by States.

44  The first rule, which is contained in Art. 26 ARSIWA, provides that circumstances precluding wrongfulness, such as self-defence, necessity (Necessity, State of), and force majeure, do not apply to breaches of norms of ius cogens. The principle is itself largely uncontroversial and is generally accepted. Yet its application is not without complications. For example, it may be (and has been) pointed out that self-defence, a circumstance precluding wrongfulness, is an exception to the ius cogens norm prohibiting the use of force. This apparent controversy can be explained by seeing the question of self-defence in the context of the use of force as one concerning the scope of the prohibition itself and not as a circumstance precluding wrongfulness. In other words, what is covered under the ius cogens prohibition on the use of force does not include the use of force conducted strictly in accordance with the requirements of self-defence. Seen from that perspective, self-defence is not a circumstance precluding wrongfulness under the rules of State responsibility but is rather part of the primary rule concerning the prohibition on the use of force.

45  A second consequence closely connected to the rules on State responsibility is that ius cogens norms are said to establish erga omnes obligations. In general, international law obligations are owed to particular States and not to the ‘international community’ as a whole. Yet, it is generally accepted that, for ius cogens, the obligations created are owed not to particular States but to the international community of States as a whole. The distinction between obligations owed to particular States and obligations owed to the international community as a whole was first articulated by the ICJ in the Barcelona Traction Case (para. 33). This position has since been reiterated in several decisions of the ICJ concerning particular norms of ius cogens.

46  That there is a relationship between ius cogens and erga omnes obligations is well-settled. What has been open to some discussion is the precise nature of the relationship between ius cogens and erga omnes obligations. There is such a close connection between them that very often the concepts are used interchangeably. Indeed, a large proportion of the accepted rules on particular consequences of serious breaches of ius cogens is based on the jurisprudence of the ICJ on erga omnes obligations and not on ius cogens norms as such. What seems to be generally agreed is that all norms of ius cogens produce erga omnes obligations. It also seems agreed that not all erga omnes obligations flow from ius cogens norms. Beyond obligations arising from rules of international law related to common spaces, it is difficult to identify other rules of international law that may produce erga omnes obligations outside the context of ius cogens.

47  The breach of ius cogens norms produces the same consequences under the law of State responsibility as those triggered by the breach of other obligations. However, in addition, international law recognizes particular consequences that are triggered by serious breaches of ius cogens. These consequences are, first, the duty on other States to ‘cooperate to bring to an end through lawful means’ the breach, second, the duty not to ‘recognize as lawful a situation created’ by the breach, and third, the duty not to assist in the maintenance of such situation. These duties are found in Art. 41 ARSIWA. Although there were some question marks over whether these duties are part of customary international law, there is sufficient evidence in both State practice and judicial decisions that these duties are accepted as customary international law. The ICJ has, for example, pronounced on the duty to cooperate to bring to an end a serious breach in, most recently, Chagos (Advisory Opinion) (2019) (see also Commentary to Draft Conclusion 19 para. 6). The Russian invasion of Ukraine and the responses by States to that invasion, especially within multilateral forums, provide support for all three duties as well as examples of the concrete application of these duties. An example of this is the General Assembly resolution of 2 March 2022 (UNGA Res ES-11/1 ‘Aggression against Ukraine’).

E.  Selected Topics and ius cogens

1.  Regional ius cogens

48  One question that might arise in the context of ius cogens is whether, as in the case of customary international law, it is possible to have regional or particular ius cogens that is applicable only as between particular States (Regional International Law). On the one hand, as a matter of general international law any group of States should be free to determine that particular norms, whether they be pre-existing general norms of international law or not, be endowed with particular consequences not ordinarily associated with such norms, including priority over other norms. It has been said, for example, that certain norms, such as democracy and freedom, have attained the status of regional ius cogens in Europe. Similarly, it has been suggested that the Inter-American system of human rights recognizes certain rights as ius cogens, such as the prohibition against discrimination broadly speaking, that are not (as yet) general ius cogens.

49  Yet, on the other hand, the notion of regional ius cogens would be difficult to reconcile with the universalist aspirations of ius cogens, ie the idea that these norms have the quality of ius cogens because the values underlying them, the values they seek to protect, and the values they protect are so fundamental to the international community as to be deserving of heightened status. If it could not be said that the (regional ius cogens) norm in question reflected or protected such fundamental values, it would be unclear on what basis it would have ius cogens status, regional or otherwise. This might suggest that its status would be based purely on the agreement of the States to which this status applied, which would make it qualitatively different from ius cogens.

50  The Fourth Report of Special Rapporteur Tladi on Peremptory Norms of General International Law (jus cogens) (2019) also identified specific practical challenges with ius cogens on a regional level. There was first the question of how ‘the region’ was to be identified. It seems the only way around this particular conundrum would be treaty law, which would suggest that the notion of regional ius cogens, if it existed, would always have to be based on treaty law, making it peculiarly consent-based. A treaty-based approach to regional ius cogens raises its own difficulties, chief amongst them the application of the consequences of ius cogens. For example, presumably a treaty concluded by a State within a region with a State outside the region that conflicted with the said regional ius cogens would not be invalid, or a general rule of customary international law in conflict with the said regional ius cogens would emerge notwithstanding the conflict. Indeed, even as between members of the region, it would be difficult to explain why two or more members could not contract out of the said regional rules. Even if there were consequences of State responsibility for violating the initial rule, it is not clear why, under international law, such States could not decide to depart from the previous treaty rule. Moreover, the obligations of non-cooperation and non-recognition would be very difficult to apply to acts that were not contrary to ius cogens in a general sense.

51  There is a way to reconcile these positions. States are always free to create their own particular rules applicable inter se. These rules may even concern secondary rules concerning prioritization (or hierarchization) where certain rules are granted a certain superiority over other rules, or rules concerning consequences for breach. But this does not make such rules ius cogens. It may make them regionally superior rules, but not necessarily ius cogens because ius cogens norms, on the strength of the values underlying them, have consequences that apply universally.

2.  Persistent Objector

52  In the case of customary international law, a State that has persistently objected to a rule (Persistent Objector) is not bound by such a rule. However, because of the nature of ius cogens, the persistent objector rule cannot apply to ius cogens.

53  That the persistent objector rule does not apply to ius cogens does not mean that persistent objection is irrelevant for ius cogens. Persistent objection may well have the effect of preventing the formation of a ius cogens norm or, put differently, persistent objection may prevent the identification of a norm as having ius cogens character, if the objection is of such a quality, and from a sufficiently large number of States, that it cannot be said that the international community of States as a whole recognize the peremptory character of such a norm. This, however, is not the application of the persistent objector rule. It is simply the application of the criteria for the identification of ius cogens.

54  The following brief points can be made with respect to the persistent objector rule. First, ius cogens norms apply to all States, including any that are persistent objectors. Second, the persistent objector rule does not, as such, prevent the emergence of a ius cogens norm based on a rule of customary international law to which one or more States have persistently objected. In other words, if a rule of customary international law subject to persistent objection from one or more States becomes accepted and recognized by the international community of States as a whole as having peremptory status, a norm of ius cogens will emerge, and the effect of the persistent objection will fall away, including in respect of the States that had persistently objected. Third, whether there is acceptance and recognition by the international community may, however, be affected by the persistent objection of States. In other words, the number of persistent objections might be sufficiently large to prevent the emergence of a norm of ius cogens.

3.  The Problem of Modification

55  It is generally accepted that ius cogens norms can be modified. The modification of ius cogens norms has, however, been said to raise conceptual difficulties. If customary international law is the most common basis of ius cogens, it might be asked how peremptory norms could be modified (or new peremptory norms could emerge) if rules of customary international law conflicting with ius cogens could not arise.

56  The problem, however, is not as intractable as might first appear. Modification of a ius cogens norm takes place whenever the international community accepts and recognizes that the modified norm has peremptory status. Where the modification in question concerns the expansion of the norm in question, no problem arises at all, since the underlying customary international law would not be in conflict with any ius cogens norm.

57  Where modification involves the emergence of a new, possibly conflicting norm or the narrowing of an existing norm, then the question of whether it is possible for the new norm to emerge does arise. While extremely difficult and unlikely, this is possible as long as the modified norm is accepted and recognized as having peremptory character. This would require that the existence of opinio iuris sive necessitatis coincides with the emergence of the opinio iuris cogentis. The prohibition on the use of force provides an example to illustrate the point. The prohibition on the use of force is a norm of ius cogens. There are two exceptions to this prohibition. To show that a new exception has emerged, eg the unilateral use of force for humanitarian purposes (humanitarian intervention), would require not only a belief that general international law permits this (opinio iuris sive necessitatis) but also an acceptance and recognition by the international community of States as a whole of the peremptory status of the permissibility of humanitarian intervention (opinio iuris cogentis), or, put differently, that the rule of humanitarian intervention has the same ius cogens status as the prohibition.

4.  Immunities and ius cogens

58  The impact of immunities on ius cogens is one of the most controversial issues in international law. On the one hand, it has been argued that immunities are provided for in customary international law or sometimes in treaties and that the main consequence of ius cogens is invalidity of rules of international law that are in conflict with ius cogens. Seen from this perspective, the application of immunity in a way that prevents the prosecution of ius cogens-–related crimes is in conflict with ius cogens and thus invalid or inapplicable. On the other hand, it is pointed out on the basis, inter alia, of the Jurisdictional Immunities case that there is no conflict between the substantive ius cogens norms and immunities.

59  There is certainly some merit in the argument that immunity and ius cogens concern different issues and are not necessarily directly in conflict. Yet, at the same time, it would be going too far to suggest that ius cogens has had, and can have, no effect at all on immunities. There is in fact some evidence that ius cogens has had some effect on immunities, curtailing it in some respects. Article 7 Draft Articles on Immunity of State Officials from Foreign Criminal Jurisdiction may be an illustration of this curtailment ([2017] UN Doc A/CN.4/L.893). This curtailment may be seen as an expression of the duty to cooperate through the development of law that reduces the reach of immunity for serious breaches of ius cogens. However, since the types of measures to be taken in giving effect to the duty to cooperate are to be determined by States, the extent of curtailment is determined not by ius cogens but by general international law.

60  The work of the ILC on the immunity of officials from foreign criminal jurisdiction would seem to suggest that, while the reach of the duty to cooperate to bring to an end serious breaches of ius cogens has not reached immunity ratione personae, it has reached immunity ratione materiae, such that immunity ratione materiae does not apply to breaches of ius cogens. This conclusion can be supported by practice and some judicial decisions (see generally ILC Special Rapporteur C Escobar Hernández ‘Fifth Report on Immunity of State Officials from Foreign Criminal Jurisdiction [2016]; ILC Special Rapporteur D Tladi ‘Third Report on Peremptory Norms of General International Law (jus cogens)’ paras 121–32).

5.  Application of ius cogens‎ to Other Entities

61  International law is traditionally addressed to States. It is thus unclear to what extent ius cogens norms apply also to actors other than States. On the one hand, given the importance of the norms, it would be expected that they apply to all actors, including non-State actors, particularly those with international legal personality. On the other hand, there is very little practice in support of the application of ius cogens to actors other than States.

62  It seems clear that ius cogens rules, and the secondary rules they produce, must apply at least also to international organizations since they act in very much the same ways as States on the international plane and are, like States, capable of creating rules of international law. Moreover, not applying the rules of ius cogens would permit States to avoid the rules of ius cogens by acting through international organizations. To consider an example, inasmuch as States are prohibited from committing genocide, so too international organizations are prohibited from committing genocide. To take an example of secondary rules flowing from ius cogens, in the same way that States may not assist in the maintenance of a situation created by a serious breach of a ius cogens norm, it would seem counter-intuitive to permit international organizations, created by States, to assist in the maintenance of that same situation. The less an entity is governed by international law, the less clear the (direct) applicability of ius cogens to that entity is possible or even necessary.

6.  A List of ius cogens‎ Norms

63  It is not possible to provide a comprehensive list of ius cogens norms. Nonetheless, there are some norms that are widely regarded as having attained the status of ius cogens. These include the prohibition on the use of force, the prohibition of genocide, the prohibition of crimes against humanity, the prohibition of racial discrimination and apartheid (Racial and Religious Discrimination; Apartheid), the prohibition of slavery, the prohibition of torture (Torture, Prohibition of), the right to self-determination, and the prohibition of war crimes. This is the list of norms contained in the Commission’s 2022 Draft Conclusions. Moreover, the norms reflected here are those norms that are most identified in lists produced by scholars.

64  With the exception of the prohibition on the use of force and, to a lesser extent, the right to self-determination, both of which have a strong State-centric logic underlying them, most of the norms on the list have a decidedly humanitarian logic to them. This might suggest that ius cogens norms are principally humanitarian in nature. Indeed, even the prohibition on the use of force and the right to self-determination, while in general addressing State-centric issues, are not unconnected with humanitarian concerns. The UN Charter’s preambular paragraph declaring the United Nations’ determination to ‘save succeeding generations from the scourge of war’ is itself an indication that underlying the prohibition on the use of force, the most State-centric norm on the list, are humanitarian concerns. This might explain why rules such as sovereignty and pacta sunt servanda are generally not recognized as ius cogens notwithstanding their fundamental role in the international legal system.

65  There are, however, other norms that are not included in the list above that deserve mention, either as norms that could meet the criteria for ius cogens or that deserve, because of the values they reflect and protect, consideration as ius cogens norms. These include the right not to be arbitrarily deprived of life, protection of the environment from massive destruction, the prohibition of discrimination generally speaking, and the prohibition of enforced disappearances (Life, Right to, International Protection).

F.  Conclusion

66  Ius cogens remains an immensely important concept in international law. Indeed, as international law matures as a legal system and takes on a more value-oriented character, the necessity for ius cogens and the tools it can offer becomes greater. Yet its full implications remain unfulfilled because of the uncertainty surrounding its content and consequences. The most recent work of the International Law Commission may go some way in alleviating some of the uncertainties concerning the identification of ius cogens and the legal consequences flowing from it, and thus nudging the car a little further out of the garage.

Further Bibliography

  • A Verdross ‘Jus Dispositivum and Jus Cogens in International Law’ (1966) 60 AJIL 55–63.

  • L Alexidze ‘Legal nature of jus cogens in Contemporary International Law’ (1981) 172 RdC 223–70.

  • MC Bassiouni ‘International Crimes: Jus Cogens and Obligatio Erga Omnes’ (1996) 59 Law&ContempProbs 63–74.

  • E de Wet ‘The Emergence of International and Regional Value Systems as a Manifestation of the Emerging International Constitutional Order’ (2006) 19 LJIL 611–32.

  • K Kawasaki ‘A Brief Note on the Legal Effects of Jus Cogens in International Law’ (2006) 34 Hitotsubashi Journal of Law and Politics 27–43.

  • M Koskenniemi From Apology to Utopia: The Structure of International Legal Argument (CUP Cambridge 2006).

  • C Tomuschat and JM Thouvenin (eds) The Fundamental Rules of the International Legal Order: Jus Cogens and Obligations Erga Omnes (Nijhoff Leiden 2006).

  • A Bianchi ‘Human Rights and the Magic of Jus Cogens’ (2008) 19 EJIL 491–508.

  • AA Cançado Trindade ‘Jus Cogens: The Determination and the Gradual Expansion of its Material Content in Contemporary International Case Law’ (2008) XXXV Curso de Derecho Internacional 3–29.

  • A Orakhelashvili Peremptory Norms in International Law (OUP Oxford 2008).

  • S Knuchel Jus Cogens: Identification and Enforcement of Peremptory Norms (Schulthess Zürich 2015).

  • R Kolb Peremptory International Law: Jus Cogens (Hart London 2015).

  • A Rohr La responsabilidad internacional del Estado por violación al jus cogens (SGN Editora Avellaneda 2015).

  • M den Heijer and H van der Wilt (eds) Jus Cogens: Quo Vadis? (2015) vol 46 Netherlands Yearbook of International Law (Asser Press The Hague 2016).

  • D Costelloe Legal Consequences of Peremptory Norms in International Law (CUP Cambridge 2017).

  • ME O’Connell ‘Self-Defence, Pernicious Doctrines, Peremptory Norms’ in ME O’Connell CJ Tams and D Tladi (eds) Max Planck Trialogues on the Law of Peace and War vol 1 Self-Defence against Non-State Actors (CUP Cambridge 2019) 174–257.

  • U Linderfalk Understanding Jus Cogens in International Law and International Legal Discourse (Edward Elgar Cheltenham 2020).

  • SB Traoré L’Interprétation des résolutions du Conseil de sécurité des Nations Unies: Contribution à la théorie de l’interpretation dans la société international (Helbing Lichtenhahn Basel 2020).

  • D Shelton Jus Cogens: Elements of International Law (OUP Oxford 2021).

  • D Tladi Peremptory Norms of General International Law (Jus Cogens): Disquisitions and Disputations (Brill Leiden 2021).

  • D Tladi The International Law Commission’s Draft Conclusions on Peremptory Norms (OUP Oxford 2024).

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