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Initial notions of the concept of international crimes began to emerge in the Roman Empire with the writings of jurist Marcus Tullius Cicero and the concept of hostes humani generis, the enemies of humanity. This concept was used by writers from the 17th century to refer to the perpetrators of international crimes such as piracy and slavery.[1] However, it was not until the Nuremberg and Tokyo tribunals - set up after the Second World War to prosecute leaders accused of serious crimes - that some crimes became criminalized under international law and the foundation for what we currently refer to as international criminal law was laid.

In the 1948 Hostages case, an international crime was defined as “such an act universally recognized as criminal, which is considered a grave matter of international concern and for some valid reason cannot be left within the exclusive jurisdiction of the state that would have control over it under ordinary circumstances”.[2]

The creation of the International Criminal Tribunal for the former Yugoslavia (ICTY), the International Criminal Tribunal for Rwanda (ICTR), the International Criminal Court (ICC) and several other internationalised or hybrid criminal courts resulted in the prosecution of international crimes and the concept of international crimes started developing. These concepts are still evolving. Over the years, this development has been guided and supported by the International Law Commission (ILC). More than six decades later, it has been generally agreed upon that certain crimes, such as war crimes, crimes against humanity, genocide and the crime of aggression are considered to have the status of ‘international crimes’. 

At the same time, the label of ‘international crime’ has proven quite controversial. Many scholars, such as Cassese,[3] Cherif Bassiouni[4] and Werle,[5] have written about the intriguing question: what constitutes an international crime? Despite many efforts, scholars disagree as to the answer to this fundamental question. Some scholars argue that international crimes are those crimes committed by individuals or state agents, while others claim that they can also include crimes perpetrated by corporations. Finally, there are also those who do not provide a definition of international crimes in their work at all.[6] 

While several acts have been recognised as international crimes and can be prosecuted in both national courts and international tribunals, it seems that the status of many other crimes remains unclear under international law. One might say that, as a result, there is neither a universally accepted definition of ‘international crimes’, nor has any general criteria been established in order to determine the scope and content of ‘international crimes’. Considering international criminal law is an evolving field, it might not even be possible to establish clear criteria at this moment. Whether this is true or not, the goal of this background piece is not to find the right answer to the question of what constitutes an international crime. Rather, by exploring the indeterminate boundaries of the label ‘international criminal law’ and the discussion that lies beneath it, an attempt will be made to provide an overview of some general characteristics of the crimes that have/or have not been labelled as an international crime.

International crimes can be divided between the so-called ‘core’ crimes and the more controversial international crimes. With respect to the former category, it is generally accepted that this label is ascribed to genocide, crimes against humanity, war crimes and the crime of aggression.[7]The more controversial international crimes include terrorism, torture and piracy (to just name a few). As this overview will discuss, scholarly opinions differ on which crimes should fall under the heading of international crimes, with some advancing that the core crimes should also include the above-mentioned crimes. Without choosing one side of the debate, the discussion and definitional controversy of what does or does not constitute an international crime will be addressed in the following section. The final section will provide a short overview of the several crimes.

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In search of a definition


A precise and widely agreed upon definition of ‘international crimes’, is—to this day—non-existent. Definitions of international crimes vary from author to author and are often so broad that they do not tell us which crimes are, or are not, included. In 1947, Wright described a crime against international law as “an act committed with intent to violate a fundamental interest protected by international law or with knowledge that the act will probably violate such an interest, and which may not be adequately punished by the exercise of the normal criminal jurisdiction of any state”, see p. 56 of this article. Further, as mentioned before, the judges in the 1948 Hostages case characterised an international crime as “such an act universally recognized as criminal, which is considered a grave matter of international concern and for some valid reason cannot be left within the exclusive jurisdiction of the state that would have control over it under ordinary circumstances”. In 1991, Orentlichter quoted Dinstein’s views as follows, that “while international crimes typically are grave offenses that “harm fundamental interests of the whole international community,” an offense becomes an international crime only when defined as such by positive international law”.[8] According to her, “the term in its broadest sense comprises of offenses which conventional or customary law either authorizes or requires states to criminalize, prosecute, and/or punish [original footnote omitted].”[9] Cassese’s 2003 handbook on international criminal law explains that international crimes can be defined as “breaches of international rules entailing the personal criminal liability of the individuals concerned (as opposed to the responsibility of the State of which the individuals may act as organs).”[10] Comparably, Werle wrote in 2005 that “[c]rimes under international law are all crimes that involve direct individual criminal responsibility under international law.”[11] More recently, ‘international crimes’ were defined as “punishable acts or conduct proscribed by international law”, see p. 141 of this 2012 paper.


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Characteristics of international crimes




Which features of international crimes set this category apart from ordinary crimes? Obviously, one can think of their particularly heinous, inhumane character that shocks the conscience of humanity,[12] but there are more possibilities. While there is no consensus here either, several attempts have been made by legal scholars to define the key characteristics of international crimes. In the second edition of his book Principles of International Criminal Law, Werle identified three cumulative conditions for an offence to fall within the scope of international criminal law: “First, it must entail individual responsibility and be subject to punishment. Second, the norm must be part of the body of international law. Third, the offense must be punishable regardless of whether it has been incorporated into domestic law [original footnote omitted].”[13] Cassese, on the other hand, identifies four cumulative elements. With respect to the requirement mentioned by Werle - that the rule must be part of international law - Cassese explicitly adds that the violated rule should have customary status in international law and should intend to protect values considered important by the whole international community.[14] Moreover, a universal interest in repressing these crimes should exist, in the sense that, subject to certain conditions, alleged perpetrators may, in principle, be prosecuted and punished by any state.[15] Finally, Cassese explains that if the perpetrator has acted in an official capacity, the state on whose behalf he has performed the prohibited act is barred from claiming immunity (with the exception for a serving head of state, foreign minister, or diplomatic agent).[16] Naqvi mentions, in the context of the exercise of jurisdiction, a list of eight features that would appear to characterise an international crime:

  • It is a norm of such a fundamental character that its violation attracts the criminal responsibility of individuals
  • Individual criminal liability exists at an international law level
  • The act is universally recognised as criminal and is considered a grave matter of international concern, i.e., it is recognised under customary law
  • The enforcement of this norm requires universal jurisdiction because it is not sufficient to leave it to the forum of primary jurisdiction
  • Such an act endangers international relations (peace and security). In this sense, the exercise of jurisdiction is not just for prosecution’s sake, but to fulfil the broader objectives of contributing to international peace and security
  • The act breaches a moral obligation fixed by international law
  • There is a collective responsibility to enforce such rules
  • International crimes are violations of jus cogens norms[17]

This list of features cna be expanded, but overall, the following five (non-cumulative) characteristics can be deduced from scholarly literature:

  • crimes which violate or threaten fundamental values or interests protected by international law and which are of concern to the international community as a whole;
  • criminal norms emanating from an international treaty or from customary international law, without requiring intermediate provision of domestic law;
  • criminal norms which have direct binding force on individuals and therefore provide for direct individual criminal responsibility;
  • crimes which may be prosecuted before international or domestic criminal courts in accordance with the principle of universal jurisdiction;
  • a treaty provision or a rule of customary international law establishing liability for an act as an international crime binds all (or a great majority of) States and individuals.[18]

It seems that the debate and legal discourse surrounding the concept of international crimes revolves around the importance given to these characteristics. Indeed, these characteristics are used in different ways by scholars when defining the concept of ‘international crimes’, resulting in the various views currently present in the legal debate. In the following section, four characteristics will be addressed in more detail: values of concern to the international community, basis in customary law or treaties, universal jurisdiction and their binding force via jus cogens.


Values of concern to the international community


With regard to the first characteristic, the focus lies on the values that are protected by rules of international law. From this more philosophical/natural law point of view, international crimes are those crimes that shock the consciousness of the world. For instance, the Rome Statute of the ICC (.pdf), in its Preamble, makes reference to “the most serious crimes of concern to the international community as a whole”, which “threaten the peace, security and well-being of the world” and which “deeply shock the conscience of humanity”. During the ICC Review Conference in Kampala in 2010, there were several unsuccessful attempts to include other crimes within the jurisdiction of the ICC. The Netherlands, for example, advocated for the incorporation of the crime of terrorism, while Trinidad and Tobago submitted that international drug trafficking be included as a crime within the Rome Statue. This could mean that these crimes are not to be seen yet as crimes of the same gravity as the four ‘core’ international crimes. Barbara Yarnolds has also considered that “[a]n international element raises conduct to an international crimes, if one of two factors is present: first, the conduct must constitute a direct threat to world peace and security; second, the conduct must either “shock the conscience” of the world community or constitute an indirect threat by rising to the level of threatening world peace and security due to the magnitude of the offense.”[19] Also, Cassese finds a violation of the values of the international community to be integral to distinguish an international crime from other crimes.[20] With respect to the content of these values, he does not explain how to identify these norms, but he remarks that they are not values “propounded by scholars or thought up by starry-eyed philosophers”,[21] but are rather laid down, although sometimes not very directly, in international instruments, such as the 1945 United Nations (UN) Charter, the 1948 Universal Declaration of Human Rights, the 1950 European Convention on Human Rights and the two 1966 UN Covenants.[22] Werle agrees with this notion and states that “[a]n attack on the fundamental values of the international community lends a crime an international dimension and turns it into a crime under international law”, which subsequently affects the international community as a whole.[23] He adds to this that “[a] connection to the most important values of the international community is established for all crimes under international law through one common characteristic, the so-called international element: All international crimes presume a context of systematic of large-scale use of force. As a rule, it is a collective that is responsible for this use of force, typically a state.”[24] According to this viewpoint, international crimes can be distinguished from all other crimes, because they reflect a core of common moral perceptions and normative standards. On this more normative level, it is, however, debated on which grounds certain crimes were given the label of “most serious concern” or of “international crime”. Mayeul Hiéramente argues that there are no substantive reasons why certain crimes – genocide, crimes against humanity and war crimes – have risen to this level while others have not. He starts by analysing the reference, in the Preamble of the Rome Statute, to “unimaginable atrocities that deeply shock the conscience of humanity” and asserts that this “indicates that the mere extensive or atrocious nature and gravity of the crime elevates genocide, crimes against humanity and war crimes to ‘international crimes’”, see p. 564 of this article. By analysing the label of ‘international crime’ through the lens of the Rechtsgutstheorie, he aims to distinguish a distinct legal good that is protected by the criminalisation of these crimes. An example of such a distinct legal good could be the notion of humanity, as referred to in the Preamble of the Rome Statute. However, he concludes that humanity is a characteristic of various other crimes, which are not considered to be international crimes. After examining several more ‘potential’ legal goods, Hiéramente concludes that there is no distinct legal good or feature that could explain the difference between ‘international crimes’ and ‘ordinary crimes’. The sole anchorage for such a distinct treatment could be the purely hypothetical and thereby abstract peace-threatening nature of these crimes. As a result, he states:

[t]hrough the lens of the Rechtsgutstheorie the label of ‘international crime’ appears to be at least questionable. A persuasive value-based explanation is inexistent. Apart from the somewhat abstract ‘threat to international peace’ one might discern in the commission of crimes such as genocide, crimes against humanity and war crimes, no distinctive feature can be established that would “[…] elevate the acts […] to international crimes […]”. Neither the state implication nor the cruelty and scale can explain how to draw a line between “[…] human rights violations on the one side and international crimes on the other side, the latter forming the nucleus of the most heinous violations of human rights representing the most serious crimes of concern to the international community” [original footnotes omitted].

For this reason, he rejects the current legal doctrine in which certain criminalised human rights abuses are singled out as ‘international crimes’. He further argues that turning away from the label ‘international crime’ should not be perceived as a setback or a problem. The international prosecution of crimes labelled as ‘national crimes’ or ‘ordinary crimes’ can be also justified in terms of jurisdiction. In other words, according to Hiéramente, the label ‘international crime’ is not needed to successfully repress international criminal acts.


Basis in customary law or treaties


The second characteristic requires the criminal norm to emanate from either an international treaty or to clearly form part of customary international law. With regard to the latter, the prohibition of genocide (see para. 161 of this ICJ Judgment) and the prohibition of torture (see para. 454 of this ICTY judgment) have been widely acknowledged to have achieved the status of customary law. Cassese has argued to limit international crimes to those crimes that have a base in customary international law, and under his definition, the following crimes fall within the label of ‘international crime’: war crimes, crimes against humanity, genocide, torture (distinct from torture as a war crime or as a crime against humanity), aggression, and some extreme forms of terrorism.[25] He excludes piracy because that crime was, and is still not, punished for the sake of protecting a community value. He also believes that illicit traffic in narcotic drugs and psychotropic substances, the unlawful arms trade, smuggling of nuclear and other potentially deadly materials and money laundering should not be seen as international crimes. The reason Cassese claims that these crimes do not fall within the scope of international crimes lies in the fact that they do not have a base in customary law, but are only provided for in international treaties or resolutions of international organisations. In addition, these offences are often perpetrated against states and are not committed by states or state agents.[26] Finally, the list would also not include apartheid, although the inclusion of this crime in the Rome Statute “might gradually facilitate the formation of a customary rule.”[27]

By excluding crimes defined only by international treaties, one of the important sources of international law is ruled out. Cherif Bassiouni took a rather different approach. Instead of focusing on customary international law, he tried to clarify the concept of ‘international crimes’ by analysing conventional international law. He reviewed 267 conventions to distil no less than 28 international crimes: aggression, mercenarism, genocide, crimes against humanity, war crimes, nuclear terrorism, theft of nuclear materials, apartheid, slavery and slave-related practices, torture and other forms of cruel, inhuman or degrading treatment, unlawful human experimentation, piracy, aircraft hijacking and unlawful acts against international air safety, unlawful acts against the safety of maritime navigation and the safety of platforms on the high seas, threat and use of force against internationally protected persons, crimes against the UN and associated personnel, taking of civilian hostages, use of explosives, unlawful use of the mail, financing of terrorism, unlawful traffic in drugs and related drug offenses, organized crime, destruction and/or theft of national treasures, unlawful acts against certain internationally protected elements of the environment, international traffic in obscene materials, falsification and counterfeiting, unlawful interference with submarine cables, and bribery of foreign public officials.[28] This results in a very different and much longer list of ‘international crimes’ compared to the list of crimes recognized by Cassese, who didn’t recognize piracy, slavery, apartheid and mercenarism – and many others – as international crimes.

The different results reached between eminent legal scholars, such as Cassese and Cherif Bassiouni, seem to be founded in the way they use various sources, such as treaties or customary law, to define the concept of international crimes, see this paper at p. 156.


Universal jurisdiction


A third way of looking at the concept of international crimes is by focusing on the prosecution of violations using universal jurisdiction as an avenue.

The identification of a crime as an international crime has often been associated with the concept of universal jurisdiction. According to Principle 1, para. 1 of the Princeton Principles on Universal Jurisdiction, universal jurisdiction is criminal jurisdiction based solely on the nature of the crime and may be exercised, see Principle 1, para. 2, with respect to serious crimes under international law. That latter concept includes piracy, slavery, war crimes, crimes against peace, crimes against humanity, genocide and torture (see Principle 2, para. 1).

Instead of relying on the notion that some crimes harm humanity as a whole and should therefore be considered international crimes, some scholars, such as Win-chiat Lee, reverse the justification by looking at legitimate jurisdiction: “Certain harm harms humanity as well in the relevant sense only because it belongs to the legitimate jurisdiction of the international community to suppress and adjudicate such harm done to the individuals.”[29] Hence, international crimes are crimes over which the international community, collectively or individually, can legitimately exercise universal jurisdiction.


Jus Cogens


Finally, the concept of jus cogens, or peremptory norms of international law, has also often been mentioned in relation to the determination of international crimes. The ICTY concluded in the Čelebići case that, “in human rights law the violation of rights which have reached the level of jus cogens, such as torture, may constitute international crimes”, see para. 172, n. 225 of this judgement. Although it remains debated which norms rise to this higher level, certain crimes, such as the prohibitions on aggression, genocide, slavery, racial discrimination, crimes against humanity, and torture, are generally characterised as having the legal status of jus cogens (with war crimes being increasingly accepted to fall within this category as well).[30] Cherif Bassiouni explains that several legal obligations flow from this superior norm, among others:

the duty to prosecute or extradite, the non-applicability of statutes of limitations for such crimes, the non-applicability of any immunities up to and including Heads of State, the non-applicability of the defense of “obedience to superior orders” (save as mitigation of sentence), the universal application of these obligations whether in time of peace or war, their non-derogation under “states of emergency,” and universal jurisdiction over perpetrators of such crimes.

Obligations established by jus cogens norms are obligations erga omnes: obligations owed to the international community as a whole, meaning that any state may invoke the responsibility of a state that is violating such obligations, see conclusion no. 37 on p. 183 of this report by the ILC.

Some crimes could gain the status of jus cogens in the future. It has been proposed in legal literature that a certain crime can develop into a customary international rule of jus cogens if “all the significant components of the international community (…) show that they perceive that principle as aiming to protect an essential common interest and therefore see its breach as indivisibly violating the rights of each and all [emphasis in original].”[31]



[1] M. Cherif Bassiouni, ‘International Crimes: The Ratione Materiae of International Criminal Law’, in: M. Cherif Bassiouni (ed.), International Criminal Law. Vol. I: Sources, Subjects and Contents, 3rd ed., Martinus Nijhoff Publishers: Leiden 2008, p. 129.

[2] Q. Wright, 'The Law of the Nuremberg Trial', American Jiurnal of International Law, Volume 41, Issue 1, January 1947, p.56.

[3] A. Cassese, International Criminal Law, Oxford University Press: Oxford 2003.

[4] Cherif Bassiouni (n. 1), pp. 129-203.

[5] G. Werle, Principles of International Criminal Law, T.M.C. Asser Press: The Hague 2005.

[6] See G. Sluiter G and A. Zahar, International Criminal Law: A Critical Introduction, Oxford University Press: Oxford 2008.

[7] Werle (n. 5), p. 26.

[8] D.F. Orentlicher, ‘Settling Accounts: The Duty to Prosecute Human Rights Violations of a Prior Regime’, Yale Law Journal, Vol. 100, No. 8 (June 1991), p. 2552, n. 57 (quoting Y. Dinstein, ‘International Criminal Law’, Israel Law Review, Vol. 20 (1985), p. 221.

[9] Ibid., p. 2552.

[10] Cassese (n. 3), p. 23.

[11] Werle (n. 5), p. 25.

[12] See W.N. Ferdinandusse, Direct Application of International Criminal Law in National Courts, T.M.C. Asser Press: The Hague 2006, pp. 177-178.

[13] G. Werle, Principles of International Criminal Law, 2nd edn. T.M.C. Asser Press: The Hague 2009, p. 29.

[14] Cassese (n. 3), p. 23.

[15] Ibid.

[16] Ibid.

[17] Y.Q. Naqvi, Impediments to Exercising Jurisdiction over International Crimes, T.M.C. Asser Press: The Hague 2010, p. 31.

[18] K.J. Heller, 'What Is an International Crime? (A Revisionist History)', Harvard International Law Journal, Vol. 58, No. 2, 2017, p. 353-420. See also A.K.A. Greenawalt, 'What is An International Crime?', in K. Heller et al (ed.), 'The Oxford Hanbook of International Criminal Law', Oxford University Press, 2020, p. 293-298.

[19] B.M. Yarnold, ‘Doctrinal Basis for the International Criminalization Process’, Temple International & Comparative Law Journal, Vol. 8 (1994), p. 90.

[20] Cassese (n. 3), p. 23.

[21] Ibid.

[22] Ibid.

[23] Werle (n. 13), p. 31.

[24] Ibid., p. 32.

[25] Cassese (n. 3), p. 24.

[26] Ibid.

[27] Ibid., p. 25.

[28] Cherif Bassiouni (n. 1), pp. 134-135.

[29] Win-chiat Lee, ‘International Law and Universal Jurisdiction’, in: Larry May and Zachary Hoskins (eds.), International Criminal Law and Philosophy, Cambridge: Cambridge University Press 2010, p. 38.

[30] Naqvi (n. 17), p. 27.

[31] L. Condorelli, “Customary International Law: The Yesterday, Today and Tomorrow of General International Law’, in: A. Cassese (ed.), Realizing Utopia: The Future of International Law, Oxford University Press: Oxford 2012, p. 153.