The Evolution of International Criminal Law: Addressing Global Crimes

When the Nazi leadership stood trial at Nuremberg in 1945, the world witnessed something that had never quite happened before: individuals being held criminally accountable under international law for acts committed in their capacity as state agents. That moment did not just end a chapter – it opened an entirely new field of law. Today, international criminal law (ICL) is a sophisticated body of norms, treaties, and institutions designed to address crimes so serious that no single nation can – or should – handle them alone. Understanding how this field developed, and what makes a crime truly “international,” is essential for anyone studying criminal justice at the global level.

What makes a crime “international”? 🔗

Not every serious crime is an international crime. The distinguishing feature of an international crime is that it either has transnational implications – crossing borders in commission or effect – or it strikes at values shared by the entire world community, such as human dignity, peace, and security. As the T.M.C. Asser Institute explains, international crimes are those that “shock the conscience of the world” – offences so grave that the international community as a whole has a legitimate interest in suppressing and punishing them.

There are four key characteristics scholars typically use to define an international crime. First, the conduct must attack values of concern to the international community. Second, the prohibition must be grounded in either an international treaty or customary international law. Third, such crimes are subject to universal jurisdiction – meaning any state can prosecute the perpetrator regardless of where the crime was committed or the nationality of the accused. Fourth, the most serious among these crimes attain the status of jus cogens, or peremptory norms of international law, from which no state can derogate.

Jus cogens and obligations erga omnes 🔗

The concept of jus cogens is central to understanding why international crimes carry such unique legal weight. As described by Cornell Law School’s Legal Information Institute, jus cogens norms are peremptory rules of international law from which no derogation is permitted – they bind all states absolutely, regardless of treaty consent. Scholar M. Cherif Bassiouni explained that when a crime reaches jus cogens status, it triggers obligations erga omnes – duties owed to the entire international community – including the duty to prosecute or extradite offenders, the inapplicability of statutes of limitations, the removal of head-of-state immunity, and the rejection of “superior orders” as a complete defence. Crimes such as genocide, torture, crimes against humanity, and aggression are generally recognized as having this elevated status.

Bassiouni’s taxonomy: twenty-five categories of international crimes 🔗

The most comprehensive attempt to map the full landscape of international crimes came from M. Cherif Bassiouni, widely regarded as the father of international criminal law. A professor at DePaul University for 45 years, co-chairman of the UN committee that drafted the Convention Against Torture, and recipient of the Hague Prize for International Law in 2007, Bassiouni conducted an exhaustive analysis of international conventions from 1815 to 1996. Reviewing these conventions for relevant penal matters, he identified twenty-five distinct categories of international crimes.

These categories include aggression, war crimes, crimes against humanity, genocide, apartheid, slavery and slave-related practices, torture, unlawful medical experimentation, piracy, aircraft hijacking, kidnapping of diplomats, taking of civilian hostages, unlawful use of weapons, drug offences, falsification and counterfeiting, theft of archaeological and national treasures, bribery of public officials, interference with submarine cables, and international trafficking in obscene publications, among others. As Bassiouni noted in his foundational 1983 article in the Case Western Reserve Journal of International Law, what unites these diverse categories is either their transnational reach or the threat they pose to shared international values.

Core crimes vs. broader categories 🔗

Not all twenty-five categories carry the same legal weight. At the apex sit the four core crimes – genocide, crimes against humanity, war crimes, and the crime of aggression – which are universally recognized as the gravest offences under international law. Beneath them lies a broader tier of crimes that, while serious, are often addressed through specific treaty regimes rather than a permanent international court. The distinction matters enormously: the four core crimes fall squarely within the jurisdiction of the International Criminal Court (ICC), while crimes like drug trafficking or piracy are handled through separate treaty mechanisms and domestic prosecutions.

The historical development of international criminalization norms 🔗

The norms governing international crimes did not emerge overnight. Their development tracks closely with the expansion of international human rights law and international humanitarian law (IHL) – a process spanning over two centuries but accelerating sharply after the horrors of the Second World War.

Early foundations: piracy, slavery, and the laws of war 🔗

The earliest recognized international crimes were piracy and the slave trade. Piracy was suppressed through the doctrine of universal jurisdiction as far back as the 17th century – pirates were treated as hostes humani generis (“enemies of the human race”), meaning any state could try them. As legal history records, the concept of universal jurisdiction – which now underpins the prosecution of genocide and crimes against humanity – traces its roots directly to these early maritime offences. The prohibition on the slave trade followed in the 19th century, establishing the precedent that certain moral wrongs transcend national borders.

The Hague Peace Conferences of 1899 and 1907 marked another milestone, as states began codifying the laws and customs of war, restricting the use of certain weapons and protecting non-combatants. These efforts laid the groundwork for modern international humanitarian law.

Nuremberg and Tokyo: the turning point 🔗

The true inflection point came in 1945. The International Military Tribunal at Nuremberg was established by the Allied powers under the London Charter to try major Nazi war criminals for crimes that had no precise geographical location. For the first time, individuals – not just states – were held criminally responsible under international law, and three new categories of crimes were formally recognized: crimes against peace (aggression), war crimes, and crimes against humanity. A parallel tribunal was convened in Tokyo for war criminals from the Pacific theatre. Though both courts were criticized as “victor’s justice,” their legacy was transformative.

The UN General Assembly affirmed the Nuremberg Principles in 1946 and mandated the International Law Commission (ILC) to codify them. Shortly after, in December 1948, the UN adopted the Convention on the Prevention and Punishment of the Crime of Genocide – the first major step toward a permanent international criminal jurisdiction. The four Geneva Conventions of 1949 followed, codifying protections for war victims and establishing grave breaches as international crimes subject to universal jurisdiction.

Cold War stagnation and the ad hoc tribunals 🔗

Despite the momentum of the late 1940s, the Cold War effectively froze the development of international criminal institutions for four decades. Geopolitical rivalries made consensus on enforcement impossible, and serious violations of humanitarian law continued with little accountability. As the Cold War ended in 1989, international criminal justice became a realistic possibility once again. The UN Security Council established the International Criminal Tribunal for the former Yugoslavia (ICTY) in 1993 and the International Criminal Tribunal for Rwanda (ICTR) in 1994 – the first international criminal tribunals since Nuremberg and Tokyo. These ad hoc courts developed vital jurisprudence on genocide, war crimes, and crimes against humanity, providing much of the legal architecture that would inform the Rome Statute.

The Rome Statute and the ICC: crystallizing international norms 🔗

The vision of a permanent international criminal court materialized on 17 July 1998, when states adopted the Rome Statute under UN auspices. Negotiated by 160 states, the Statute entered into force on 1 July 2002 and created the International Criminal Court (ICC) – the world’s first permanent court with jurisdiction over the gravest international crimes. The Rome Statute formally established four crimes within the court’s jurisdiction: genocide, crimes against humanity, war crimes, and – following the 2010 Kampala Conference – the crime of aggression.

The significance of the Rome Statute extends beyond jurisdiction. The Statute provides that these crimes “shall not be subject to any statute of limitations,” and that no official – not even a head of state – enjoys immunity from ICC prosecution. These provisions directly reflect Bassiouni’s framework on jus cogens obligations. The ICC operates on the principle of complementarity: it intervenes only when states are unable or unwilling to genuinely prosecute, reinforcing rather than replacing domestic criminal justice systems. As of early 2026, 125 states are party to the Rome Statute.

The role of human rights and humanitarian law in shaping criminalization norms 🔗

The development of international crime norms cannot be understood in isolation from the parallel growth of international human rights law and international humanitarian law. The two bodies of law are deeply intertwined. Human rights law establishes the fundamental protections owed to individuals; humanitarian law governs the conduct of armed conflict. When violations of these protections reach a threshold of gravity – systematic, widespread, or directed at civilian populations – they cross into the territory of international criminality.

The expansion of human rights treaty law from the 1960s onward – through instruments like the International Covenant on Civil and Political Rights (1966) and the UN Convention Against Torture (1984) – gradually elevated new categories of conduct to the status of potential international crimes. The ICTY’s landmark ruling in the Čelebići case confirmed that violations of human rights norms that have reached jus cogens status, such as torture, may constitute international crimes. This jurisprudence has been critical in expanding the normative perimeter of ICL beyond the four core crimes to encompass a broader range of human dignity violations.

Ongoing debates and the limits of criminalization 🔗

Despite the remarkable progress of the past 80 years, the boundaries of international criminalization remain contested. Scholars differ sharply on what should count as an international crime. Antonio Cassese, for instance, argued for a narrower definition limited to crimes firmly grounded in customary international law – excluding treaty-based offences like drug trafficking from the category of “true” international crimes. Bassiouni’s broader framework, by contrast, emphasizes the functional role of international conventions in creating criminal norms, even without a customary law base. The tension between these approaches directly affects questions of jurisdiction, immunity, and the reach of institutions like the ICC.

There are also practical gaps. The ICC has no police force of its own and depends entirely on state cooperation for arrests. Major powers – including the United States, Russia, China, and India – are not parties to the Rome Statute, limiting the court’s effective reach. And the crime of aggression, only activated in 2018, remains politically sensitive, with its application constrained by the veto power of permanent Security Council members. These limitations remind us that the development of international criminal norms, while impressive, is still an unfinished project.

What do you think? Given that the ICC can only act when states are “unable or unwilling” to prosecute, do you think the principle of complementarity adequately addresses the problem of impunity for powerful states? And with major nations like the United States and China outside the Rome Statute, how effective can international criminal law realistically be in holding all perpetrators accountable?

References

1. https://www.asser.nl/nexus/international-criminal-law/international-crimes-introduction/
2. https://www.law.cornell.edu/wex/jus_cogens
3. https://www.derechos.org/intlaw/doc/bassiuscogens.html
4. https://www.icc-cpi.int/publications/core-legal-texts/rome-statute-international-criminal-court
5. https://www.cambridge.org/core/journals/international-journal-of-legal-information/article/abs/international-criminal-law-edited-by-m-cherif-bassiouni-2nd-edition-ardsley-ny-transnational-publishers-1999-3-volumes-isbn-1571050175-1571050183-1571050191-us49500/8C7799CF0EA02F6E6F211B254A9B98B8
6. https://scholarlycommons.law.case.edu/cgi/viewcontent.cgi?article=1795&context=jil
7. https://en.wikipedia.org/wiki/Universal_jurisdiction
8. https://www.icc-cpi.int/sites/default/files/NR/rdonlyres/ED2F5177-9F9B-4D66-9386-5C5BF45D052C/146323/PK_20060930_English.pdf
9. https://blogs.loc.gov/law/2023/07/falqs-international-criminal-court-25th-anniversary-of-the-adoption-of-the-rome-statute/
10. https://www.ebsco.com/research-starters/history/rome-statute-international-criminal-court-adopted
11. https://www.un.org/en/chronicle/article/role-international-criminal-court-ending-impunity-and-establishing-rule-law