What is an International Crime? (A Revisionist History)

The question “what is an international crime?” has two aspects. First, it asks us to identify which acts qualify as international crimes. Second, and more fundamentally, it asks us to identify what is distinctive about an international crime – what makes an international crime different from a transnational crime or an ordinary domestic crime.Considerable disagreement exists concerning the first issue, particularly with regard to whether torture and terrorism should be considered international crimes. But nearly all states, international tribunals, and ICL scholars take the same position concerning the second issue: an act qualifies as an international crime if – and only if – that act is universally criminal under international law. The international-law aspect of the definition distinguishes an international crime from a domestic crime: although some acts that qualify as domestic crimes are universally criminal – murder, for example – their universality derives not from international law, but from the fact that every state in the world has independently decided to criminalize them. The universality aspect of the definition, in turn, distinguishes an international crime from a transnational crime: although a transnational crime such as drug trafficking involves an act that international law deems criminal through a suppression convention, international law does not deem the prohibited act universally criminal, because a suppression convention does not bind states that decline to ratify it. This definition of an international crime, however, leads to an obvious question: how exactly does an act become universally criminal under international law? Two very different answers are possible – and the goal of this article is to adjudicate between them. The first answer, what I call the “direct criminalization thesis” (DCT), is that certain acts are universally criminal because they are directly criminalized by international law itself, regardless of whether states criminalize them. Nearly every modern ICL scholar takes this position, as does the ILC. The second answer, what I call the “national criminalization thesis” (NCT), rejects the idea that international law bypasses domestic law by directly criminalizing particular acts. According to the NCT, certain acts are universally criminal under international law – and thus qualify as true international crimes – because international law obligates every state in the world to criminalize and prosecute them. No modern ICL scholar has taken this approach, although intimations of it date back to Grotius.Which thesis is correct? This article argues that it depends on whether we adopt a naturalist or positivist approach to international law. Although every international criminal tribunal has insisted that international crimes are positivist, not naturalist, phenomena, no extant theory of positivism – not even so-called “instant custom” – is capable of justifying the idea that certain acts are directly criminalized by international law. On the contrary: if we take positivism seriously, the NCT provides the only coherent explanation of how international law can deem certain acts to be universally criminal. Maintaining fidelity to the DCT, therefore, requires rejecting positivism in favour of naturalism – with all of naturalism’s inherent limitations.