Abstract/Summary

This article tests the Kampala compromise on the aggression amendments to the Rome Statute of the International Criminal Court against the principle of legality, nullum crimen sine lege, requiring criminal law to be reasonably clear and prohibiting its retrospective application. It outlines three possible legality-based challenges to criminalizing aggression: the supposed indeterminacy of the jus ad bellum and the lack of a criminalization under customary international law; the vagueness of the definition of the crime of aggression introduced in Article 8bis; and uncertainty regarding the application of this definition to situations in which the ICC's jurisdiction over a particular individual arises only ex post facto. The article argues that it is the last of these three challenges, based on retroactivity rather than vagueness, that is most serious. A fundamental ambiguity about the legal nature of the Rome Statute has direct bearing on this issue: it is either substantive in nature, directly creating the crimes it defines, or jurisdictional in nature, in that it merely sets out the subject-matter jurisdiction of the Court over offences which are substantively defined elsewhere, in customary international law. The main practical consequence of this distinction is in the further question whether defendants charged before the Court have the right to challenge the legality of the charges against them on the basis that they do not comport with customary law. The article argues that this ambiguity about the nature of the Rome Statute was if anything only exacerbated in Kampala, discusses the substantive scope of application of Article 8bis as well as the intricate jurisdictional regime introduced by the aggression amendments, and finally turns to the question whether the definition of aggression adopted in Kampala departed from custom.