Customary International Law and the Principle of Legality in International Criminal Justice: Judicial Interpretation and the Limits of Genocide Accountability

International criminal justice must reconcile the need to prosecute genocide with the principle that criminal responsibility may be imposed only on the basis of law existing at the time of the conduct. Although genocide is codified in the Genocide Convention 1948 and the Rome Statute 1998, customary international law remains relevant to identifying the historical and legal status of the prohibition and to interpreting its elements. Existing discussions often affirm both the importance of custom and the principle of legality without sufficiently specifying the evidential and temporal tests by which a customary rule may ground individual criminal liability. This article addresses that problem through doctrinal and critical analysis of treaty provisions, the International Law Commission’s 2018 Conclusions on the Identification of Customary International Law, and selected decisions of the ICTR, ICTY and ICJ. Materials were selected because they directly address the definition of genocide, specific intent, legality, or the distinction between individual and State responsibility; they are assessed for legal authority, temporal relevance, consistency of reasoning, and implications for foreseeability. The article argues that customary international law may support individual criminal responsibility only where the offence and its essential elements were sufficiently established and reasonably foreseeable at the material time. Judicial decisions can clarify those elements but cannot substitute later judicial preference for proof of pre-existing law. The article’s contribution is a practical framework that separates identification of the customary rule, interpretation of the offence, and proof of individual guilt, while keeping State responsibility analytically distinct from criminal responsibility.