Fragmentation and methodological inconsistency are already evident in the jurisprudence of the International Criminal Court (ICC). Part of the solution is to use the principle of systemic integration to interpret crimes consistently with customary international law. This book provides novel data on the ICC’s compliance with the Vienna Convention on the Law of Treaties, focusing on how the failure to interpret war crimes consistently with custom leads to unnecessary fragmentation and misses important opportunities to clarify and develop the law. It offers a detailed case study on the war crime of denying a fair trial, tracing its interpretive foundations from the Vienna Convention to the Rome Statute to the administration of justice by rebel groups in Timbuktu.