Since after the Second World War, the crime of aggression is – along with genocide, crimes against humanity and war crimes – a “core crime” under international law. However, despite a formal recognition of aggression as a matter of international criminal law and the reinforcement of the international legal regulation of the use of force by States, numerous international armed conflicts occurred but no one was ever prosecuted for aggression since 1949. This book examines the evolution of aggression as an internationally wrongful act of State and a corresponding individual crime. After a cross-cultural historical introduction to the subject, it offers an overview of contemporary international law on the use of inter-State armed force, and makes an original proposal for the development of Draft Articles on the use of force by States. The book makes a case for a judicial review of the inter-State use of force – by the International Court of Justice or, as the case may be in the future, by the International Criminal Court. It further scrutinises in a detailed manner the relevant jurisprudence of the Nuremberg and Tokyo Tribunals as well as of the Nuremberg follow-up trials, and makes proposals for a more successful prosecution for aggression in the future. In identifying customary international law on the subject, the volume draws upon a wealth of applicable sources of national criminal law and puts forward a useful classification of States' legislative approaches towards the criminalisation of aggression at the national level. It also offers a detailed analysis of the current international legal regulation of the use of force and of the Rome Statute's substantive and procedural provisions pertaining to the exercise of the International Criminal Court's jurisdiction with respect to the crime of aggression, after 1 January 2017. It is hoped that the book would be useful to both practitioners and students of international law and relations in that it brings together, in a comparative fashion, the normative experience of various States representing the major legal systems of the world, and of relevant international organs, and seeks to identify ways for reinforcing individual criminal responsibility for the use of inter-State force in contravention of international law.