Aggression, Crime and International Security examines the concept of aggression in international relations and how it has been dealt with by international law and collective security organisations. This book analyses the evolution of the concept of aggression in international relations from World War I to the post-Rome Statute era. It charts the emergence of two competing visions of this notion: on the one hand, as a triggering mechanism for collective security enforcement among states, and, on the other, as an international crime giving rise to individual responsibility. The author argues that despite certain contemporary international trends suggesting a shift away from traditional, state-centric power structures towards a more cosmopolitan, globalized polity, the history of the concept of aggression demonstrates just how far away this is in reality. By examining aggression in theory and practice at the League of Nations, the Nuremberg and Tokyo Trials, the United Nations, the conference establishing the Rome Statute, and beyond, the book reveals the recurring moral, political and legal challenges this concept poses - challenges which continue to be at the forefront of thinking about international relations today.