What does a cosmopolitan criminal law look like? Can it live up to challenges of human agency in war? In this monograph, Dr. Song Tianying substantiates the persistent yet underexplored ethos of cosmopolitanism in international criminal justice. She also draws out philosophical and psychological tensions between such a cosmopolitan criminal law and war. She reflects on the mobilizing force of war crimes law within a cosmopolitan community: how the law can (fail to) align motivations of both the global public and participants in war. Dr. Song highlights the special moral, social and psychological dynamics within a cosmopolitan war crimes law: as human beings we can be both cosmopolitan and tribal, compassionate and hostile towards fellow human beings. Cosmopolitanism confronts its alter ego in the conceptualization and implementation of criminal law in war. Tensions between the rationalist moral appeal of criminal law and fluctuating human agency, between cosmopolitanism and war persist throughout the book. Dr. Song constructively integrates different research fields, including criminal law theory, moral philosophy, moral psychology, war crimes trials and war studies, to develop a multidisciplinary analytical framework. This enables her to explain different perspectives within war crimes law itself: how the law is perceived differently in the courtroom and on the battlefield; by international lawyers and military lawyers. She concludes that both the strengths and weaknesses of a cosmopolitan war crimes law lie in the moral nature of human beings.