This book makes a simple but consequential claim: individuals who decisively influence a state's decision to use armed force should be held criminally responsible for aggressive war. Challenging the prevailing orthodoxy, it reinterprets the leadership clause in the definition of the crime of aggression and offers a new responsibility framework that reflects how decisions about war are actually made. Under this approach, liability is not confined to heads of state and top military leaders but extends to civilian power brokers, private actors, and third-state officials who are present at key moments of deliberation and, as reason-giving and reason-demanding participants, exercise decisive influence over whether a state goes to war or continues one. By clarifying who counts as a 'leader' and why, the book charts a principled middle path between overcriminalizing ordinary participants in war, such as common foot soldiers and private citizens, and exempting those who set unlawful wars in motion. Drawing on an eclectic set of sources, the book offers a timely rethinking of individual responsibility for the unlawful uses of force.