This book aims straight at the tangled heart of international law and politics. Obvious atrocities like mass killing and torture are excused by international law's commitment to state consent as the foundation for obligations. Cronin turns this on its head by drawing on history, law, and politics to show that new rules against atrocities have arisen through consensus rather than consent. The book is essential reading at the boundaries between law and politics, human rights and state power, and theory and practice.