This book theorizes collective trauma as a foundational force in international politics—a shock to political cultures that can both make and break international institutions. Though scholars of international relations and related disciplines have historically paid outsize attention to the onset of mass violence, as well as the changes it causes in the balance of power or security calculations, far less attention has been paid to its indirect longer-term impacts, particularly as they manifest as collective trauma. This book argues that collective trauma can not only shape the divisions between “us” and “them” that constitute the international system but also frame logics of interaction over the course of generations. The first half of the book develops a theoretical framework for understanding collective trauma as an emergent phenomenon, outlining both how it translates from individual to social (and vice versa) and how it interacts with diverse political conditions and competing priorities. The second half turns to three historical cases examining colonialism as collective trauma in post-independence India, the Holocaust’s constitutive role in Israeli foreign policy imaginaries, and the influence of the post-traumatic stress disorder diagnosis on the US global war on terror. Taken together, these cases demonstrate collective trauma’s foundational role in international politics, as well as the larger potential benefits of a “trauma turn” for the international relations discipline. This reorientation, the book demonstrates, is particularly vital as scholars work to combat the discipline’s Western bias and better account for the legacy of structural injustice and oppression.