“Cataclysm” is a comparative socio-environmental history of the Holocaust and mass violence in the greater German Reich and Hungary (1933 to 1945) and the postwar aftermath. Using water as the focal point of analysis, this book-length project brings together social history and environmental history to understand how the environment and natural processes shaped reactions to and experiences, practices, and spaces of violence and persecution in an integrated history of the Holocaust that considers the perspectives of perpetrators, victims, and witnesses of violence as well as that of the environment(s) they inhabited. Heretofore, environmental approaches to the history of World War II and the Holocaust have largely focused on environmental ideas in fascism, landscape history, or post-Holocaust spaces and memory. “Cataclysm” explores how the environment and environmental processes shaped experiences of persecution, and how, in turn, violence made its mark on the environment, revealing the extent and limits of human power and nature’s agency in the history and memory of Central and Eastern European genocidal regimes.