From crusading in the Middle Ages to genocide in the twentieth century, from ancient blood feuds to modern urban riots, from tribal warfare to suicide terrorism, revenge has long been recognized as a root cause of violence in human societies. Developing a novel theory linking individual vengefulness to state behavior, Rachel M. Stein brings the study of revenge into the field of International Relations. Stein argues that by employing strategically crafted rhetoric, leaders with highly vengeful populations can activate their citizens' desire for revenge and channel it into support for war, thereby loosening the constraint of democratic accountability and increasing their freedom to use military force as a tool of foreign policy. This book will change the way scholars think about how citizens form their opinions regarding the use of military force and about the role those opinions play in shaping when and how democracies go to war.