What is the significance of the First World War to International Relations (IR) theory ? We argue that World War I planted the seeds for International Relations theory, but it was World War II and the Cold War that fostered its growth. By exposing shocking deficiencies in thinking about world politics, World War I drastically expanded the audience for IR thought, stimulated academic study of the subject, and set the scene for the ferment that followed. The First World War remains a critical case in IR research and a fertile basis for arguments on the causes and course of war and peace. But its central place in the field is increasingly contested because of the nuclear revolution, the disintegrative force of nationalism, and the salience of terrorism.