Many theories of international conflict are based on the premise that war can occur by accident. The basic idea of accidental war is that crisis situations can spiral out of control, leading to the outbreak of a war despite no one having decided to go to war. Although World War I is often claimed to be the prime example of such an accidental war, modern research suggests that it began as a result of deliberate choices, not by accident. Nonetheless, accidental war continues to be used as an important building block in many theories, including theories of deterrence, bargaining, and the spiral model. In this article, I examine the validity of the assumption that wars can begin by accident, as well as the implications that the lack of accidental wars has for international relations theory.