Does attrition behavior, defined as waiting for the other side to give in despite the costs of delay, help explain the duration of interstate wars? To answer this question we develop a war and bargaining model that integrates a dynamic progression of the war and allows the rivals to make offers at any time of their choosing while they fight. The model predicts that, in equilibrium, states choose to fight without making significant offers in the hope that the other side will give in to outstanding demands. The model also predicts that each side’s flow cost of war increases with the probability that the other side gives in to its demands. This is the testable consequence of attrition behavior. Our statistical analysis suggests that both challenger and defender include attrition behavior in their conduct of warfare, and that this behavioral element significantly affects the duration of interstate wars.