Why do some armed groups commit massive wartime rape, whereas others never do? Using an original dataset, I describe the substantial variation in rape by armed actors during recent civil wars and test a series of competing causal explanations. I find evidence that the recruitment mechanism is associated with the occurrence of wartime rape. Specifically, the findings support an argument about wartime rape as a method of socialization, in which armed groups that recruit by force—through abduction or pressganging—use rape to create unit cohesion. State weakness and insurgent contraband funding are also associated with increased wartime rape by rebel groups. I examine observable implications of the argument in a brief case study of the Sierra Leone civil war. The results challenge common explanations for wartime rape, with important implications for scholars and policy makers.