Civil war is fought between two political organizations, the state and a rebel group. Myriad theories of civil war have examined the role of state institutions and state strength, but little attention has been devoted to theorizing about rebel organizations themselves. The organizational structure of rebel groups is examined to understand patterns of recruitment and allegiance. Drawing on principal-agent analysis of participation and incentive compatibility constraints and the analytical tradition of rent-seeking contests, a model is developed to demonstrate that three factors—geography, ethnicity, and ideology—play an important role in determining military success, deterring defection within the rebel group, and shaping recruitment.