Why do some winning rebel groups build obedient and effective state militaries after civil war, while others suffer military defections? When winning rebels face intense security threats during civil wars, rebel field commanders are more likely to remain obedient during war-to-peace transitions. Intense security threats incentivize militants to create more inclusive leadership structures, reducing field commanders’ incentives to defect in the postwar period. Intense security threats also reduce commanders’ capacity for postwar resistance by forcing insurgents to remain mobile and adopt shorter time horizons in rebel-governed territory, reducing the likelihood that field commanders will develop local ties and independent support bases. The plausibility of the argument is examined using a new list of winning rebel groups since 1946. Two case studies—Zimbabwe and Côte d'Ivoire—probe the causal mechanisms of the theory. The study contributes to debates about the consequences of military victory in civil war, the postwar trajectories of armed groups, and the conditions necessary for civil-military cohesion in fragile states.