This article examines the relationship between gender, state-building, and military reform after the Mexican Revolution. It argues that military reform was one of the most visible and politically significant attempts by the new regime to dictate gender in the interests of national development and uphold sexual differences—that is, to modernize patriarchy. The article identifies the main phases in policies aimed at reshaping military habits, comportment, sociability, physique, and family life. Reform reflected broader trends in Mexican politics and social policy and faced abundant obstacles: the army was powerful, secretive, and riven by factional, generational, and ethnic divisions; officers and soldiers clung to their own ideas about work, family life, and leisure. Nevertheless, by the 1950s, military reform had successfully reshaped gender roles in military families, moderated officers’ public behavior, and produced a more disciplined and physically fitter soldiery. These changes helped to reshape the army’s public image in lasting ways.