When authoritarian regimes face serious mass challenges, they often rely on military backing to remain in power. In revolutionary moments, institutional instability and competing legitimacy claims can severely undermine the principle of civilian control. With civilian leadership in question, soldiers must rely on the military’s organizational culture, a set of shared values, assumptions and beliefs, to guide their decision making. Military responses, from violent repression to complete disengagement, are driven by soldiers’ shared understandings about their proper roles and missions, duties and responsibilities, and relationship to both ruler and ruled. The paper identifies three initial sources of organizational culture in postcolonial armies: the military’s institutional origins, role in national independence, and relationship to the ruling party. Subsequently, organizational learning takes place as the military interacts with foreign military sponsors and the population at home. The theory is elaborated through a case study of Tunisia during the Arab Spring. The role of the Tunisian Armed Forces in producing Tunisia’s largely nonviolent, prodemocratic regime transition has been widely lauded by scholars and policymakers alike. As such, Tunisia represents a “best case” scenario for civil-military relations during revolution. However, the conventional explanations for this outcome – the military’s weakness, the lack of regime patronage toward military officers, and Tunisia’s ethnic homogeneity – fail to predict the observed processes of revolution and military response. Original interviews with retired Tunisian military officers, government ministers, and security experts offer strong evidence of a cultural mechanism guiding the army’s response to mass protests in 2010-11. A cross-national quantitative analysis generalizes the theory to a global sample of nonviolent revolutions from 1950-2013. The analysis employs original data on military responses to revolution, which record the use of violent repression by military forces during anti-regime campaigns. The data are designed to be compatible with the Nonviolent and Violent Campaigns and Outcomes (NAVCO) Data Project (Chenoweth and Lewis 2013). The analysis confirms that organizational culture explains military responses to revolution better than alternative arguments based on capacity, patronage, and ethnicity.