What explains armed groups’ extensive violence against civilians in the Syrian civil war? Existing theories of violence against civilians in civil war offer indeterminate predictions about why it occurs. I compile the largest available micro-dataset on civilian death in civil war, consisting of data on the dates, locations, and causes of over 100,000 civilian deaths in the Syrian war, along with fine-grained data on armed groups’ territorial control, the locations of arrests during protests in 2011, and a novel measure of regime threat. Using this data, I systematically evaluate existing theories’ abilities to explain violence in Syria. I find little support for prominent theories of violence against civilians that emphasize territorial control, regime threat, or differences between types of violence. Instead, strategic logics of deliberate civilian violence and mass violence in areas of anti-regime mobilization better explain casualties in Syria.