This article explores the causes of nationalist civil war, a subtype of ethnic civil war in which anti-state actors fight for greater communal autonomy. It presents a theoretical framework claiming that grievances over lost communal autonomy commonly motivate nationalist civil war, but that other conditions are needed to put this motive into action: Nationalist frames and expectations must make communities sensitive to lost autonomy, and mobilizational resources must be available so actors can organize nationalist movements. Nation-state building, in turn, commonly promotes reductions in communal autonomy, and British colonial pluralism frequently strengthened nationalist frames, expectations, and mobilizational resources, suggesting that nationalist civil war should be common in former British colonies after transitions from empire to nation-state. To test the framework, this article provides a comparative historical analysis of Zomia, a region that has the highest concentration of nationalist civil wars in the world and in which half of the countries are former British colonies. The analysis provides strong evidence supporting the theoretical framework.