There are currently over one hundred stateless nations pressing for greater self-determination around the globe and, while most will never achieve independence, many will receive some accommodation over self-determination, many will engage in civil war over self-determination, and, in many cases, internecine violence will plague these groups. This book examines the dynamic internal politics of states and self-determination groups, which significantly affect information and credibility problems faced by these actors, as well as the incentives and opportunities for states to pursue partial accommodation of these groups. Using new data on the internal structure of all self-determination groups and on all accommodation in self-determination disputes, this book shows that states with a moderate number of internal divisions are best able to accommodate self-determination groups and avoid civil war. More internally divided groups are both much more likely to be accommodated and get into civil war with the state, and are also more likely to have fighting within the group. Detailed comparison of three self-determination disputes in the conflict-torn region of northeast India reveals that internal divisions in states and groups affect when these groups achieve accommodation, which groups violently rebel, and whether actors target violence against their own co-ethnics. Understanding the relations between states and SD groups requires looking at the politics inside these actors.