Wars within states have become much more common than wars between them. A dominant approach to understanding civil war assumes that opposition movements are unitary, when empirically, most of them are not. I develop a theory for how internal divisions within opposition movements affect their ability to bargain with the state and avoid conflict. I argue that more divided movements generate greater commitment and information problems, thus making civil war more likely. I test this expectation using new annual data on the internal structure of opposition movements seeking self-determination. I find that more divided movements are much more likely to experience civil war onset and incidence. This analysis suggests that the assumption that these movements are unitary has severely limited our understanding of when these disputes degenerate into civil wars.