Prominent formal theories of conflict provide considerable insight into how civil wars begin and end, but offer little understanding of how they proceed during wartime. One prevalent pattern is that rebel strategies vary significantly within conflicts over time, from guerrilla to conventional tactics. Why do rebels switch between different fighting strategies? How does the transition affect civil war negotiations? I develop a model of rebel–government negotiation in which rebels choose fighting strategies throughout a multiperiod war. The analysis shows that rebels switch from guerrilla to conventional tactics after gaining strength, and the expectation of growth delays rebels' transition to conventional fighting. The potential switch between different fighting strategies hurts the prospects for peace and prolongs civil wars. I identify the generic conditions under which peace is infeasible, no matter how belligerents negotiate. These conditions characterize the incipient stages of many rebellions, thereby explaining the lack of serious negotiations early on.