Rebel groups exhibit significant variation in their treatment of civilians, with profound humanitarian consequences. This book proposes a new theory of rebel behavior and cohesion based on the internal dynamics of rebel groups. Rebel groups are more likely to protect civilians and remain unified when rebel leaders can offer cash payments and credible future rewards to their top commanders. The leader's ability to offer incentives that allow local security to prevail depends on partnerships with external actors, such as diaspora communities and foreign governments. This book formalizes this theory and tests the implications through an in-depth look at the rebel groups involved in Liberia's civil war. The book also analyzes a micro-level dataset of crop area during Liberia's war, derived through remote sensing, and an original cross-national dataset of rebel groups.