Why do some civil wars terminate soon, with victory of one party over the other? What determines if the winner is the incumbent or the rebel group? Why do other conflicts last longer? We propose a simple model in which the power of each armed group depends on the number of combatants it is able to recruit. This is in turn a function of the relative "distance" between group leaderships and potential recruits. We emphasize the moral hazard problem of recruitment: fighting is costly and risky so combatants have the incentive to defect from their task. They can also desert altogether and join the enemy. This incentive is stronger the farther away the fighter is from the principal, since monitoring becomes increasingly costly. Bigger armies have more power but less monitoring capacity to prevent defection and desertion. This general framework allows a variety of interpretations of what type of proximity matters for building strong cohesive armies ranging from ethnic distance to geographic dispersion. Different assumptions about the distribution of potential fighters along the relevant dimension of conflict lead to different equilibria. We characterize these, discuss the implied outcome in terms of who wins the war, and illustrate with historical and contemporaneous case studies.