Combatant recruitment and the outcome of war

Why do some civil wars terminate soon, with victory of one party over theother? What determines if the winner is the incumbent or the rebel group?Why do other conflicts last longer? We propose a simple model in whichthe power of each armed group depends on the number of combatants itis able to recruit. This is in turn a function of the relative 'distance' between group leaderships and potential recruits. We emphasize the moralhazard problem of recruitment: fighting is costly and risky so combatantshave the incentive to defect from their task. They can also desert alto-gether and join the enemy. This incentive is stronger the farther away thefighter is from the principal, since monitoring becomes increasingly costly.Bigger armies have more power but less monitoring capacity to preventdefection and desertion. This general framework allows a variety of interpretations of what type of proximity matters for building strong cohesivearmies ranging from ethnic distance to geographic dispersion. Di¤erentassumptions about the distribution of potential fighters along the relevantdimension of conflict lead to di¤erent equilibria. We characterize these,discuss the implied outcome in terms of who wins the war, and illustratewith historical and contemporaneous case studies.