I study civilians’ cooperation with an armed group in an irregular war. In the model, civilians differ in their valuation of siding with the armed group and make cooperation decisions without knowing others’ motivations or cooperation choices. I find that a superior military force is not sufficient to bring high cooperation and that full cooperation can only be attained if military power is complemented by expectations of punishment for helping the enemy. The model challenges the idea that random violence aimed at punishing enemy cooperators is used when selectivity is difficult to implement, and it shows that indiscriminate reprisals induce lower levels of cooperation, even when enemy cooperators are less likely to be punished with selective methods. Finally, I find that communities that have a highly centralized process of decision making are expected to give their support to only one group of combatants and to be exposed to less violence.