By using innovative microlevel data from Colombia’s civil war, this article examines protests, one of many actions that civilians employ to resists peacefully armed actors during civil war. By using a cost-benefit analysis, the author argues that inhabitants can mobilize and solve their collective action problem when the costs of migrating or supporting one side in a war are too expensive. By examining the civil conflict on Colombia between 1988 and 2010, the author finds that civilians are likely to protest armed actors under three conditions. First, autonomous communities, which enjoy benefits associated with their territory, are likely to protest because migration would be costly; moreover, the institutional strength of these communities can decrease the costs of mobilization against armed actors. Second, foreign aid distributed through nongovernmental associations can empower individuals to take action by providing selective incentives for mobilization, thus decreasing costs. Last, high levels of violence, up to a certain threshold, increase the costs of nonparticipation and induce civilian action.