Studies of cohesion focus on pre-war networks of insurgency organizers and war-time socialization processes, but do not account for cohesion in civil wars involving spontaneous mobilization, where leaders lack sufficient integration in communities for mobilization and socialization of fighters. This paper shifts attention from insurgency organizers to fighters and disaggregates the concept into horizontal cohesion, or the risks taken by fighters for one another, and vertical cohesion linking fighters to local and central commanders, or the risks taken as part of the unit. While quotidian and local ties bond fighters to one another and local commanders in the small group context, units might fight to protect their own members rather than contribute to the broader struggle. This commitment to the insurgency depends on how fighters understand the benefits of victory and costs of loss in the war. The argument is supported by fieldwork-based analysis of the Georgian-Abkhaz war of 1992-1993 and has implications for cases of spontaneous mobilization characterizing the post-Soviet space and, more recently, the Arab Uprisings. It suggests that most mobilization takes place in a social setting, but insurgent organizations are not the only setting for collective decisions to join the fighting and develop cohesion among fighter groups.