What effect does state violence have on the cohesiveness and fragmentation of insurgent organizations? This article develops a theory of how state violence against civilians affects insurgent cohesion and fragmentation in civil war. It argues that the state-led collective targeting of an armed group's alleged civilian constituency increases the probability of insurgent fragmentation, defined as the process through which insurgent organizations split into distinct entities, each with its own social composition, goals, and leadership. This effect is driven by the interaction of several mechanisms at the individual, group, and organizational levels: state-led collective targeting enlarges the supply of fresh recruits, strengthens the bonds between immediate group members (interpersonal cohesion), and disrupts intra-organizational coordination, strategic unity, and institutional arrangements that underpin the commitment of individual fighters to the organization as a whole (ideological cohesion). The implications of this argument are empirically tested in an analysis of armed groups fighting against their governments between 1946 and 2008. The results suggest that campaigns of massive state violence directed against the civilian constituency of rebel groups increase the overall risk of insurgent fragmentation, a finding that has important implications for the duration and escalation of civil wars.