This dissertation focuses on the organizational structures of insurgent movements and the capacity for collective action of the nonviolent allies of clandestine armed groups in explaining why the Provisional Irish Republican Army (IRA) disengaged from violence in Northern Ireland and why Basque Homeland and Freedom (ETA) failed to do so in Spain. In so doing, it broadens the focus of research on conflict transformation and peacemaking to include not only the interactions between armed groups and states (which have been thoroughly investigated), but also the relations between the military and political wings of insurgent movements and the effects of state counterterrorism strategies on the political capacities of nonviolent movement groups and leaders. I make two principal arguments. First, the centralized Irish republican movement successfully coordinated political action during peacemaking efforts and ensured the IRA’s commitment to agreements made by its allied political party, Sinn Fein, while the decentralized Basque separatist movement persistently failed to coordinate peacemaking efforts, giving ETA a veto over its allies’ initiatives. Second, Spain’s broad legal definition of the “terrorist organization ETA” criminalized nonviolent separatist groups, thereby hindering their capacity for peacemaking, while the United Kingdom’s narrower legal definition of the “proscribed terrorist group,” which in practice focused on the IRA’s rank-and-file, facilitated and encouraged the participation of nonviolent republican leaders and cadres in the Irish peace process. Bargaining between armed groups and states is critical to peacemaking, but it represents only one aspect of conflict transformation. Peacemaking requires mass mobilization and collective action, as well as cooperation with rivals and opponents, tasks for which nonviolent insurgents are better suited than are clandestine armed groups. This dissertation therefore emphasizes the critical role played by the nonviolent movement allies of armed groups in the initiation, development, and ultimate success or failure of peace processes, and it investigates how their capacity as peacemakers is shaped by the organizational structures of insurgent movements and by state counterterrorism strategies.