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Existing research finds that cooperation among militant groups is common and contributes to both capabilities and lethality. Comparatively little is known, however, about how militant alliances are maintained and how they break apart. We argue that leaders are critical to sustaining alliances among militant groups. As a consequence, organizational disruption in the form of leadership targeting can lead to the breakdown of militant alliances. To test this argument, we pair original data on militant alliances with data on leadership targeting to reveal that decapitating an organization’s leader, and particularly its founder, increases the probability that an organization’s alliances terminate. We find that leadership decapitation spurs alliance termination by incapacitating targeted groups, stoking fear among allies, and inducing preference divergence between targeted groups and allies over strategy.