Power-sharing settlements to civil wars are often reached in the shadow of external intervention. We argue that the spectre of external intervention undermines the effectiveness of power sharing in general, and territorial autonomy in particular. Shifts in the power balance between a government and former rebels induced by changes in actual or anticipated foreign support induce commitment problems that undermine power-sharing arrangements. Due to the generally weaker position of rebel actors, gaining or losing an outside option proves particularly severe for rebel organizations. Shifts in foreign support further increase the risk of conflict recurrence in the context of territorial power sharing where the inability to monitor the other side compounds the underlying commitment problem. In a mixed methods design, we test for differences in the recurrence rate of intrastate conflicts that ended in territorial power sharing with and without prior intervention. We then provide process-tracing evidence from four cases of armed conflicts that reveal how foreign intervention contributed to the failure of autonomy settlements. Our key finding—that the effectiveness of territorial power sharing as a conflict resolution strategy is conditional on patterns of external intervention—has important implications for the management of ethno-territorial civil wars.