The international community has been increasingly interested in the ways by which external actors help end civil conflicts. This study compares and contrasts the conflicts in Sri Lanka and Indonesia (Aceh) in order to understand why international intervention has failed in Sri Lanka but has been successful in Aceh. In Sri Lanka, the intensely fractured nature of domestic politics has been one of the most significant obstacles to successful peace talks. Although international intervention has become necessary to overcome the trust barriers between the two parties, it is seen as a threat to national sovereignty and therefore rejected. In contrast, the democratization process in Indonesia created new opportunities for peace. The political will of principal actors and their receptivity to external assistance enabled a peace agreement in 2005. The findings of this study show that both domestic political consensus and a supportive international environment, which protect the reputational concerns of the negotiating state, are necessary for resolving protracted ethnonational conflicts.