Why do interstate rivalries end? In pursuit of this question, we advance a territorial theory of rivalry, which bridges the limited theoretical work on rivalry termination, the logic of commitment problems, and recent research associated with the territorial peace. We argue that unsettled borders are caused by an underlying commitment problem that contributes to the formation and perpetuation of rivalry. Ending the rivalry (by settling the borders) requires overcoming this fundamental commitment problem. We suggest that the signing of an international border agreement therefore represents a tangible indicator that states have overcome this commitment problem. After such agreements, we consequently expect rivalry termination to hasten and the rivalries that persist to experience fewer, shorter, and less severe conflict episodes. Empirical analysis of rivalries during the period 1816-2001 confirm these expectations. Such findings offer support to our unified territorial theory of rivalry and suggest that those seeking to manage rivalries might first focus upon unsettled borders as they attempt to create more peaceful interstate relations.