Maintaining peace with adversaries is costly, yet conflict models typically depict cooperation as a costless byproduct of not fighting. This article provides a formal model where peace is costly and countries can diplomatically compete over the bargaining surplus. Against conventional wisdom, repeated interactions of patient countries can destabilize peace. The likelihood of war depends on fundamentals of the international order, such as the persistence of outcomes and the level of competition in diplomatic affairs. Even when cooperation is mutually preferred, wars are inevitable due to a coordination problem induced by costly peace. Competition over favorable peace settlements does not directly instigate attacks, but it reduces the gains from peace and makes inadvertent war less likely. The article offers new explanations for war, highlights the importance of institutional design in conflict resolution, and sheds light on which international orders are most likely to fare well over time.