Maintaining peace is costly. To understand what this implies about war outbreak and frequency, this article provides a dynamic crisis bargaining model where peace is costly and countries can take diplomatic action to compete over the bargaining surplus. Against conventional wisdom, repeated interactions of patient countries can destabilize peace. The likelihood of war relies on fundamentals of the international order, such as the persistence of war outcomes and peace agreements, as well as the severity of competition in diplomatic affairs. Even when countries prefer to cooperate, inadvertent wars are inevitable in the long run due to a coordination problem induced by costly peace. Diplomatic competition reduces the gains from peace and affects the probability of inadvertent war, but does not directly instigate attacks. The model offers new explanations for war, highlights the importance of institutional design in averting conflict, and sheds light on which international orders are most likely to fare well over time.