When and under what conditions do norms and rules change? Dominant conceptions of institutional change in International Relations theory are based on the idea that it is the result of a shift in power: new actors become able to impose their vision on the world. However, the source of change need not be the power or preferences of actors in society, but could come from the internal dynamics of the rule system governing these actors. This article develops recent research in this area by linking Sandholtz’s model of norm change to recent dynamic institutionalist work and exploring and specifying particular mechanisms, or types of tensions, in rule systems that produce change. Institutions and complexes of rules exhibit rule tensions: inconsistencies, ambiguities, and inadequacies that can lead to disputes over the application of the rules. Actors then have to solve problems or disputes over rule interpretation. Change can thus occur without the introduction of new actors or a shift in the power of existing actors. I apply these ideas to a significant change in the rules of war in the early 20th century: the shift from the rule ‘to the victor go the spoils’ to the Stimson Doctrine, or the rule that states should not profit from aggression.