How do domestic political institutions affect the way states interact in international crises? In the last decade we have witnessed an explosion of interest in this question, thanks largely to the well-known claim that democratic states do not fight wars with one another. Work on the “democratic peace” has generated a number of theoretical arguments about how practices, values, and institutions associated with democracy might generate distinctive outcomes. Although the level of interest in this topic has focused much-needed attention on the interaction between domestic and international politics, the proliferation of competing explanations for a single observation is not entirely desirable. Progress in this area requires that researchers devise tests not only to support different causal stories but also to discriminate between them.