ew findings in political science have received as much attention as the "democratic peace," the discovery that democracies almost never fight other democracies (Doyle 1986;Russett 1993). To some, the absence of military conflict among democracies is so consistent that it approaches the status of an "empirical law" (Levy 1988).Some authors have attempted to explain the democratic peace by highlighting the role of public opinion. They observe that democratic leaders are beholden to voters and claim that voters oppose war because of its human and financial costs. This argument, which dates to Immanuel Kant, predicts that democracies will behave peacefully in general-avoiding war not only against democracies but also against autocracies. History shows, however, that democracies frequently fight autocracies.A different possibility is that democratic publics are primarily averse to war against other democracies. If leaders are responsive to voters and voters are more reluctant to fight democracies than otherwise equivalent autocracies, then public opinion could play an important role in the dyadic democratic peace. To date, however, surprisingly few studies have investi- gated whether democratic publics are more reluctant to attack democracies than autocracies.1 Moreover, the small body of existing work has not accounted for variables that could confound the relationship between shared democracy and public support for war, nor has it investigated the mechanisms by which the regime type of the adversary affects the public mood. Despite decades of research on the democratic peace, we still lack convincing evidence about whether and how public opinion contributes to the absence of war among democracies.We used experiments to shed new light on these important questions.2 Our experiments, embedded in public opinion polls that were administered to nationally representative samples of British and American citizens, involved a situation in which a country was developing nuclear weapons. When describing the situation, we randomly and independently varied four potential sources of peace: the political regime, alliance status, economic ties, and military power of the adversary. We then asked individuals whether they would support or oppose a preventive military strike against the country's nuclear facilities.Participants in our experiments were substantially less supportive of military strikes against democracies than against otherwise identical autocracies. Moreover, because we randomly and independently manipulated the regime type of the adversary, the observed preference for peace with other democracies was almost certainly causal, rather than spurious. Our findings therefore provide empirical microfoundations for the 1 For exceptions, see Geva, DeRouen, and Mintz (1993); Johns and Davies (2012);Rousseau (2005). See also Lacina and Lee (2012), who examine how regime type affects perceptions of threat, and Geva and Hanson (1999), who focus on sociocultural similarity. 2 For other recent examples of experiments about internatio...