Previous research has used observational data to study the link between public opinion and decisions about military force. We take a complementary approach, by using experiments to examine two mechanisms—responsiveness and selection—through which opinion could shape policy. We tested responsiveness by asking members of the Israeli parliament to consider a crisis in which we randomized information about public opinion. Parliamentarians were more willing to use military force when the public was in favor, and believed that contravening public opinion would entail heavy political costs. We tested selection by asking citizens in Israel and the U.S. to evaluate parties/candidates, which varied randomly on many dimensions. In both countries, foreign policy proved as electorally significant as economic and religious policy, and far more consequential than non-policy considerations such as gender, race, and experience. Overall, our experiments imply that citizens affect policy by incentivizing incumbents and shaping who gets elected.