Since the rise of mass politics, the role national identities play in international relations has been debated. Do they produce a popular reservoir easily tapped for war or bestow dignity thereby fostering cooperation and a democratic peace? The evidence for either perspective is thin, beset by different conceptions of identity and few efforts to identify its effects independent of situational factors. Using data drawn from new national surveys in Italy and the United States, we advance a three-dimensional conception of national identity, theoretically connecting the dimensions to conflictive and cooperative dispositions as well as to decisions to cooperate with the United Nations in containing Iran's nuclear proliferation and Sudan's humanitarian crisis in Darfur. Attachment to the nation in Italy and the United States is found to associate with less support for militarist options and more support for international cooperation as liberal nationalists expect. This depends, however, on containing culturally exclusive conceptions of the nation and chauvinism.