Contemporary debates over the appropriate balance of unilateralism and multilateralism in U.S. foreign policy reflect disagreements not simply about the practical effectiveness of these alternative options but also about their legitimacy. Advocates of multilateral and unilateral action alike tend to bundle prudential calculations with normative claims, making assessments about costs and benefits difficult to disentangle from ethical arguments about fairness, justice, morality and obligation. Greater clarity may be possible by classifying U.S. foreign policy into six analytical categories, based on whether (...) the aims pursued are nationalist, internationalist, or cosmopolitan and the strategies adopted to realize them are unilateral or multilateral. Each set of aims has different ethical justifications that generate and help to explain divergent attitudes and judgments about the role of multilateral cooperation in U.S. foreign policy. The article sheds new light on alleged U.S. unilateralism, showing that the U.S. decision to go it alone–or to act with others–can be motivated by the desire to advance the narrow interests of the United States, to advance the interests of all states, or to advance the interests of humanity at large. The article suggests that purely nationalist policies, whether pursued through unilateral or multilateral means, will become increasingly untenable and illegitimate as world politics becomes institutionalized and as humanity becomes integrated, albeit slowly, into a single cosmopolitan community. (shrink)