Abstract: This essay critiques the "cosmopolitan duty" argument that is raised in criticizing the United States for its failure to take affirmative steps that would help other nations and their peoples. This argument maintains that the United States should ratify global treaties and intervene more vigorously to stop human rights abuses, even if doing so would lower net U.S. welfare. This essay argues that underappreciated theoretical, practical, and moral factors limit the duty of liberal democracies to engage in cosmopolitan action, and that there cannot be a coherent ideal of liberal democracies' cosmopolitan duties unless these realistic limits on what liberal democracies can do is understood. However, this essay does not criticize the cosmopolitan stance per se; rather, it suggests ways that cosmopolitan sentiments can be more fully realized by being more realistic.