The post-Cold War era has seen the increased significance of moral argument as a force in international relations. Arguments such as those developed in Michael Walzer's Just and Unjust Wars have shaped debates about the relative weights to be given to non-intervention and human rights as core values of international law over the past three decades. This article analyses the form of moral internationalism that is exemplified by Walzer's work, and the ways in which that moral internationalism has sought to justify humanitarian intervention, foreign involvement in civil wars, regime change, and, most recently, the responsibility to protect concept. It concludes by exploring the political stakes of the turn to what Walzer calls "practical morality" as a basis for reforming international institutions and laws, and the ways in which new forms of internationalism are redrawing the realism/moralism map.