Recent works on the role of argumentation in international politics have enriched our understanding of the discursive construction of international legitimacy. Many scholars have recognized the pervasiveness and privileged status of legal claims. Building on these insights, I advance the proposition that the international legitimacy of the use of force has legalized. Legalization implies that successful (de-)legitimation depends on the strategic use of international law, and that alternative legitimacy discourses (such as morality) have been marginalized and play a negligible role in the construction of legitimacy. Thus, the use of force is legitimate to the extent that it conforms to international law. I test this “legalization thesis” against the “hard” case of NATO's intervention in Kosovo in 1999. By revisiting the arguments used by state representatives, I show that, as expected in a legalized legitimacy system and contrary to what has become common knowledge about this case, legitimacy was gained through, not despite, international law. I analyze NATO's strategy of legitimation in detail and reconstruct it as a set of seven strategic moves, all of them appealing exclusively to the international legal discourse.