This study deals with one of the most fundamental problems regarding the use of force – the relation between law and power. More concretely, can illegal humanitarian wars be legitimized, or does a lack of legality doom any chance for legitimation? And why is legitimacy so important for the successful use of force? These questions are tackled by using the legitimization of NATO’s contentious humanitarian war over Kosovo in 1999 as a comprehensive case-study. Thus, this study analyzes how the Atlantic Alliance sought to wage a successful and, at the same time, legitimate war against the Federal Republic of Yugoslavia (FRY) without prior authorization of main source of international legitimacy, the United Nations Security Council (UNSC).