This article first considers how to explain the applicability of international humanitarian law to NATO's armed intervention in the Federal Republic of Yugoslavia during the Kosovo crisis. Despite the fact that NATO, as a regional governmental organization, is not a party to the various treaties of humanitarian law, the individual States participating in the campaign were bound to respect international humanitarian law in its entirety. In the author's opinion the United States, though not party to the 1977 Additional Protocol I, has unilaterally agreed to respect the content of these new rules on the conduct of military operations — it has incorporated many of them in its own military regulations - and was therefore bound to abide by them. In this context, the author also discusses the latest developments with regard to the applicability of international humanitarian law to forces acting under UN authority. The article finally examines the way the international rules affecting the conduct of military operations were respected by the intervening forces, with particular emphasis on the well publicized failures.