The NATO Operation Allied Force against the Federal Republic of Yugoslavia is highly relevant to the question of the legal assessment of humanitarian intervention. Not only can the current state of law be identified through the various phases of the Operation; the campaign also provides a significant impetus for the development of future law. This study focuses on the question of the intervention threshold: what are the conditions that must exist for the right to intervene to be exercised? In this vein, the facts of the Kosovo case before the intervention are laid out, complemented by a discussion of the law on the recourse to force. Thereafter, the law is applied to the facts.