The United States and its NATO allies have defended the air strikes against Yugoslavia on moral grounds (to stop atrocities) and security grounds (to prevent the conflict from spilling over to neighboring European countries), but curiously they have never articulated a legal justification for the intervention. The nearest the NATO countries have come to articulating a legal rationale has been to cite various resolutions of the Security Council, in which the Council has determined that the actions of Yugoslavia in Kosovo constitute a threat to peace and security in the region and, pursuant to Chapter VII of the UN Charter, demanded a halt to such actions. Notably, however, these resolutions do not employ the talismanic phrase, 'States may take all necessary means..' which would constitute an express Security Council authorization of the use of force.
The failure of the NATO countries to articulate a legal basis for their humanitarian intervention in Kosovo is puzzling in that there are in fact several compelling legal arguments that could be made to justify the Kosovo intervention. The reason for this silence may be that each possible legal underpinning carries with it the specter of a practical consequence that the NATO countries traditionally hope to avoid. Unfortunately the policy of silence is a blunt and weak tool for navigating these concerns and, in the long term, frequently exacerbates the concerns and validates the objections to the legitimate use of force for humanitarian intervention.
Williams, Paul and Scharff, Michael, "NATO Intervention on Trial: The Legal Case that Was Never Made" (2000). Articles in Law Reviews & Other Academic Journals. 532.