In the course of NATO's 1999 air campaign in Kosovo, most international lawyers remained conspicuously silent on the strict legality of the intervention. Of those who gave on-the-record comments, many couched their opinions in terms of “traditional international law,” observing that it provided no clear basis for the intervention but usually refraining from condemning the intervention as illegal. Subsequent legal analysis has seen less ambiguity – and greater divergence of opinion. Authors with more or less similar understandings of the factual situation in Kosovo in the early months of 1999 – widespread and escalating persecution of the non-Serb population by a government and army with a history of mass atrocities elsewhere – arrive at sometimes starkly different conclusions, both on the law as it stood at the outset of the intervention, and on the potential for legal change.

Normally, when academic lawyers come to starkly different conclusions, this indicates that the questions of law are difficult ones on which experts reasonably can disagree. On the basis of traditional approaches to the interpretation of treaties and the analysis of customary rules, however, the Kosovo intervention was clearly illegal, and regarded as such by enough states that it could not possibly have contributed to a change in the law. It is therefore tempting to dismiss those who come to any other conclusion as mistaken or opportunistic in their interpretation.