There are actually two Kosovo precedents: (1) the 1999 war, and (2) the 2008 declaration of independence. Putin cites both–the first for the military intervention in Crimea, the second for the subsequent secession/annexation of Crimea.
Focusing on the first, much of the debate has turned on whether Crimea is like Kosovo, with western critics arguing that the Kosovo intervention was justified by humanitarian considerations not present in Crimea. The problem with this argument–if understood as a legal argument–is that no one believes that the Kosovo intervention was legal. The U.S. government has not made such a claim. Here is an account from a state department lawyer named Michael Matheson:
NATO decided that its justification for military action would be based on the unique combination of a number of factors that presented itself in Kosovo, without enunciating a new doctrine or theory. These particular factors included: the failure of the FRY to comply with Security Council demands under Chapter VII; the danger of a humanitarian disaster in Kosovo; the inability of the Council to make a clear decision adequate to deal with that disaster: and the serious threat to peace and security in the region posed by Serb actions.
This was a pragmatic justification designed to provide a basis for moving forward without establishing new doctrines or precedents that might trouble individual NATO members or later haunt the Alliance if misused by others. As soon as NATO’s military objectives were attained, the Alliance quickly moved back under the authority of the Security Council. This process was not entirely satisfying to all legal scholars, but I believe it did bring the Alliance to a position that met our common policy objectives without courting unnecessary trouble for the future.
Matheson could have argued that the UN Charter’s flat ban on the use of military force should be interpreted away, perhaps in light of the Charter’s reference to human rights or some such thing. The UK would make such an argument. The U.S. never did. It still does not. The translation of Matheson’s exquisitely tortured statement is that we broke the law; we won’t do it again; and you better not, either.
Putin may be forgiven his skepticism about the “we won’t do it again” claim. The U.S. would do it again in 2003 (Iraq) and (arguably) 2011 (Libya). It did it before in 1989 (Panama) and 1983 (Grenada). Jack Goldsmith says, plausibly, “International law drops out because both actions were illegal, leaving only a fight over ‘legitimacy,’ which is even more in the eye of the beholder than legality.”
We can go farther. The U.S. did not advance a humanitarian intervention exception to the ban on the use of force because it did not believe that such an exception served its interests. It would open the door to other countries intervening whenever they believed humanitarian considerations justified intervention, leading to a surely impossible debate about what conditions constitute a humanitarian emergency and depriving the Security Council veto of its value. Russia and China also rejected the humanitarian intervention exception. So it could never become a part of international law.
Can the United States nonetheless argue that, while it broke the law in Kosovo and never paid a penalty, now, henceforth, it and everyone else must follow it? The problem with this argument is that the current use of force rules embodied in the UN charter can be mutually acceptable to all countries only in fact if all countries follow them. If only the United States can break them by citing “pragmatic” considerations, the United States alone possesses a veto. Russia, China, and the rest need to decide whether Kosovo was a one-off event or not. They appear quite rationally to have concluded that it was not one-off, and that the United States doesn’t take seriously the UN rules. And so they will not, either.