NATO intervention in Kosovo created a precedent in international law, we can see now its consequences US supported every secessionist group that wanted to break away from Yugoslavia, they bombed Yugoslavia into smitherness in 1999 (80 days of intensive bombing),to split it in two, and they declared the Yugoslav Army to be an occupier on its own territory.
The Assembly of Kosovo, a province of Serbia, approved an unilateral declaration of independence from Serbia on 17 February 2008. Kosovo was soon recognized as a sovereign state by the United States, Germany, Italy, France, the United Kingdom, and others ...
The West cannot have it both ways. They can’t say, for example, that the right of Albanians to self-determination in Kosovo trumps Serbia’s right to territorial integrity, and then turn around and say that Ukraine’s right to territorial integrity trumps the right of Russians in Crimea to self-determination.
Kosovo leaving Serbia is like "Aztlan" leaving the US.