Kosovo
... je Srbija!
it was seemingly OK to arbitrarily change borders
Yip. Some idiots came to the conclusion that borders in post-WW2 Europe are merely suggestions, and if migrant people breed like rabbits they will be given their own state even if it happens to be a part of an independent country's sovereign territory. A rather imprudent decision, taking into account the number of migrant population in some of the Western European countries.
Putin said to the West: "If you don't like what Russia did in Crimea, you only have yourselves to blame - on the basis of the precedent in Kosovo in 1999". You may not like him (and have very good reasons for it) but on this point he was quite right.