It’s true that the UN did not agree to NATO’s bombing of Serbia, but that does not mean it was against international law. America has a veto in the UN so if the UN wanted to condemn NATO’s action, we would just veto it. NATO’s attack on Libya was authorized by the UN.
The UN doesn’t make international law. It preexists the UN. There is nothing under international law that makes it legal for NATO or anyone else to decide to just depose governments we don’t like, let alone give part of a country away to rebels. And that was the original posters point about Putin. If Putin’s actions are illegal then so was our attack on Serbia. And Russia also has a UN veto.
Countries have a right to act when they or other countries are attacked, or when their security, or that of other countries are threatened. The Serbs in Kosovo were no threat to any other country, and they were fighting Muslim terrorist rebels in their own territory. There was no legal justification for us doing what we did. But it didn’t matter because we were strong enough to do so, and nobody could stop it. Well, what goes around comes around.