Crimea was annexed, but was once part of Russia in the past. That is a bit like England invading France is order to reclaim Normandy, which was part of England for hundreds of years. Past sovereignty does not justify current invasion.
As for the Ukrainian Nazis you mention, Ukrainians faced a difficult choice: fight for a Totalitarian Dictatorship or against it? They chose to fight against the USSR, and so were forced to ally with the lesser evil of Hitler’s Germany. The justification is an old one: “The enemy of my enemy is my friend.”
I know it is leftist dogma that the USA put a Fascist into power in Ukraine but that really isn’t true either. The problem for Ukraine is that Russia regards not just the Crimea, but all of Ukraine as being part of Russia, having starved 4.5 million Ukrainians under Stalin and replacing them with ethnic Russians. Russia needed a western port for its Navy Fleet, so it took it. It doesn’t want NATO bases on its border so cannot accept Ukraine joining NATO, and will invade to stop this happening. That is the facts behind the rhetoric.