The parallel between Hitler and Putin is so close. Both saw their countries humiliated, Germany after WW1, and Russia after the fall of the USSR and the Berlin wall. And they both take that personally.
In the 30's, the world stood by and let Germany take Czechoslovakia thinking that was all they wanted. Putin wants it like it was in his good old KGB days and there is no one to stop him in in his own country. At least Khrushchev had the Politburo he had to deal with, Putin has an agenda and few to stand in his way. He is used to the old sword rattling on the Cold War days and is putting the Allies to the test. And he has already played the nuclear arsenal card.