NATO and the EU have committed a lot of mistakes too, by ignoring the warnings by political and diplomatic experts over the past two decades. Heck, even Yeltsin was wary of NATO's expansion and made reference to the Two Plus Four Treaty pertaining to Germany’s reunification in 1990. That treaty was signed under the condition that a further expansion of the NATO zone into the East would be excluded. From a Russian perspective, NATO's eastward expansion is an act of aggression so they feel like they are defending themselves. Not to mention that Eastern-Ukraine has a majority of Russian sympathizers among the population.
History is the reason why Putin is feeling strong enough to make this bold move, a severely weakened EU, Eastern-European countries moving away from the West and inching closer to Russia, energy dependency prohibiting a strong response from Western-Europe. History is the reason why Germany is refusing to openly take a strong stance, being almost the only European country to not send any military to the Ukraine. Hence
Klitschko's negative response:
NATO's biggest mistake was believing that the cold war had truly ended, by recklessly backing Russia into a corner. The reason why Putin feels like he can pull this off now is because he believes that Europe's response will be quite timid. The sanctions that were imposed since Crimea's annexation have been toothless and didn't hurt Russia all that much.
Sure, Biden can flap his wings as much as he wants, but Putin knows that ultimately natural resources are more important than economy. He's also aware that as long as Germany remains reluctant, there won't be a unified strong response from the EU.
Bucharest Summit Declaration