You really aren't paying attention, are you?
You didn't think he'd invade Ukraine, based on some naive idea you had that Putin's word could be trusted.
If we let him get away with this, he will then chance his arm further.
And you are changing the playing field a little. You talked about Europe, not NAto countries here:
Is Russia aiming to overrun Europe?
As Ukraine is part of Europe then I'd have to say yes, he has started.
As pointed out in the article I linked to, the arguments used by Putin for Ukraine can equally be used for the Baltic states, Finland & Poland.
As it is already well known that he is unstable then he might think NATO is weak enough to just stand by if we don't stop him now. He cannot be allowed to succeed in Ukraine.