The difference between now and WW2 is that all of Europe west of Ukraine ( excluding Switzerland), and most of it north, are in a single military alliance, which has nuclear weapons ( and France also does, not under the NATO chain of command).

Sure. That military alliance was just recently kicked out out Afghanistan, in a very embarrassing way. That Alliance's most powerful member, the US, aren't as strong as they were in the past neither for various reasons. And Putin, it seems, wants to find out when that block is willing to go to war against a peer (or near-peer) aggressor. Because I wouldn't take it for granted that NATO would actually start a shooting war with Russia over the almost indefensible Baltics (beyond some honor saving measures) or that the US would actually got to war with China over Taiwan. What prevented those scenarios was, IMHO, the perception of the Wests power, and the real soft power backing that perception up. Now a lot of that soft power is gone, and the perceived power decreased considerably. At the same time the perceived, and soft, power of countries like Russia and China increased a lot. The balance changed, and naturally people want to gauge, actively, by how much exactly that balance changed. Ukraine will just be the beginning if it isn't stopped.

You seems to see Russia as expanding, they perception, I think, it's exactly the opposite. Putin is not thinking in expanding is thinking in making an stand.

Everything is a question of perspective, isn't it? You managed to put my thoughts into one sentence, thanks for that. Because I think Putin is exactly thinking like that. Let's hope all he wants is to make a stand, and not revenge.