I dont know what to make of your post-WW2 sense of history in Europe. Take events of 1968 for example:
en.wikipedia.org
I expect for you the key difference is the sense you have of the 'unprovoked' nature of things on this occasion, and your failure to understand Putins rationale. I dont think I currently know the whole story on that, but for a start whats happening now can easily be filed under 'unfinished business'.
In terms of justifications for invasion where no credible military provocation is available, we've already seen examples of that this century such as the invasion of Iraq. If a world power decides to mount such an invasion then they will do so even if the pretext is flimsy and does not stand up to proper scrutiny, even if everyone really knows that the emperor has no clothes on.
I dont agree with claiming that there can only be one ending as well, that it can only end badly for Putin and/or Russia. There are scenarios where it ends badly and scenarios where it doesnt. And predicting these things isnt easy. Take the invasion of Czechoslovakia that I linked to, which was eventually seen as both an event which managed to end up contributing to a form of peace with the USA in the 1970s, but also fractured some global communist solidarity and propaganda in ways that may have contributed heavily to the eventual dissolution of the Soviet Union. And if making comparisons to that and what could go wrong for Russia this time, some of those things are things that Russia doesnt have in the first place this time, so arent theirs to lose in this conflict.
In some ways its surprising that its taken Putin this long to go this far with the agenda. He's always been painted as someone with these agendas on his mind, claims that history ended when the Soviet Union collapsed were laughable, and it never seemed likely that Russia would be content with only limited ambitions for all eternity. With the benefit of hindsight I am not surprised that Ukraine and Belarus turned out to be beyond the limit of what they would take on the chin, and now there is bloody pushback.