Might actually not be that bad a thing for Europe if it happened. Most of the recent immigrants/invaders would probably not care to fight but would behave like the rats most of them are - run out of the place as fast as they can. Might not even need an actual war for that, a lot would presumably decide that their origin countries weren't that bad after all even if there were serious and believable rumors that all of Europe might end up as a war zone in which they would not be the winners.

But yes, I doubt Putin is that stupid.