The Russian military would almost certainly not send millions of soldiers to take territory in Poland or other NATO countries, but would launch hundreds of long-range cruise, ballistic and hypersonic missiles at targets it deems militarily important, setting back their economies by decades. No sane leader of an independent (or at least partially independent) country would want that. And Europe doesn't really have a way of responding without escalating the conflict into a thermonuclear exchange, one that it would most certainly lose, as it can't even maintain its strategic arsenal in peacetime.