If Russia do decide to invade NATO territory, the Baltic states (Estonia, Latvia, Lithuania) will capitulate quickly. That won't even be particularly bloody, save for some brave but futile resistance. Poland is where it gets messy.

It won't happen though. To invade Poland, or the Baltics, is to declare war on NATO. I don't see how Russia can hope to win a war against NATO. All it can do is end the world as we know it if they want to be remembered as the worst regime in history, worse even than the Nazis.