Not agreeing with the Trumpers here, but NATO won't do diddly other than offer an official statement of protest. 90% of NATO forces are US soldiers, as is NATO funding. Or was, as Trump made some sort of attempt to make you are the countries pay more during his last term, I'm not sure what the details on the final outcome on that are.
That being said, as if invading wouldn't be difficult enough in such a vast territory, occupying it sure will be one heckuva mess, and very costly as well. We had a hard enough time with Iraq and Afghanistan, and you can probably fit both those countries in Lake Ontario. Ok, maybe not, but almost.