NATO will bomb and possibly even occupy Belarus in this scenario. Granted, deterring this is probably why russia placed nukes there, so perhaps NATO will provide 48 hours warning for Russia to remove its weapons. It will also provide Ukraine with aircraft and long range missiles. I don’t see how losing Belarus in order to solidify control over Eastern Ukraine and Crimea is a good strategic play.
Russian (and presumably Belarusian) nuclear doctrine is: strike when home territory is substantially threatened/invaded. There's no way that NATO wouldn't give warning of its intent and risk a nuclear exchange.
My guess is what would likely happen is NATO would yell loudly, look like it was ineffective and couldn't decide what to do, but in reality suddenly "Ukraine" would suddenly sink some expensive big ships in the Black Sea or take down a strategic bomber or two, using advanced US/European weaponry, and NATO would shrug and say "Huh. Neat. Wasn't us tho"
