I not naive enough to think Pudtin is incapable of invading a NATO country. Pull the same bluff of using his nukes if counter attacked or feels threatened. He is a bully and knows intimation is working.
Nobody that hasn't already. Belarus and Syrian mercenaries.
NATO could run the Russians out of the Ukraine in two days. China would probably launch an all out conventional attack on Taiwan. The fight would stay in that theater.
And that would be it. It would not go nuclear. Not even tactical nukes.