How far do we let Putin go forward with whatever he has in mind? Would the average America want our troops to engage with the Russian military? How long would the conflict last? What nations would commit to total victory over Russia?
I don't really think that the Ukraine is going to be the starting point for WWIII. If anyplace, I think it will start somewhere in the Mideast, and it will be a coalition of Islamic countries against a coalition of Western countries.