It's all about public support. Before Putin invaded, there was still alot of anti-American cynicism in Europe, ie that we were alarmists who were unfairly demonizing poor Pootie, whose oil/gas they love. But now that everything we said has come to pass, the West is thoroughly united. It's just a matter of time before the West would be unable to stand by - whether or not Ukraine is in NATO (which they begged to be for many years, and this would not be happening if they were). NATO is simply a military alliance - not some final word on which countries go in or not.

Putin/Russia are bluffing about nukes... to scare the West out of the conflict. Even if there is direct conflict between the West and Russia, Putin will not use nukes, knowing we have many aimed right at all of Russia's major cities and it would be an instant end to him and his country.