Two Points of view

The U.S. point of view is that Ukraine being an independent and sovereign country separate (governmentally but not culturally) from Russia should not have it's territorial integrity violated by Russia.

Putin's point of view is more about the "Survival of Russia and the Russian Empire". This is something likely that the U.S. doesn't understand very well. So, NATO making allies of countries like Poland and others and NATO missile shields Russia gets very sensitive about reminiscent of the Cold WAR. So, in some ways (For Russia) and for Putin the Cold War has never ended and (at least psychologically) the power of the Soviet Union still has to be defended (by Putin).

So, would Putin be willing to use nuclear weapons to protect the integrity of "Mother Russia". Maybe.

So, there may be no point at all in which Russia will back down short of using nuclear weapons in some limited way.

This is why I was saying in other articles this is the single most serious thing that has happened on earth since the Cuban Missile Crisis in Fall 1962.

This is how seemingly relatively innocent changes politically in Ukraine have precipitated this. When the government changed in Kiev that was something Putin couldn't live with because he was going to lose his Crimean ports. But since he needs a way to send trucks to the ports he needs Eastern Ukraine to so he can more easily ship weapons to Syria to help Assad stay in power so he can keep his port there in Syria as well. And the Crimean and Syria ports are seen by Russians as strategic to their survival here on earth as a nation because they are the only warm water ports they have.

So, no matter what the U.S. thinks at this point I don't see now how all this is going to resolve itself in any easy way in the short or long run. However, during the Cuban Missile Crisis many people on earth thought earth was toast then too.

One of the potential names for this crisis might be: "The Ukraine and Anti-missile missile crisis"
But let history name it in the end.