Why is America so committed to battling Russia over Ukraine?

What difference does Ukraine make to American security or the American national interest?

In my opinion absolutely none.

So why is America literally risking a Third World War, fought with thermonuclear weapons, that would literally result in the destruction of much of America, over something of zero significance to America?

I asked, via email, a U.S. senator who has influence over U.S. policy with regard to Ukraine, 

and he (more likely his staff) responded with a statement about how bad the Russians are, how Ukraine is an innocent victim of Russia, and how we must fight authoriatism and autocracies.

Is Ukraine the only victim of aggression in the world?

Of course not.

So why get involved in this one case?