The Ukraine is in the middle of a civil war and the United States is itching to pick sides. The problem is that both sides have legitimate reasons for their views, this isn't a standard power grab. The Ukraine straddles the Euro-Asia divide and so does its politics. In the East most people speak Russian and identify more with Russia whereas those in the West primarily speak their own language, Ukrainian, and identify more politically with Europe than Russia.
The European Union was dangling some carrots in front of the Ukraine, but not enough to make its (former?) president comfortable with pissing off Mother Russia. So he made the "safe" choice and went with Putin's offer. The western Ukrainians didn't like this at all and have seemingly taken over the nation's capital, Kiev.
The ideal solution to their problems would be to divide the country since their political division follows a geographic divide. Kiev is where the East meets the West and should stay a neutral city-state, much in the way Hong Kong has been treated.
Whatever they decide, the U.S. should not jump too fast to condemn Putin for sending in troops to aid in his cause. That's exactly what we would do in the same situation.
Imagine if Iran was courting Japan with some new trade deals that the U.S. would not approve of. After a little diplomatic flirting, the Japanese prime minister bows out and decides to strengthen ties with the U.S. instead. But half of Japan really wanted that Iranian trade deal, so they storm the capital and banish the prime minister. What would the U.S. do?
American troops would be immediately deployed to Japan. Congress would hold an emergency session giving the president full authority to act as the media hawks screamed from the pundit pulpit for 'Merica to go "defend democracy" in Japan, since that's what it's called when we impose our views militarily upon the world.
All the while the U.S. would be warning Iran that it's none of their business and if they choose to get involved it will be considered an act of war.
So what should the United States do about the Ukraine?
Just let Europe and Russia sort things out on their own.