With Russian troops already in the country, any direct military intervention by the U.S. at this point would likely precipitate World War III. Even less direct intervention, like U.S. intervention in Afghanistan during the 1980s against the Soviet Union, would run risks that would need to be very carefully analyzed.
There's also the fact that Crimea has historically been a Russian region, has a majority ethnic Russian population with mainly pro-Russian Tatars, and already is under Russian control which suggests that its link to Ukraine is really tenuous at best at this point, from many different standpoints.
_________________
"You have a responsibility to consider all sides of a problem and a responsibility to make a judgment and a responsibility to care for all involved." --Ian Danskin