The United States has no good options with regard to Crimea.
The best outcome would be for Crimea to remain an integral part of Ukraine. This will not happen. The next best outcome would be to give Crimea even more autonomy within Ukraine than it now has, federalism on steroids. This is highly unlikely.
The next best outcome would be a confederal arrangement in which Crimea remains formally a part of Ukraine, but would have autonomy over elements of both its foreign and domestic affairs. This is possible but will require adroit diplomacy — not just threats and sanctions from the United States and the West.
The worst outcome would be for Crimea to be incorporated into Russia. On the current track, this may be where we end up.
For Russian President Vladimir Putin, not just Crimea, but Ukraine as a whole, is a core interest. Russia will not want to make its naval basing rights in the Crimea hostage to a Ukrainian state that is ever more tightly aligned with the West.
Putin will also want to satisfy the aspirations of his core constituents, who see Ukraine as closely aligned with, if not part of, Russia. Backing down at this point would undermine the tough image that Putin has so carefully cultivated.