In the end, the Obama Administration is making a lot of idle threats, and GOP hawks are condemning them for not making even more. Though the US has military forces in the region, it is unthinkable that they would risk a full-scale land war with Russia over a peninsula with a majority Russian population which is almost certain to back secession in the pending referendum.
http://news.antiwar.com/2014/03/02/kerry-threatens-russia-all-options-on-the-table/
http://news.antiwar.com/2014/03/02/kerry-threatens-russia-all-options-on-the-table/