Can the U.S. stop Crimea from rejoining Russia?

President Obama and President Putin spent about an hour speaking on the phone this evening. According to former national security adviser Stephen Hadley, talk of "diplomatic solutions and off-ramps" is likely having little impact on Putin's plans in Crimea. Hadley also says there is nearly nothing the U.S. can do to stop Crimea from rejoining Russia; at this point the best strategy may be to de- legitimize the referendum.