By Nandita Bose and Steve Holland WASHINGTON (Reuters) -Russian President Vladimir Putin’s massive troop buildup along Ukraine’s border has left the West struggling to pinpoint if or when he might actually launch a military campaign. U.S. intelligence has been closely monitoring troop, equipment and other moves in the region, while trying to factor in international events, like the Beijing Olympics and an upcoming meeting of Western allies. Only Putin knows for sure what he will do, U.S. officials concede. But that hasn’t stopped the United States and its allies from trying to make more specif…