Russia really doesn't have any good cards to play. Uncle Sam is so much stronger than Russia on the conventional front that the US could deal with the problem without breaking a sweat, and that means merely arming the Ukrainians. Putin is bluffing on the assumption that Obama doesn't want any foreign policy entanglements. If Obama decides to act, Putin will fall all over himself backing down. And that's why Putin has been measured in his actions - he doesn't want to rouse Obama out of his languor.