If the situation were to escalate from an energy/trade war into military war, what would happen? Obviously Europe would be dragged into the conflict, but who else would most likely join the efforts of either side?
Probably the same thing that happened in Hungary 1956 and Czechoslovakia 1968: nothing, aside from the absolute crushing of the country threatening to break away from the bloc. Except this time NATO has even less of a reason to intervene, with a majority of their own population against intervention. Simply put: Obama is not Romney or Bush, otherwise, I believe, we might be in the midst of World War 3 about now. And if the US does not intervene, none of the European NATO states would want to either, as nobody in Europe has the capability nor the will.
The only 'red line' Putin has to watch out for which would significantly increase the minute chance of intervention is if he crosses the Dneiper and brings back the rest of Ukraine into the arms of Russia. The reason I state this is because Ukrainians on the west bank of the Dneiper have more pro-EU stances and will likely cause problems for the Russians, who will likely respond in force.