I think a more plausible implausible scenario would be something like this:
2011-12: Greeks revolt over austerity measures. Tax revenues fall and the Greek government is unable to meet the demands of its creditors. Under the guise of restoring order to rebelled areas, the Greek government declares a state of emergency dispatches the army. The army is given two specific tasks: restore order and collect back taxes. Greek civilians begin fighting with the army.
2012, February: France and Germany try to get European support for a "peacekeeping" operation in Greece. Italy, torn by internal problems and fearful of retribution, goes along with the Franco-German proposal. Britain, Spain, Poland, and other European states reject and the proposal does not go forward. Three weeks later, France unilaterally declares it will intervene in Greece. Merkel decides to try and play both sides and appease domestic opinion by only offering "logistical support" but no combat troops. Almost immediately, a vote of no confidence is held and Merkel is ousted -- the German support never arrives. German taxpayers celebrate.
2012, April: French intervention in Greece continues and the Greek government remains largely unable to control much of its territory. Greek troops begin to resent French troops, who are increasingly taking command of their own army with the compliance of the flaccid Greek government. Sarkozy is defeated in the first round of the French presidential election. Hollande (PS) and Le Pen (FN) are the two run-off candidates.
2012, May: French voters, skeptical of Hollande's pledge to return French troops from Greece, elect Marine Le Pen. Le Pen, upon assumption of office, orders an immediate evacuation of French troops from Greece. The Greek government collapses and is replaced by an "interim" military administration. Greece withdraws from the frail European Union and declares bankruptcy. France quickly follows suit and issues francs at 1:1 to the euro and leaves the EU. Germany, unwilling to foot the bill for the other eurozone states, follows France and reissues the mark at 1:1 too, but tries to in vein rescue the EU. Britain leaves the EU and is soon followed by Poland, Romania, and Bulgaria.
2012, June: The battered EU votes to dissolve itself in six months. The remaining eurozone currencies try to pick up the pieces. The Baltic states discuss a single currency but are unable to reach an agreement. The United States, thankfully, has avoided intervention altogether. Life goes on.