Citizens of the eurozone countries didn't know when they formed the monetary union that they were not only losing their sovereign and democratic rights to control their most important macroeconomic policies. They had also ceded this power to people with an anti-social-Europe agenda. Now Greece is trying to get some of that democracy back.
Restated for the 21st century, "socialism" simply means that a people's judgments about its own economic life come before the supposedly iron rules of the international economy. It would be fair to call it "economic democracy." The condescending view of the Greeks as somehow not understanding economic reason and the direction of history writes off this kind of economic democracy as infeasible, archaic, and probably senseless. Syriza's government has a chance to reverse the lens. Economic democracy (or, as Syriza calls it, socialism) is politics that puts human needs first and accepts that market-based destabilization, impoverishment, and humiliation are not natural disasters or comeuppance for bad behavior but forms of political violence.
If Greece leaves Europe, the EU dream will be gone for good. Second, other European countries like Portugal, Spain and Ireland, also under the humiliations of debt, might imitate Greece and exit the EU. It's in the political interests of the U.S. to intervene decisively and prevent the break up of the Western world.