Paul Krugman writes: Suddenly, it has become easy to see how the euro — that grand, flawed experiment in monetary union without political union — could come apart at the seams. We’re not talking about a distant prospect, either. Things could fall apart with stunning speed, in a matter of months, not years. And the costs — both economic and, arguably even more important, political — could be huge.

This doesn’t have to happen; the euro (or at least most of it) could still be saved. But this will require that European leaders, especially in Germany and at the European Central Bank, start acting very differently from the way they’ve acted these past few years. They need to stop moralizing and deal with reality; they need to stop temporizing and, for once, get ahead of the curve.

I wish I could say that I was optimistic.

The story so far: When the euro came into existence, there was a great wave of optimism in Europe — and that, it turned out, was the worst thing that could have happened. Money poured into Spain and other nations, which were now seen as safe investments; this flood of capital fueled huge housing bubbles and huge trade deficits. Then, with the financial crisis of 2008, the flood dried up, causing severe slumps in the very nations that had boomed before.

At that point, Europe’s lack of political union became a severe liability. Florida and Spain both had housing bubbles, but when Florida’s bubble burst, retirees could still count on getting their Social Security and Medicare checks from Washington. Spain receives no comparable support. So the burst bubble turned into a fiscal crisis, too.

Europe’s answer has been austerity: savage spending cuts in an attempt to reassure bond markets. Yet as any sensible economist could have told you (and we did, we did), these cuts deepened the depression in Europe’s troubled economies, which both further undermined investor confidence and led to growing political instability.

And now comes the moment of truth.

Greece is, for the moment, the focal point. Voters who are understandably angry at policies that have produced 22 percent unemployment — more than 50 percent among the young — turned on the parties enforcing those policies. And because the entire Greek political establishment was, in effect, bullied into endorsing a doomed economic orthodoxy, the result of voter revulsion has been rising power for extremists. Even if the polls are wrong and the governing coalition somehow ekes out a majority in the next round of voting, this game is basically up: Greece won’t, can’t pursue the policies that Germany and the European Central Bank are demanding.

Nouriel Roubini writes: The Greek euro tragedy is reaching its final act: it is clear that either this year or next, Greece is highly likely to default on its debt and exit the eurozone.

Postponing the exit after the June election with a new government committed to a variant of the same failed policies (recessionary austerity and structural reforms) will not restore growth and competitiveness. Greece is stuck in a vicious cycle of insolvency, lost competitiveness, external deficits, and ever-deepening depression. The only way to stop it is to begin an orderly default and exit, coordinated and financed by the European Central Bank, the European Commission, and the International Monetary Fund (the “Troika”), that minimizes collateral damage to Greece and the rest of the eurozone.

Greece’s recent financing package, overseen by the Troika, gave the country much less debt relief than it needed. But, even with significantly more public-debt relief, Greece could not return to growth without rapidly restoring competitiveness. And, without a return to growth, its debt burden will remain unsustainable. But all of the options that might restore competitiveness require real currency depreciation.

The first option, a sharp weakening of the euro, is unlikely, as Germany is strong and the ECB is not aggressively easing monetary policy. A rapid reduction in unit labor costs, through structural reforms that increased productivity growth in excess of wages, is just as unlikely. It took Germany ten years to restore its competitiveness this way; Greece cannot remain in a depression for a decade. Likewise, a rapid deflation in prices and wages, known as an “internal devaluation,” would lead to five years of ever-deepening depression.

If none of those three options is feasible, the only path left is to leave the eurozone. A return to a national currency and a sharp depreciation would quickly restore competitiveness and growth.

Of course, the process would be traumatic – and not just for Greece. The most significant problem would be capital losses for core eurozone financial institutions. Overnight, the foreign euro liabilities of Greece’s government, banks, and companies would surge. Yet these problems can be overcome. Argentina did so in 2001, when it “pesofied” its dollar debts. The United States did something similar in 1933, when it depreciated the dollar by 69% and abandoned the gold standard. A similar “drachmatization” of euro debts would be necessary and unavoidable.