“If Europe does for Greece and potentially other peripheral countries what West Germany did for East Germany, it will cost dearly, politically and economically,” a senior European diplomat said, speaking on the condition of anonymity because of the sensitivity of the issue.

But if it does not and Greece leaves the euro zone, the diplomat added, the cost could be even higher. Past breakups of currency regimes led to messy transitions, often involving bank runs, capital flight, emigration and some measure of default.

When Austria-Hungary collapsed in 1918, after World War I, and with it a currency zone covering part of today’s euro zone, the new governments of the region created national currencies by simply stamping the Austro-Hungarian krones circulating in their country with a national marking. Armed troops patrolled the borders to stop people from ferrying krones to the country they thought would have the strongest currency to get the most valuable stamp.

In 2012, much of the conversion back to Greek currency, the drachma, would happen electronically, during a banking holiday that would temporarily freeze online transfers out of the country, but the borders would still have to be sealed to prevent people from smuggling euros out of Greece after the devaluation has taken place, an awkward undertaking in postwar Europe. A substantial default on Greece’s public and private debt would almost inevitably follow: the value of Greek liabilities would surge overnight as the revived currency would trade at an estimated 50 percent to 80 percent discount to the euro, economists say.

The lesson from the United States’ moving away from the gold standard in 1933 and from Argentina’s abandoning its dollar-peg in 2001, said Nouriel Roubini, a professor of economics at New York University, is that Greece’s euro debts — public and private — would have to be “drachmatized.”

After depreciating the dollar by 69 percent, the U.S. Congress voted to invalidate any promise to pay debt in a unit referenced in gold. Argentina “peso-fied” not just the government’s dollar liabilities but also those of banks and companies, in effect decreeing a private sector default, without which much of the economy would have been bankrupted.

Another lesson from the Argentine case, said Simon Johnson, a professor of economics at the Massachusetts Institute of Technology, is that if you’re planning to leave a currency regime do it sooner rather than later because the costs are likely going to rise.