The political costs are also rising. In parliamentary elections earlier this month, Greece’s voters rejected candidates from the two major political parties that had agreed to a German-dictated “rescue” package, and the country has been unable to form a government since. In that vote, the far-right party, Golden Dawn, whose xenophobic members perform Nazi salutes, did frighteningly well — a warning that no responsible political leader in Europe can afford to ignore.

Meanwhile, the unthinkable becomes increasingly imaginable: Greece fails to meet the conditions of its bailout and drops out, or is forced out of the euro zone. The financial chaos could quickly spread, spooking investors and destabilizing the banks and economies of other struggling European nations, with knock-on effects for the global financial system and the world economy.

We take no comfort in recent reports that European finance officials have been preparing contingency plans for Greece’s exit from the euro, or in the proclamations of Germany’s central bank that the effects of a Greek exit would be manageable. Let’s remember that in 2008, American officials also believed that they and the markets were prepared for the collapse of Lehman Brothers, though the global credit crunch that ensued quickly disabused them of that notion.

The financial system is no less interconnected now, and the weakened European and American economies are more vulnerable to shocks.

Ideally, Europe’s leaders would recognize that growth measures are crucial to resolving the debt crisis — giving struggling economies a chance to recover even as the euro-zone nations work toward strengthening European institutions for political and fiscal integration. A sensible plan for euro-zone members to jointly issue bonds — championed by Mr. Hollande and the International Monetary Fund — would be coupled with a mechanism for strict monitoring of sovereign budgets.