So why should Americans care about any of this? The first reason is that, with American consumers still in the doldrums of deleveraging, the United States badly needs buoyant exports if its economy is to grow at anything other than a miserably low rate. And despite all the hype about trade with the Chinese, U.S. exports to the European Union are nearly three times larger than to China…

But there’s more. Europe’s problem is not just that governments are overborrowed. There are an unknown number of European banks that are effectively insolvent if their holdings of government bonds are “marked to market”—in other words, valued at their current rock-bottom market prices. In our interconnected financial world, it would be very odd indeed if no U.S. institutions were affected by this. Just as European institutions once loaded up on assets backed with subprime U.S. mortgages, so most big U.S. banks have at least some exposure to eurozone bonds or banks. One institution—MF Global, run by former Goldman Sachs CEO Jon Corzine—just blew up because of its highly levered euro bets. Others are biting their fingernails because it is suddenly far from clear that the credit default swaps they have bought as insurance against, say, a Greek default are worth the paper they are written on.

But the third reason Americans should care about Europe is more important even than the risk of a renewed financial crisis. It is the danger that what is happening in Europe today could ultimately happen here. Just a few months ago, almost nobody was worried about Italy’s vast debt, which amounts to 121 percent of GDP. Then suddenly panic set in, and Italy’s borrowing costs exploded from 3.5 percent to 7.5 percent.