Nobody really knows what caused the Great Depression of the 1930s, but its specter is haunting us now. Will the financial meltdown spreading around the world lead to a re-run of the events that followed the Wall Street crash of October 1929? The answer to this question lies ahead of course, but the clues we have are unsettling.

"History," said Henry Ford, the great American automobile magnate, "is bunk," and he was right in that it rarely if ever repeats itself. But we can still learn valuable lessons from it, and the events of 80 years ago need to be at the front of politicians' minds as they grapple with a financial crisis that threatens to turn into a worldwide economic slump.

The crucial point is that even though historians argue over the complex causes of the Great Depression, they generally agree that it wasn't the Wall Street crash itself. Dramatic as that was, the panic in which corporate shares collapsed and financiers leapt from skyscraper windows was a fairly short-lived affair. By the early 1930s American capitalism was back in business and forging ahead once more. Wall Street had, after all, suffered similar shakeouts every 20 years or so since the 1840s.

One of the major reasons that America's 1929 financial crisis turned into a global economic slowdown was shortsighted policymaking, first by the U.S. Congress and then by America's main trading partners. The Smoot-Hawley tariff introduced in 1930 by a Republican senator from Utah and a fellow-Republican congressman from Oregon to protect the United States from both farm and industrial imports triggered a global trade war. Britain retaliated with its system of imperial preferences. The protectionism that spread rapidly made scores of millions jobless, homeless and hungry across the planet.

The law of unintended consequences quickly made itself felt, with the United States arguably hit hardest by its own misguided policy. American banks had lent generously to European industries after World War I, and when the latter were denied their U.S. export earnings they could no longer service their debts. By 1933 almost 9,000 U.S. banks had failed.

Now even more than then, thanks to globalization, international trade is the key to economic health everywhere. The failure this summer of the Doha Development Round of trade liberalization negotiations between the 153 countries in the World Trade Organization could signal the onset of a new cycle of protectionism. And if the world's great trading nations begin to erect barriers to one anothers' goods and services, then they'll be doing far more damage to the global economy than any financial crisis ever could, however enormous the banks' and investment houses' recent losses.

If there is to be a resumption of tit-for-tat protectionism, then the signs are that it will start in the United States. The perception among many American voters is that "unfair" competition from low wage countries in Asia is robbing the U.S. of jobs. Similar resentments in Europe have of late seen the European Union resorting to anti-dumping actions against allegedly underhand practices in China and Vietnam.

The seeds of a fresh round of protectionism may already be sown, yet no economies benefit more from booming international trade than those of the rich countries. American labor unions complain about the 7 million jobs lost every year to foreign competition, but in fact the last 12 years have seen a net jobs gain in the United States with an extra 23 million people employed.

The reason isn't hard to find. America and Europe have the know-how and the retail markets that are the main drivers of new industrial bonanzas like that of China. On average, only 15 cents in every dollar of Chinese exports' added-value stays in China, and for goods like laptop computers and other electronic gizmos that share can drop as low as 3 to 4 percent. Experts talk of the "smiley curve," in which the upward parts of the smile are a product's design cost, brand value and retail profit and the lowest portion of the curve the actual manufacturing process.

With luck, the weeks and months ahead will see containment of the panic that is making global financial markets so dangerous and unpredictable. Still more luck as well as good judgement will be needed to stop the crisis spreading to world trade. Let's hope the policymakers have learned the lessons of the "hungry thirties."

This article was published in the International Herald Tribune on October 3.