Here is France's newest trade policy. In October it decided that all videotape recorders imported from Germany, Japan and the Netherlands should pass through a tiny customs post ill-equipped to handle large volumes. Give the French and A for ingenuity, but not for originality.

Protectionism is engulfing the postwar trading system. World trade stagnated last year and probably is declining now. When trade ministers meet in Geneva on Nov. 24, 25 and 26 under the auspices of the General Agreement on Tariffs and Trade, they will have difficulty repairing the damage. Indeed, they will do well to prevent further deterioration.

What is vanishing is America's postwar vision of political prosperity. In the 1940s, the economic warfare of the Depression was widely thought to have prolonged the slump, encouraged fascism and contributed to the onset of World War II. The postwar trading system aimed to minimize economic conflicts between nations. It made prosperity the bedrock of alliance.

Now the converse is true: Trade disputes poison political relations. The United States and its European allies wrangled over the Soviet natural gas pipeline for months. Japan seems locked in perpetual conflict with Europe and America on trade. Developing countries, suffering from sharp price declines of commodity exports, feel increasingly victimized.

Not that restrictions account for most of the trade slump. Last year's stagnant trade contrasts sharply to the roughly 8 percent annual growth rate between 1950 and 1975 and the 5 percent rate between 1975 and 1979. But most of the decline apparently stems from the recession itself.

Economists C. Fred Bergsten and William R. Cline of the Institute for International Economics find that trade grows disproportionately in an expariding world economy and contracts disproportionately in a slump. But the danger now is that protectionism will accelerate this downward spiral.

The immutable reality of trade, of course, is that countries need to export in order to import. If they can't sell, they can't buy. If the Japanese don't sell cars, steel and electronic goods, they can't buy American wheat, soybeans and timber. In turn, American farmers and mills won't buy new tractors and saws.

The postwar trading system -- institutionalized in the GATT -- recognized this logic by establishing rules governing when countries could invoke trade restrictions. The mind-boggling complexity of these rules is almost beside the point, because the GATT never achieved its ostensible purpose: creating a court to settle disputes according to universal rules.

What it fostered instead was a climate of self-restraint, and it was on this self-restraint (rather than rules) that the system rested. Not anymore: Self-restraint is collapsing. Worldwide recession has made countries quicker to adopt restrictions. The main reason these restrictions don't seem to have affected the overall volume of trade yet is its very magnitude.

Last year, roughly one-fourth of the output of U.S. goods (not services) was exported. This included one-third of the printing presses, one-sixth of the pipes and valves and one-third of the power generators. Ratios elsewhere are even higher: France (57 percent), Germany (60 percent), Japan (30 percent) and Canada (87 percent).

So far, protectionism in steel, autos, electronics and textiles has barely impeded these huge flows. The high export ratios -- and comparable import ratios -- reflect the very success of the postwar trading system and, in particular, strong intra-European and U.S.-Canadian trade.

But the paradox of so many countries being thrown together in commerce is that, sooner or later, they were bound to notice their differences. And these perceptions undermined the trading system's political basis.

Once this starts, it becomes a snowballing process. When one industry wins protection, others imitate. When one country does it, others feel justified in following suit. The GATT is simply side-stepped.

What compounds this problem is the plight of less-developed countries. Nearly two-fifths of American exports go to developing countries. A collapse in this demand would retard recovery further and erode the trading constituency.

What to do?

Bergsten and Cline suggest that the GATT meeting halt the drift toward protectionism by declaring a "standstill" on new trade barriers. The best argument for this is that trade complaints usually mask more fundamental problems. In the United States, industries suffering from foreign competition -- steel, autos -- have acute internal problems: high labor costs, short-sighted management, a disinclination to face new competition.

But Bergsten, an assistant Treasury secretary in the Carter administration, surely understands the Long odds on his proposal. Carter's vice president, Walter F. Mondale, is plugging protectionism. Mondale used to be a free trader. His conversion is symbolic: A system designed to promote stability now seems to be doing just the opposite.