Although the Soviets talked of pulling out of Afghanistan for more than six years, they suddenly became serious about it one year ago, and U.S. diplomats say they believe U.S. anti-aircraft "Stingers" helped lead to a negotiated settlement of the war.

The easy-to-carry, surface-to-air missiles not only were a technological breakthrough in the war, but they were a visible and painful symbol to the Soviets that there was a major U.S. commitment to supplying the Afghan rebels with whatever was required to fight the Soviets.Administration officials have never acknowledged the size and the details of the U.S. covert military supply operation for Afghan rebel groups, but they are willing to discuss it privately and it is no secret to the Soviets.

One State Department official, aboard the plane that took Secretary of State George Shultz to Geneva for the signing of the Soviet pullout agreement, said the shoulder-fired heat-seeking missiles were the difference, costing the Soviets 400 aircraft and crew. The missiles were accurate enough to force the Soviets to change their low-level tactics, and fly at higher altitudes, where the bombing was less effective.

Although other surface-to-air missiles have been used for 20 years, the Stinger was revolutionary because of its accuracy and sensitivity to even minor sources of heat.

At first, it was thought the Stinger, which involves a small battery-powered refrigeration system to chill the missile just before firing, was too complex for the unsophisticated rebel groups to use.

But the CIA argued it was no more difficult than the priming and loading of a muzzle-loading musket.

The weapons, without American markings, began showing up in Afghanistan abouttwo years ago, and the results were immediate: the destruction of an average of one Soviet aircraft a day.

The supply operation became semipublic last year. As a low-key but direct message to the Soviets, the U.S. identity on the missiles was displayed, making it appear to both the Soviets and the Afghan rebels that the United States was prepared to increase steadily the cost to the Soviets for their Afghan adventure.

The Afghan war has cost an estimated 20,000 Soviet lives and those of another 1 million Afghans since the December 1979 Soviet invasion.

The cost of that U.S. support has never been publicly declared, but congressional sources have put it at more than $600 million a year.

The Stingers have an electronic identification system built into them, which means they will not home in on a "friendly" American military aircraft that puts out a radio signature. But that would not protect a civilian airliner. At least two Soviet passenger planes have been shot down by ground-fire in Afghanistan, apparently by Stingers.