PARIS -- The Soviet Union needs American help to carry out the orderly withdrawal it desires from its war against Afghanistan. As soon as American aid to the Afghan mujaheddin stops, Soviet withdrawals will begin. Nothing else stands in the way of a settlement.

That is the message that Soviet diplomats are transmitting to Washington and to other capitals as the most serious diplomatic effort yet undertaken to end the war begins. But there is an important section of small print in the Soviet formulation that suggests that this is more diplomatic strategy than a genuine negotiating "offer," and that peace is probably not at hand.

"We will leave Afghanistan, but we will not leave clinging to the skids of helicopters lifting off the roof of our embassy," a Soviet official said with a hint of defiance in his voice in Washington a few weeks ago while Mikhail Gorbachev was out winning the hearts and minds of Connecticut Avenue. I found it more revealing than Gorbachev's own comments about the war.

The choice of images to represent or to obscure reality is nearly as important in diplomacy as it is in poetry. Both arts depend on the power of suggestion to convey complex meanings that disappear when reduced to a literal formula. In using Vietnam as an analogy to Afghanistan, the Soviets seem to emphasize the double-edged nature of a strategy that still boils down to leaving on their own terms if they leave now.

By denying that they will submit to the sort of humiliation that Americans experienced in Saigon in 1975, the Soviets implicitly concede that such an outcome in Afghanistan is now an idea that has to be confronted, and avoided.

The Vietnam reference is thus a measure of Moscow's awareness that the military situation on the ground in Afghanistan is shifting to stalemate or worse, a view that has to be reinforced by the battles around the garrison town of Khost. The Soviet Army has shown this week that it can keep the road to Khost open, but only at the price of a major offensive.

It is possible that Khost could represent a Soviet version of the gunslinger backing out the saloon door with both guns blazing as a way of getting out of town alive. But placed against the background of the rejection of the Saigon syndrome, it suggests instead a Soviet determination to keep on fighting to preserve its client forces, even in marginal situations.

Analysis of the results around Khost will feed a muted debate that has been going on in western capitals about Soviet intentions in Afghanistan since the Washington summit. For the optimists, Gorbachev provided new hope that a settlement can be reached in the final year of the Reagan administration by personally committing himself in Washington to a Soviet withdrawal within 12 months or less -- if the United States shuts off the arms flow to the guerrillas.

This helped trigger the trip by Undersecretary of State Michael Armacost to Pakistan this week, as Soviet Foreign Minister Eduard Shevardnadze flew to Kabul on an undisclosed mission.

The Soviets have positioned themselves to follow a double-track strategy. If a retreat "with honor" can be arranged, the cost in Afghanistan has become so high that the Soviets would take it. If it cannot, they will fight on and blame the Americans for making them stay by refusing to cut off supplies to the mujaheddin.

What does "honor" mean in this context? The Soviets seem to want an outcome that at a minimum keeps the mujaheddin from becoming a threat to the local forces that the Soviets would leave in place under an ineffective, politically mixed central authority. They would stage their withdrawal valley by valley, intensifying Afghanistan's fragmentation.

While admitting that their costs are high and growing, the Soviets do not seem to be hurting enough -- yet -- to make the kind of true withdrawal that the Reagan administration rightly demands. The next U.S. president is likely to be in office before such a deal ripens.

The first step the next administration should take is to reverse Reagan's well-meaning but wrong decision to lump Afghanistan with Angola, Cambodia and Nicaragua, to be discussed with the Soviets as "regional conflicts."

The Soviet war on Afghanistan is a superpower invasion of a small neighbor and deserves to be singled out as such. By placing it alongside the other conflicts, the United States plays into the Soviet strategy of portraying the flow of weapons to guerrilla forces as the fundamental dynamic of the war in Afghanistan.