NO SOONER had Moscow announced that it is withdrawing some of its forces from Afghanistan than the Carter administration announced that if the Soviet Union will remove them all, the United States will help it out the door by supporting arrangements that protect its "legitimate security interest." This display of solicitude has its gratuitous and curious sides: gratuitous in that the United States always has been ready to facilitate a Soviet withdrawal and curious in that any discussion of modes of withdrawal tends to credit Moscow's highly dubious pullback announcement. But no matter: the West's need is not to keep the issue of Soviet aggression alive for purposes of alliance mobilization and propaganda but to end it -- the "it" being the aggression.

Is this likely, or even possible? One point has to be kept front and center in any discussion of rolling the clock back to say, 1978 and restoring Afghanistan as a neutral buffer state. The thrust of the American proposal is that at the end of whatever "face-saving" process is set in motion to cover a Soviet withdrawal, the Afghan people will have a government of their own choice. This has to be seen against unlikelihood that the Afghan people would choose the government currently being imposed on them by Soviet tanks. The thrust of the Soviet Union's countering proposal, however, is that Soviet troops will go home only in a context of foreign acceptance of and foreign guarantees for a regime like the one now sitting, uneasily, in Kabul. One does not have to believe that the Kremlin knew exactly what it was doing when it invaded -- that it intended either to stabilize a border region, expand the socialist camp or march a step closer to the Persian Gulf -- to understand that it has a heavy investment in making its invasion stick. Certainly there should be no sanguine expectations that its considerable losses, in casualties and in diplomatic consequences, are inducing it to reverse field.The safest bet is that Moscow, like Washington, wants to see what the traffic will bear.

A prediction: it will not be long before some, in Europe and elsewhere, will start murmuring, and then saying aloud, that since Moscow has accepted the principle of withdrawal, the West should relax its pressures in order to "let diplomacy work." But, of course, this is the way to let diplomacy down. Moscow should not be paid off for its opening feint. This is a time to keep the heat on and, meanwhile, to make it clear that the crisis lingers only because the Soviet Union refuses to explore the "decent and constructive" solution that the United States now proposes -- one providing the Afghans with a government of their choice and the Soviet Union with security on its border.