CAN IT BE that, more than eight years after they invaded Afghanistan, Soviet troops are to leave? Mikhail Gorbachev has set a starting date, by May 15. He promised to complete withdrawal in 10 months and to show the necessary good faith by taking big units out early. He put his personal stamp on Moscow's promise not to demand prior ''national reconciliation'' -- a condition that, in existing circumstances, could postpone withdrawal indefinitely. He reports, without detail, that ''mutual satisfaction'' has been attained on the issue of ending American aid to the resistance as Soviet troops depart.

Mr. Gorbachev says he is waiting only for Afghanistan and Pakistan to reach a peace agreement by March 15 under United Nations auspices. This condition requires careful inspection to make sure it is not used to nullify withdrawal. Moscow has conveyed an impression of some readiness to deliver its Afghan client regime to a U.N. settlement -- a settlement in which the regime would be largely on its own to compete, no doubt militarily as well as politically, for its place in the Afghan sun. But if Kabul drags out the U.N. talks, the suspicion is bound to arise that Mr. Gorbachev's withdrawal notice was simply a ruse to draw extra, unwarranted concessions from the resistance.

This leaves the sensitive question of whether Washington is contributing (perhaps with Pakistan) to deliver the resistance to a settlement. The charge has been made. In broad terms, the answer appears to be no. Rather, it has been the American responsibility, accepted by two presidents and successive Congresses, to sustain a brave people fighting to repel a foreign aggression. As the possibility of a good settlement and a Soviet withdrawal nears, it becomes the clear American interest to see that possibility fulfilled. Deference to Afghan guerrillas and refugees is central to American policy. It need not be inconsistent with encouraging them to take advantage of a Soviet initiative that, after all, removes the principal prop -- the Red Army -- of the detested Kabul regime.

The Soviet invasion was a moral monstrosity, a political adventure and a strategic blitz. Its termination, on terms that do not secure a communist regime or a Soviet lodgment, would amount to a tremendous reversal, one brought about by Afghan valor and by the support of a hundred nations. In a real withdrawal Mr. Gorbachev will be tacitly acknowledging and publicly accepting as much.