With regard to The Post's Dec. 13 editorial and Anthony Davis' subsequent story {Dec. 21}, certainly the Soviets deserve to be condemned for their invasion of Afghanistan. And certainly we can claim some of the credit for turning the invasion into a bloody stalemate and major propaganda disaster. But it won't help either us or the Afghans if we start to believe and act upon the myths we've created in the process.

The primary myth is that the Soviets sent their troops into Afghanistan in December 1979 to gobble up a small, peaceful, unoffending neighbor. Few seem to remember that they actually marched in to shore up an already existing communist regime, one that -- with no help from the Soviets -- had taken power almost two years earlier by overthrowing the right-wing dictatorship that deposed King Zahir Shah. I have heard a senior U.S. diplomatic official say flatly that the Soviets not only didn't order that coup but probably didn't even have advance word of it.

The regime was a wretched one; it hopelessly botched things, pressing such wildly unpopular reforms as women's rights. As it began to collapse, the Soviets moved in to prevent anticommunist fundamentalists from taking over -- in the process executing the Afghan communist who had done the most to create the mess. Totally unjustified, to be sure, but doesn't it have a familiar ring? Change a few names and circumstances -- Hafizullah Amin for Ngo Dinh Diem, for example -- and you have a parallel to the events that brought us into Vietnam in 1965.

We'll all be delighted when, inevitably, the Soviets withdraw from Afghanistan. But anyone who thinks this will permit the Afghan people to sit down peacefully and choose their own government simply doesn't know Afghanistan. Soviet withdrawal will be followed by a continuation of the bloodbath, as the half-dozen rival mujaheddin factions happily use their shiny U.S.-supplied weapons to slaughter one another -- an improvement over continued Soviet occupation, but hardly a millennium for the Afghan people. EDWARD C. INGRAHAM Bethesda

The writer, a retired Foreign Service officer, served in Pakistan (1971-74).