AFGHANISTAN was supposed to be Russia's Vietnam. Behind our quickly receding outrage at Russia's invasion over four years ago was a certain smugness. The Afghan guerrilla fighters' record was legendary. Pictures of hardy warriors, old rifles in hand, looking out over rough mountain terrain, appeared in virtually every news publication.
There were mountains instead of jungles, but it was a sophisticated Army against a wily native force. And we all knew how that would turn out.
Despite the recent flurry of reports from disaffected Russian defectors, it is not working out as we expected. The Afghans are indeed experienced guerrilla fighters and deeply nationalistic. They are, however, up against an enemy who not only understands them but who is not bound by the same rules that America chose to use in Vietnam.
The differences between the American approach to Vietnam and the Russian approach to Afghanistan are best understood in the light of standards and definitions established in the 1820s by German military philosopher Karl von Clausewitz.
Clausewitz described how a guerrilla force could be successful against a conventional army. According to his book ''On War,'' the conditions ''under which alone a people's war can become effective'' are these:
* That the country is of a broken and difficult nature, either from being mountainous, or by reason of woods or marshes, or from the peculiar mode of cultivation in use.
* That the war is carried on in the heart of the country.
* That the theater of war embraces a considerable extent of country.
* That it cannot be decided by a single catastrophe.
* That the national character is favorable to the measure.
The situations in both Vietnam and Afghanistan would appear to fit these criteria. In addition, the Viet Cong and North Vietnamese adhered strictly to Clausewitz's detailed conditions for winning a people's war while the Afghans appear to be instinctively following the same principles.
The difference lies in the fact that the Russians too have read Clausewitz. In fact, his dictums (the most famous of which is ''War is a continuation of politics with an admixture of other means'') have been an integral part of all communist military and military-political theory since the time of Lenin, who was an enthusiastic disciple of Clausewitz.
In the chapter previous to the one quoted above, Clausewitz described how to defeat a people's war. He entertained ''no exaggerated ideas of the omnipotence of a people's war.'' The primary thrust of the chapter is that in order to defeat a people's war, you must establish an atmosphere of terror. He stated that ''it is an undeniable fact that a people's war cannot be kept up in an atmosphere too full of danger.''
It is apparent that the Russian policy in Afghanistan is based on that crucial chapter American military and political leaders, consciously or otherwise, ignored in Vietnam.
In Afghanistan, the Russians are not only willing to establish the atmosphere of terror, it is doctrinal for them to do so. This is the important and consistent lesson available in the stories of the defectors. The Russian use of ''yellow rain,'' the scattering of toy-shaped bombs in villages, and the executions of civilians are not aberrations. A Russian counterpart of Lt. William L. Calley Jr. would be acting in accordance with the game plan.
In addition to a strong theoretical base for their conduct of the war, the Russian military-political bureaucracy has the patience - and, thanks to a controlled press, the time - necessary to wage and win a counterguerrilla war.
Barring an organized and sizable injection of support from the West, there is a very real danger the outcome in Afghanistan will be different from that in Vietnam.
Unless the current balance of power is changed, the Afghans may find that victory is unattainable and that accommodation is necessary for survival.