It is 10 years this March since the last American troops left Vietnam. Within a decade, other wars get summarized and more or less satisfactorily catalogued by history so that their survivors can live in some measure of peace with some measure of honor. Not Vietnam. Vietnam remains a tangle of barbed question marks.
An interpretation in a CBS documentary has stung Gen. William C. Westmoreland to sue the network - for defamation of the war itself, it almost seems. Even the raising of the Vietnam War Memorial - the ultimate ritual for making the present accepted as the past - proved to be an ambivalent gesture. The monument became part of the controversy.
There was something deeply affecting about the Vietnam veterans who assembled along the Potomac last Veterans Day to run their hands over the nearly 58,000 names on the black granite wall and spontaneously sing whatever they could sing, from ''America, the Beautiful'' to ''Amazing Grace.'' But the ceremonies improvised by these pilgrims for fallen comrades did not finally resolve the tensions either.
Meanwhile, in the universities and elsewhere the industry of revisionism goes on. The New York Times magazine recently made ''The New Vietnam Scholarship'' its cover story. The scholarship pronounces the Vietnam war ''more successful than was recognized at the time,'' and ''no more unpopular than the Korean war'' - until the casualties began to climb. The new scholarship declares the United States failed because it fought ''on the cheap,'' attempting to ''nibble the bullet rather than bite it.'' The new scholarship denies that Vietnam was an act of hubris - a mistake of political and military arrogance, tragically paid for. The new scholarship calls Vietnam ''the most misunderstood war'' and ''morally ambiguous'' - ''a war nobody won,'' but a war that might have been won if only we had thrown a little more fire in the lake, one more time.
If the new scholarship makes the old scholarship seem a bit simplistic, it falls short of convincing us. We are left wondering: a ''cheap'' war, a ''successful'' war compared to what?
It's all a bit like seeing as a mad double bill Jon Voight in ''Coming Home'' and John Wayne in ''Green Berets.''
The war can hardly be less confusing from a Vietnam perspective. A war correspondent of 15 years ago, H.D.S. Greenway, returned to the scene just about the time the veterans were gathering in Washington at the base of the new memorial. He saw the hulks of American personnel carriers rusting along the roadside and the camouflaged skeletons of American planes on the edges of nearly abandoned airfields. He visited a network of old Viet Cong tunnels - now a tourist attraction, complete with guides.
The grass grows over battlefields, even in defoliated Vietnam. But does the perspective grow clearer?
There are terrible ironies here. According to the thoroughly disillusioned Truong Nhu Tang, once a minister of justice for the Viet Cong Provisional Revolutionary Government, 10,000 Soviet ''advisers'' are now stationed in Vietnam, replacing the Americans before them, and the French before them. From his exile in Paris, he writes for the pages of the New York Review of Books: ''I shared the romantic notion that those who had fought so persistently against oppression would not themselves become oppressors.''
And so we cannot help asking ourselves: What is the lesson of Vietnam? If only we could answer, we could put this haunting war behind us at last.
When the Vietnam War Memorial was dedicated, Defense Secretary Caspar Weinberger said the lesson was that America must never again fight a war it does not intend to win.
Truong Nhu Tang, surveying the cycle of foreign and domestic conquerors who have torn apart his land, says the lesson is that such things get worse and worse. ''It is a lesson,'' he concludes, ''that must eventually move the conscience of the world.''
The ''lessons'' - and one hears so many of them these days! - vary in their wisdom and their tendency to be self-serving in one way or another. But Tang's lesson seems to cut wider and deeper than most. As long as we allow ourselves to believe that some clever diplomatic coup or some extraordinary military tactic would have made the difference, we have not learned all there is to learn about Vietnam - or about history.