"Since shortly after the first days of liberation, escape by boat had been the single great topic of conversation throughout the South."

These words, about Saigon's liberation in 1975, have the appearance of a deliberate irony: One's first thoughts after liberation should not be of escape. These words, however, are the earnest reflections of Truong Nhu Tang, not an ironist, but a founder of the Vietcong and minister of justice of its Provisional Revolutionary Government.

Exactly how provisional that government was to be, he -- and we -- were not to know until after the "liberation," at which point Tang realized to his horror what had happened. South Vietnam had been conquered, then annexed, by the armies of the North. The South's revolutionaries were shunted aside, then repressed, and finally consigned to the gulag or, like Tang, to the South China Sea.

Such are the consequences of liberation. These consequences, and the other crimes of Indochina's victors, have had a remarkable effect in America. They have restored moral ambiguity to the Vietnam war. Ten years ago we were all so certain. "What future possibility could be more terrible than the reality of what is happening in Cambodia now?" wrote Anthony Lewis on March 17, 1975. The Khmer Rouge were quick to answer. Exactly one month later, they captured Phnom Penh. Within two days, they had ordered the evacuation of the city. Within four years, they had killed one in three Cambodians.

Going from moral certainty to moral ambiguity is not easy. The new Vietnam revisionism has met resistance. The resistance has generally taken two forms.

The first line of defense was to deny the facts. True, itis not easy to deny the murder of 2 million people, but that will not stop some from trying. This is from a front-page review by Richard Dudman (of William Shawcross' book on Cambodia) in The New York Times Book Review, April 22, 1979: "Unfortunately, Mr. Shawcross accepts uncritically the testimony of Cambodian refugees about life . . . under the Pol Pot regime." He "passes along" the story of the Cambodian holocaust "based on interviews with only a thousand refugees." Only a thousand interviews! Versus a mere thousand, Dudman counterposes the testimony of one American -- himself: "Two weeks of recent eyewitness observation inside Cambodia indicated that food, clothing and shelter were adequate under the strange regime and that working conditions, while hard, seemed by no means intolerable."

The second line of defense: When it was no longer possible to deny the facts, explain them away. Search for extenuating circumstances, preferably American circumstances. Speculate that the Khmer Rouge were driven to their barbarities by the war, or the privation, or the massive bombing (a subtle theme of "The Killing Fields," for example).

However much we Westerners pride ourselves in the belief that the natives must learn their cruelty from us, that is not the view of an observer closer to the scene, Cambodia's Prince Norodom Sihanouk. He was recently asked how the Khmer Rouge could have done what they did. He answered by quoting Khieu Samphan, leader of the Khmer Rouge: "We want to have our name in history as the ones who can reach total communism with one leap forward. So we have to be more extremist than Madame Mao. . . . We want to be known as the only communist party to communize a country without a step-by-step policy, without going through socialism."

So they went through genocide instead. They murdered by the class: merchants, intellectuals, civil servants, teachers, doctors, princes and bourgeois alike. This was nt, as some would like to pretend, simple madness. It had a logic. The Khmer Rouge, explained Sihanouk, saw their mission as serving the poorest class. And that meant having "to liquidate all the enemies of the people, that is to say all the enemies of the poorest."

The logic, Sihanouk advised, is to be found in the library of the Sorbonne, in Khieu Samphan's book, where the idea of uprooting the cities and eliminating class enemies is laid out. It was written not under a rain of B52 bombs, but in the bourgeois ease of his student days in Paris. It was his doctoral thesis. "May I add this," said Sihanouk in a final attempt to explain Khieu. "There are people like Hitler. . . . There are cruel people." Hardly news. But 10 years ago, if those words were spoken at all, they were directed not at our adversaries in Indochina. They were directed at us.

The crimes of the victors do not necessarily vindicate the war. There is still the question of means, the immensely destructive means required by a war that we had only the most uncertain chance of winning. The ends may still not justify such means. But such a judgment has lost its obviousness. It is now more problematic than ever.

The moral calculus of the war, once so clear, has shifted. It continues to shift all the time, because, while the toll of the war ended 10 years ago, the toll of the peace mounts daily with every dry- season offensive in Cambodia, with every boat lost in the South China Sea. That shift -- and the moral revisionism it has forced -- began 10 years ago this month, when Indochina was liberated and all thoughts turned to escape.