THREE YEARS AGO, when the Cambodians and Vietnamese marched into Phnom Penh and Saigon, the prospect of another Indochina War was unimaginable. The victors had fought as allies, albeit wary ones, in civil wars which exhausted their people and tore apart their countries. Yet last winter and throughout the spring their armies have been battling over their common border and the conflict has been brutal. Reports and photographs from Vietnam have shown civilians disemboweled and dismembered, villages razed and fields burnt to cinder.

The theories immediately adduced to explain this war touch on ideological differences between on two regimes and especially on the Cambodians historic fear that the Vietnamese will overrun their neighboring provinces as they did after the fall of the Angkor empire. But these explanations are not enough. Hindering all understanding is our basic ignorance of Democratic Kampuchea, the new name of Cambodia; its policies on the border war are as mysterious as everything that has happened in that country since April 17, 1975.

Cambodia: Year Zero by Francois Ponchaud gives the first serious picture of the new Cambodia and it is an unsettling one: "I was compelled to conclude, against my will, that the Khmer [Cambodian] revolution is irrefutably the bloodiest of our century . . . A terrifying one for all who have any respect for human beings."

Ponchaud is a French cleric who lived 10 years in Cambodia and mastered its language and culture. For his book he carefully pieced together refugee accounts - 94 from literate Khmers (32 of whom wrote their stories in Khmer) and hundreds from his own interviews with illiterate Cambodians of the laboring classes - with official documents from Democratic Kampuchea and Radio Phnom Penh broadcasts he monitored.

Ponchaud begins by explaining his initial impulse of hope: "Having lived with the Cambodian peasants from 1965 to 1970, I was painfully aware of their exploitation at the hands of the administration under the corrupt Sihanouk regime. From 1970 to 1975 I shared the lives of the poor in the suburbs of Phnom Penh under the Lon Nol regime . . . So I welcomed the revolutionaries' victory as the only possible means of bringing Cambodia out of its misery."

The rest of the book is overtaken by his despair at what did happen. He details the mass exodus from the cities which was ordered by the Communists in the first days of peace. Cambodians of all professions, of every class - old, infirm and healthy alike - were sent to work in the countryside building cooperative farms and huge new water projects. They were placed on exhaustive work schedules and given little food. The conditions described in the book are plainly inhuman. Thousands died from starvation and disease; and thousands more were executed for their roles in the old Lon Nol government or capriciously, the refugees say, for disobeying "Angkar" (organization) as the new regime was known.

This "self-slaughter," as Ponchaud calls it, seems indisputable on the basis of the reports coming out of Cambodia. One incident conveys the mood of the book: It describes an execution which took place in October, 1975, and is told by a former clerk from pailin, a mining town in Western Cambodia. "We had loaded [a truck] and were about to turn around when we saw a military truck enter the forest, carrying about ten young men and girls. A moment later we heard shots, then the truck came back empty. We were very frightened, and harnessed up to go home. Then we heard moaning and somebody calling for help. One of our group . . . went over and saw a young man with bullet wounds in both arms and one thigh, and his arms still tied behind his bacm. That young man told me that the people who had been shot hadn't done anything wrong, they had simply gone to look for food in the forest, so they weren't working with their group. That's why they were killed.

Ponchaud claims that these executions were not accidents. "The liquidation of all towns and former authorities was not improvised, nor was it a reprisal or expression of wanton cruelty . . . But this total purge was, above all, the translation into action of a particular vision of a man; a person who has been spoiled by a corrupt regime cannot be reformed," he writes. Poncaud adds slogans from Radio Phnom Penh which he believes support his contention: "What is infected must be cut out . . . It isn't enough to cut down a bad plant, it must be uprooted."

The revolution Ponchaud describes is so radical that it is without precedent in Asia. He says it is dedicated to an immediate reordering of society, beginning at "year zero." It is thus quite unlike either the Chinese or Vietnamese revolutions, in which leaders used extensive political indoctrination and re-education camps to convince their people to accept new techniques and goals. The Cambodians have not done so, according to Ponchaud, because their flat refusal to believe that a man can be reformed - that "what is infected must be cut out" - precludes them from accepting the idea of re-education.

Their revolution is not only extreme, but thorough, touching every part of Cambodian life. Marketplaces, the stage for everyday joys in the old culture, were entirely eliminated and the new economy was set up as a barter system among cooperatives. Ponchaud contends that the Cambodian regime took Maoist slogans like "Capitalism is bad" and pushed them to the extreme, abolishing money altogether. In the same manner, the Cambodians interpreted the Maoist slogan, "Wars are won by eincircling the towns by the countryside," to mean that towns must be eliminated, which they accomplished.

The vocabulary of the new regime gave Ponchaud a vehicle for documenting how "human relations themselves have been completely transformed by the revolution!" He studied transcripts of radio broadcasts and found that the complex linguistic code that once conveyed courtesy had been destroyed. Now phrases to denote child, father, mother, uncle-younger-than parents, uncler-older-than parents, sage, and many more are gone. They have been replaced with the word met or comrade. All adults, not only one's parents, are "mother" and "father" to children. All children are "comrade child," not just one's own child. Youngsters are "junior comrades," peers are "senoir comrades."

The fate of Cambodia has taken on greater significance now than when the civil war was raging there. For many, Cambodia appears to be something akin to an Asian Albania: a small nation whose leaders are imposing a revolution in obscurity, allowing only a few sympathetic foreign visitors, most of them Chinese, to see their country. But unlike Albania, the course of Cambodia was directly influenced by the United States - by American diplomacy during the war in Vietnam.

Because of the legacy of that war, and because of the staggering accounts of executions and starvation, Camboida has become the subject of a public propaganda debate in America equal in intensity to those of the Cold War era. Over 100,000 Cambodians have fled to neighboring Communist Vietnam than to Thailand - and American politicians of many ideologies have begun demanding some action to curb the excesses of the new regime. Others have taken the stories of the Cambodians' plight and used them to rekindle denunciations of the "Communist system." Against the background of these emotional debates, Ponchaud's work is outstanding, both for his painstaking analysis of available information and for placing it in its proper historical context.

John Barron and Anthony Paul use a different approach in Murder of a Gentle Land. The two Reader's Digest writers build their book around the assumption that the stories of thousands of refugees must contain some truth. In this sense, their book is valuable: It forces us to listen to and acknowledge those stories.

But the authors do not stop there. They pepper their book with facile polemics. The tragedy results from "the tyranny of communism," they say; and "Someone, perhaps Joseph Stalin, is reputed to have said, 'One death is a tragedy: a million deaths are a statistic.'" With these and similar remarks, they successfully turn their book into a Cold War propaganda piece.

The refugee accounts are converted to melodrama. The authors say that their work is based on interviews with some 300 refugees - done with the aid of translators, since neither author speaks Khmer - but fewer than 50 are cited in the text as eyewitness sources. But that does not prevent Barron and Paul from making sweeping statements like this one on the methods of execution in 1975:

"Relatives had to watch while their loved one was put to death by stabbing, bludgeoning, axing, garroting on decapitation. Women and children most commonly were killed by being knifed in the front of the throat or struck in the back of the neck with hoes. As the doomed person knelt to die, the relatives struggled to muffle their moans and wails lest the expressions of grief be construed as hostility toward the executioners."

The authors also extrapolate, with little evidence and considerable sensationalism, that the worst can be blamed on an obscure sexual problem of one leader: "The origins of some of the more extreme policies may lie in the personality of the impotent ideologue Khieu samphan . . . Perhaps some of the deathly hostility [the Communist organization] was to visit upon the Cambodian people, such as the savage slaughter of women and children as well as men, the ferocious assault on the Khmer traditions of love, courtship and family, the draconian punishment of extramarital sex, was spawned by hostility of the unloved and unloving Khieu."

In their treatment, history plays only aminor role. The authors refuse to discuss the civil war which spawned the Khmer Rouge victory. "It was not our purpose to detail, much less debate, the history of the Indochina War that preceded the communist conquest," they write. To include such material might have complicated an otherwise undisciplined political tract.

For an understanding of that history, readers must turn to Ponchaud on the legacy of the war: the corruption of the Lon Nol regime as it wasted much of the $1.9 billion in wartime aid it received from America: the 439,129 tons of bombs dropped on the country by U.S. airplanes; and the 200,000 casualties from the war. He suggests that the Communists won almost by default, the benefactors of America's mistaken and callous war diplomacy.

"Beyond all doubt," he writes, "we, the French and the Americans, hear part of the responsibility for the Cambodian drama."

Ponchaud was one of the few foreigner who witnessed the Khmer Rouge entrance into Phnom Penh at the beginning of their regime. During his final days in the country he witnessed a scene that told him too much about the new Cambodia.

An elderly woman was arguing with a young Khmer Rouge soldier in Phnom Penh. "The woman was imploring the soldier, her hands joined together in the highly elegant gesture of Khmer courtesy." The soldier responded, "Why are you putting your hands together like that, like the imperialists do? From now on nobody raises his hands to anybody anymore, because we are no longer slaves!" With that brief reply, he was dismissing not just an old woman, but his own centuries-old culture.