AT THE END of the '60s Cambodia was a small neutral kingdom as happy and peaceful as any weak country, caught up in the intrigues and ambitions of the superpowers, could be. Then an unending nightmare began: the American intervention of 1970, oppression by a frightening travesty of Marxism, and now occupation by the Vietnamese. Ten years of horror have culminated in the possible extinction of this small nation (numbering scarcely 7 million inhabitants in 1970). In those 10 years the Pol Pot regime and the subsequent intervention by the Vietnamese probably wiped out more than a quarter of Cambodia's citizens. This systematic genocide bears comparison to that of the Jews from 1935 to 1945. In Sideshow William Shawcross attacks the root of the Cambodian sickness -- the American invasion. This is why his book should be required reading for Westerners who denounce, with reason, the crimes of the Khmer Rouge without a full understanding of what it was that made this gentle people go mad.

Anyone attempting to coimpare Cambodia at the end of the '60s with Cambodia in 1979 must search for the turning points between its relative happiness and its extreme suffering. Two can be identified: either March 18, 1970, the date of the removal of Prince Sihanouk, or the following April 30, the day the American and South Vietnamese troops entered Cambodia to save Sihanouk's "conquerors" from defeat. It is perhaps simplistic to make the Khmers' recent sufferings a direct result of Sihanouk's defeat, or to see in the recent Vietnamese intervention anything but a response to those who, among other objectives sought to expel the Vietnamese forces rooted in Cambodia. It would also be simplistic to see the tyranny of the Khmer Rouge as the inevitable result of the American military operation in the spring of 1970. But an exhaustive analysis needs to be made of these two Cambodian ordeals.

In a country where relative abundance reigned and where huge private estates were rare, it took the suicidal attack launched by President Nixon to bring a regime like Pol Pot's to power. The hundreds of thousands of victims of the April 1970 invasion, by a predictable reaction, encouraged innumerable enlistments in the revolutionary maquis, bringing about a rapid reenforcement of the most extremist positions: these were later the victims of Pol Pot.

But Sideshow offers more than a vital key to understanding a major Southeast Asian crisis and the martyrdom of a people. It is also a superb description of the corruption of a democracy by what the 18th-century French royalists used to call "secret du roi." To the American public (and to others), this book will be an indispensable complement to the ever-growing Watergate literature with its exhaustive detailing of domestic corruption. Shawcross describes another kind of corruption -- a kind of diplomatic Watergate. This he denounces with admirable competence, precision, factual richness and talent for description.

William Shawcross, a former correspondent for The Sunday Times of London, is not merely familiar with the political scene in Washington. He also knows Southeast Asia, the places, the people and the leaders. His polrtrait of Sihanouk is not only brilliant, but original and moving. He doesn't portrary this versatile, technicolor character as the caricature favored by the popular press in the West (a little card-board Khrushchev), but subtly evokes a man whose political sense, patriotism, and ability to charm are as important as his fantasies and his hypocrisy.

In comparison with this seemingly mad Oriental prince, how clumsy, cynical and short-sighted appear the strategists who committed the strongest world power to the absurd and disastrous invasion of the little kingdom on the Mekong! From Nixon to Kissinger, from Alexander Haig to Thomas Enders, these men seem to limit their political skill to two words: toughness and secrecy. The portraits that Shawcross draws of these men in authority, scarcely using more than their own words (notably those on the famous tapes), are devastating. Since the Watergate investigations and the interviews with David Frost, one can hardly have any illusions about the kind of individual that Richard Nixon is. But in his book Shawcross focuses on Henry Kissinger, and with a ferocity that requires a few observations.

Washington's Cambodian advanture can be broken down into four phases. It seems to me that the responsibility of Kissinger in each of these must be more carefully delimited. The first phase is that of the border bombings by the U.S. air force, which began in 1965: at this time, when Kissinger was still teaching to Harvard, Sihanouk showed the bombarded Cambodian villages to the world. The second phase, that of the famous "Breakfast" operation, later enlarged to "Menu," was launched in 1969 under the direct authority of Kissinger. The third was the overthrow of Sihanouk on March 18, 1970, which opened the way for the fourth phase, the invasion. Kissinger has never denied being one of the initiators of the invasion. But the question remains open whether the former Harvard professor was directly implicated in the coup d'etat of March 18 -- which was perhaps the most important of these four phases.

In the course of an interview at his office in the White House at the end of July, 1971, just a little after his first trip to Peking (which, by chance, 1 had visited at the same time as he), I asked him about this: He firmly answered that this Phnom Penh operation had taken place with neither his knowledge nor his consent, that he regretted the elimination of Sihanouk which produced more problems than solutions, and that he felt it regrettable that Sihanouk, who was in Paris in March, 1970, had not followed his instincts and -- instead of going, to Moscow and Peking -- returned directly by plane to Phnom Penh where he would certainly have turned the situation around in his favor.Self-propaganda or sincerity? The tone of his voice impressed me, as it appeared to have impressed Joeph Kraft who participated in the interview.

Some time later, I asked Anthony Lake, new head of policy planning for the State Department, about the question of Kissinger's responsibility in the Phnom Penh coup. Lake, after having resigned from Kissinger's staff in protest against the invasion of Cambodia, had become an advisor to Senator Muskie, then Democratic candidate for the presidency. Unlikely to have much sympathy for Kissinger, Lake answered that, as far as the Phnom Penh coup was concerned, his former boss did not seem directly implicated.

These few observations about Side-show are not intended to overshadow the qualities of this superb book. Sideshow should be studied as much by those who want to know how and why a small nation in Asia was destroyed, as by those, even more numerous, who are interested in, and worried about, the functioning of democracy and its regulatory mechanisms in that nation which counts for more than any other in the peace of the world. CAPTION: Picture 1, Cambodian refugees, UPI; Picture 2, Richard Nixon and Henry Kissinger; Map, COMMUNIST LOGISTICS IN CAMBODIA 1969-70, "Sideshow"