William Shawcross didn't know what the nine colonels wanted, but they wanted him at the Pentagon, and there were nine of them. It was a hit disconcerting. Shawcross always got lost in the Pentagon, and besides, here he was a Britisher, a foreigner, asking the American Government to pony up 120-odd documents about the destruction of Cambodia, a lot of them classified Top Secret.

"I walked into the room, and there they were, all in Air Force blue, sitting around the table. They told me I was causing them a great deal of trouble; they'd never had a request like this. They said: 'We can only get those documents here by top-secret courier, and that courier flight only comes in once a month, so we hope you can understand that it will take some time to help you.'"

Shawcross, sitting over iced jasmine tea in the Viet Huong Restaurant in Georgetown, smiles in astonished delight, or delighted astonishment.

"This couldn't happen in any other country. I couldn't have written this book in England. I couldn't get the documents. What I can't understand is why no American wrote this book."

The book is "Sideshow: Kissinger, Nixon and the Destruction of Cambodia." It has won raves from reviewers, and revived hideous memories which, somehow, we hoped, got tossed away with the bandages after the "binding of the wounds" in the Ford Administration.

"I didn't think it should be forgotten. Memories are so short in this country," Shawcross says.

In fact, the power of the book is such that it disrupts that beloved American pastime of lionizing all our old rulers, making belated royalty of them, such as Truman a few years ago, and now Kissinger.

Time magazine even reacted by treating the book as a news story in its Nation section, rather than reviewing it. In its eagerness to defend Kissinger, Time reversed field on exiled Norodom Sihanouk, who it once mocked as "Snookie," and quoted him as saying after a recent Peking dinner that Kissinger was "a charming man."

So passions still run high, including Shawcross'. After two years of researching 20,000 pages of documents, he still seems startled by the pattern of American bombing and intervention which eventually led, he says, to the terrible triumph of the Khmer Rouge Communists.

"I was constantly surprised, yes," he says, leanint with a boyish fatique against the wall. He is 32, dressed in unbelted corduroy jeans and shirt-sleeves with the fillip of cuff links. He is a bit of a vrai naif who stumbled into journalism after a holiday in Czechoslovakia in 1968.

"I'd just left Oxford. My sister and I decided to go to Czechoslovaka-it was a wonderful time, an exhilarating time, with (First Secretary Alexander) Dubcek introducing all the reforms. Then shortly after we left, the Russians invaded. I wanted to go back and see what had happened to all the friends I'd made."

He got a job at the Sunday Times of London as a researcher, then a 100-pound advance for a book on Dubcek, then a job as correspondent for the Sunday Times in Eastern Europe, Indochina and Washington, where he helped write the Sunday Times Book entitled: "Watergate; The Full Inside Story."

He was a new kind of British journalist who started making waves in the late 60s, when the first Sunday Times Insight team arrive here.They tended to be young and irreverent and fascinated with America as a culture, intrigued by American power, rather than resigned to it.

"This country is much more invigorating than England, much more vital," Shawcross says.

In 1977, he became curious about the rise of the Khmer Rouge, which led him back through the Nixon/Kissinger policies since 1968, and the piles of documents including, finally, the one message that began the end for 'an idyllic, antique land unsullied by the brutalities of the modern world," as he writes. "The overriding impression was of fecundity and greenness, and of chocolate-brown waterways where buffalo and sometimes elephants steamed gently in the heat . . ."

But on March 17, 1969, labeled TOP SECRET, and EYES ONLY, an NOFORN (no foreigners to see it), the message to Gen. Creighton Abrams in South Vietnam ordered the B-52s to attack. The communication contained precise instructions for evasion in case the press asked about the attack on a neutral country: "U.S. spokesman will state . . . he has no details and will look into this question."

His operation was called "Breakfast" after a morning conference which preceded the decision to stage it. Subsequent operations were called "Dinner," "Dessert," and "Snack," with the whole plan summed up as "Menu." The idea was to deprive the Communists of sanctuaries. But, Shawcross says, all the United States did was drive them farther into Cambodia, ultimately destroying it for the sake of a war American could't win.

"Here it is," Shawcross says, and holds out a facsimile of that 1969 cable, all acronyms and capital letters, an incredible anomaly on this hot mid-afternoon in a Georgetown restaurant, both so secret and so flagrant that it seems obscene.

Shawcross calls the book "an investigation," and he concludes it by writing: "Cambodia was not a mistake; it was a crime."

His father, it is recalled, was British prosecutor at the Nuremburg war crimes trials, but Shawcross claims no moral inheritance. Perhaps watching the destruction of that brief Eden in Czechoslovakia under Dubcek has provided a capacity for anguish.

Educated at Eton and Oxford, and with spy novelist John LeCarre as a friend, Shawcross is sensitive to the terrible naivetes and meannesses of ruling classes. But ultimately, his perception seems grounded in a more frightening reality.

"I love this country," he says. "But when things go wrong here, other countries, smaller countries, suffer very, very badly. CAPTION: Picture, William Shawcross, by James A. Parcell-The Washington Post.