BY 1974, even in the farthest corners of the globe, they knew who Charles Colson was and what Woodward and Bernstein said he had done. They shook their heads as the president of the United States and his court tried to batter and beat the Constitution into submission. Everyday there was some new report out of Washington; Haldeman leaves the White House, Dean takes the stand, Nixon dodges the courts--the 18-minute gap heard round the world. Watergate captured imaginations in places they had never heard of Checkers or the Potomac and sent journalists scurrying to their wire machines to figure out what was going on.

In Washington, D.C., a young British journalist had come to the United States to work as a Congressional Fellow in Ted Kennedy's office, to watch the American system of checks and balances "at work." Today, William Shawcross says, he found Watergate "marvelously interesting. It was impossible not to be interested in," he says in a quiet, rational voice.

But Shawcross saw more in Watergate than wiretapping and ratfucking and money-laundering. He carried his suspicions with him when he left Washington to go back to work for the Sunday Times of London. "Watergate has been seen as a series of these domestic abuses," he says today, shaking his head, "while Kissinger"--the name of the enemy slips out easily--"is seen as the greatest secretary of state." Kissinger: the name hurts Shawcross more than it haunts him, but there's nothing personal here.

Willie Shawcross, as his friends call him, was just another war correspondent dodging machine-gun fire in Southeast Asia in 1975. This was no place for an Oxford-educated man who looks at home in a tweed three-piece suit, especially for one who didn't enjoy the strains of daily journalism. "Daily journalism is very bad in terms of giving scope and finding out what the hell is going on," says Shawcross. So in 1975, when the Khmer Rouge overran neighboring Cambodia, and Shawcross began to get horror stories from the refugees he talked with, he returned to London wondering "who these people were and how they had gotten this way." He persuaded his editors at the Sunday Times to let him report further.

What eventually became Sideshow was really Willie Shawcross, obsessed with two cultures he knew little about, sifting through thousands of pages of documents, looking for something but not knowing what he would find. He started with the people in the American embassy in Phnom Penh, amazed by the "unanimity with which they spoke" and with what they saw as "callous disregard" for human lives. Somebody in Washington was watching the CIA reports that showed North Vietnamese troops hiding in sanctuaries on the Cambodian side of the border. Somebody decided that the best way to flush the North Vietnamese out was to destroy the sanctuaries. But, as Shawcross demonstrates in Sideshow, they forgot that the bombs might hit a few Cambodians.

Willie Shawcross is not the chain-smoking, scrappy-looking reporter you'd imagine as Sideshow's author. Shawcross, tall, quiet, soft-spoken, strikes one as a British academic, looking for the fact that may reveal the truth, but not aiming to promote himself.

He shrugs off any glorification of his approach, or his mastery of the Woodstein technique. "Investigative reporting is a stupid term," he says definitively. "Decent reporting is by definition investigative." For Shawcross, Sideshow was an extension of his earlier days on the Times' insight team.

For a man who wrote what many say is history at its best, Shawcross doesn't like to look behind. He worries about what is going on in Cambodia now. He worries about British reporters who, bound and gagged by that country's Official Secrets Act, will never gain access to documents that Americans can poke into.

The essentially British Shawcross adopted the United States, and his experience with Sideshow hardened an already-suspicious nature. Disturbed by President Carter's recent call for increased CIA activities, he declared that the Freedom of Information Act--the lifeblood of his work--must be broadened. Angered by the Supreme Court's decision to keep Kissinger's memoirs under wraps in the sacred tombs of the Library of Congress, he hopes that other people will do for the rest of Kissinger's work what he did for Cambodia.

When Shawcross spoke this week to a crowd at the Kennedy School, a few in the audience nodded off, bored by the author's lengthy textual analysis of White House Years. If Shawcross displayed his wit at times, there was none of the emotion that the crowd had come to see. Shawcross read whole passages, compared them to others and concluded that Kissinger doesn't always tell the truth. Kissinger, his speech told us, is a target it will take more than one Sideshow to uncover. Shawcross realizes this and he goes about his task ponderously, like a coach looking at the films, shaking his head and stopping at every frame to find out where the mistake was made. He shudders when he talks about former President Gerald R. Ford's promise that Kissinger would be secretary of state if he were in the White House.

IT IS a frustrating task, for Shawcross has seen the American system at its most perverted. Willie Shawcross went to Washington and to Phnom Penh as an outsider, trying to find out what the hell was going on. Eight years later Shawcross has seen the guts of our political system turned inside out, pock-marked by the Cambodian experience and the work of some not-so-moral men. Shawcross has gone back to Washington to produce a series of long articles on the current situation in Cambodia for The Post; "it's hard to walk away from Cambodia," he admits. But the endless work, he adds quickly, is never boring, only depressing. "Cambodia was not a mistake; it was a crime," Shawcross writes at the end of Sideshow. "If the world is diminished by the experience," as Shawcross's last sentence tells us, the author's world--and ours--was opened wide.