Of course Katharine Gun was free to have a conscience, as long as it didn't interfere with her work at a British intelligence agency. To the authorities, practically speaking, a conscience was apt to be less tangible than a pixel on a computer screen. But suddenly -- one routine morning, while she was scrolling through e-mail at her desk -- conscience struck. It changed Katharine Gun's life, and it changed history.

Despite the nationality of this young Englishwoman, her story is profoundly American -- all the more so because it has remained largely hidden from the public in the United States. When Katharine Gun chose, at great personal risk, to reveal an illicit spying operation at the United Nations in which the U.S. government was the senior partner, she brought out of the transatlantic shadows a special relationship that could not stand the light of day.

By then, in early 2003, the president of the United States -- with dogged assists from the British prime minister following close behind -- had long since become transparently determined to launch an invasion of Iraq. Gun's moral concerns were not unusual; she shared, with countless other Brits and Americans, strong opposition to the impending launch of war. Yet, thanks to a simple and intricate twist of fate, she abruptly found herself in a rare position to throw a roadblock in the way of the political march to war from Washington and London. Far more extraordinary, though, was her decision to put herself in serious jeopardy on behalf of revealing salient truths to the world.

We might envy such an opportunity, and admire such courage on behalf of principle. But there are good, or at least understandable, reasons why so few whistleblowers emerge from institutions that need conformity and silence to lay flagstones on the path to war. Those reasons have to do with matters of personal safety, financial security, legal jeopardy, social cohesion and default positions of obedience. They help to explain why and how people go along to get along with the warfare state even when it flagrantly rests on foundations of falsehoods.

The e-mailed memorandum from the U.S. National Security Agency that jarred Katharine Gun that fateful morning was dated less than two months before the invasion of Iraq that was to result in thousands of deaths among the occupying troops and hundreds of thousands more among Iraqi people. We're told that this is a cynical era, but there was nothing cynical about Katharine Gun's response to the memo that appeared without warning on her desktop. Reasons to shrug it off were plentiful, in keeping with bottomless rationales for prudent inaction. The basis for moral engagement and commensurate action was singular.

The import of the NSA memo was such that it shook the government of Tony Blair and caused uproars on several continents. But for the media in the United States, it was a minor story. For the New York Times, it was no story at all.

At last, a new book tells this story. "The Spy Who Tried to Stop a War" packs a powerful wallop. To understand in personal, political and historic terms -- what Katharine Gun did, how the British and American governments responded, and what the U.S. news media did and did not report -- is to gain a clear-eyed picture of a military-industrial-media complex that plunged ahead with the invasion of Iraq shortly after her brave action of conscience. That complex continues to promote what Martin Luther King Jr. called "the madness of militarism."

In a time when political players and widely esteemed journalists are pleased to posture with affects of great sophistication, Katharine Gun's response was disarmingly simple. She activated her conscience when clear evidence came into her hands that war -- not diplomacy seeking to prevent it -- headed the priorities list of top leaders at both 1600 Pennsylvania Avenue and 10 Downing Street. "At the time," she has recalled, "all I could think about was that I knew they were trying really hard to legitimize an invasion, and they were willing to use this new intelligence to twist arms, perhaps blackmail delegates, so they could tell the world they had achieved a consensus for war."

She and her colleagues at the Government Communications Headquarters were, as she later put it, "being asked to participate in an illegal process with the ultimate aim of achieving an invasion in violation of international law."

The authors of "The Spy Who Tried to Stop a War," Marcia and Thomas Mitchell, describe the scenario this way: "Twisting the arms of the recalcitrant [U.N. Security Council] representatives in order to win approval for a new resolution could supply the universally acceptable rationale." After Katharine Gun discovered what was afoot, "she attempted to stop a war by destroying its potential trigger mechanism, the required second resolution that would make war legal."

Instead of mere accusation, the NSA memo provided substantiation. That fact explains why U.S. intelligence agencies firmly stonewalled in response to media inquiries -- and it may also help to explain why the U.S. news media gave the story notably short shrift. To a significant degree, the scoop did not reverberate inside the American media echo chamber because it was too sharply telling to blend into the dominant orchestrated themes.

While supplying the ostensible first draft of history, U.S. media filtered out vital information that could refute the claims of Washington's exalted war planners. "Journalists, too many of them -- some quite explicitly -- have said that they see their mission as helping the war effort," an American media critic warned during the lead-up to the invasion of Iraq. "And if you define your mission that way, you'll end up suppressing news that might be important, accurate, but maybe isn't helpful to the war effort."

Jeff Cohen (a friend and colleague of mine) spoke those words before the story uncorked by Katharine Gun's leak splashed across British front pages and then scarcely dribbled into American media. He uttered them on the MSNBC television program hosted by Phil Donahue, where he worked as a producer and occasional on-air analyst. Donahue's prime-time show was cancelled by NBC management three weeks before the invasion -- as it happened, on almost the same day that the revelation of the NSA memo became such a big media story in the United Kingdom and such a carefully bypassed one in the United States.

Soon a leaked NBC memo confirmed suspicions that the network had pulled the plug on Donahue's show in order to obstruct views and information that would go against the rush to war. The network memo said that the Donahue program would present a "difficult public face for NBC in a time of war." And: "He seems to delight in presenting guests who are antiwar, anti-Bush and skeptical of the administration's motives." Cancellation of the show averted the danger that it could become "a home for the liberal antiwar agenda at the same time that our competitors are waving the flag at every opportunity."

Overall, to the editors of American mass media, the actions and revelations of Katharine Gun merited little or no reporting -- especially when they mattered most. My search of the comprehensive LexisNexis database found that for nearly three months after her name was first reported in the British media, U.S. news stories mentioning her scarcely existed.

When the prosecution of Katharine Gun finally concluded its journey through the British court system, the authors note, a surge of American news reports on the closing case "had people wondering why they hadn't heard about the NSA spy operation at the beginning." This book includes an account of journalistic evasion that is a grim counterpoint to the story of conscience and courage that just might inspire us to activate more of our own.