I've never met Julian Assange. And the WikiLeaks founder, who was still a teenage hacker in Australia when I F Stone died in 1989, never met Stone. Yet, in his foreword to American Radical, my biography of America's greatest investigative journalist, Assange writes that Stone was "an important influence" on him. Why should a writer whose best-known creation, the four-page newsletter I F Stone's Weekly, ceased publication the year Assange was born still exert such an influence more than two decades after his death?

One clue comes from a New Statesman/Frontline Club debate that was held on 9 April. The motion was: "This house believes whistleblowers make the world a safer place." Assange, speaking in favour, began by conceding that some government secrecy may be legitimate. The problem is that, since governments decide what information can be kept from the public, there is no way for citizens to challenge their decisions. "All systems of censorship have that problem encoded within them," he said. "The only way we can know whether information is legitimately kept secret is when it is revealed."

Assange then described "some situations in history that have led to war". His first example was the Vietnam war, which was "triggered by the Gulf of Tonkin incident - a lie about an American boat off the coast of Vietnam, which the US government claimed had been attacked by theVietnamese. That claim was a lie."

Assange knows his history. On 4 August 1964, President Lyndon B Johnson told the American people that the North Vietnamese navy had been guilty of "aggression on the high seas" and asked Congress for the authority to "take all necessary measures in support of freedom and in defence of peace in south-east Asia". Most of the world learned that the Johnson administration had been lying when Daniel Ellsberg leaked the "Pentagon Papers", a secret history of US involvement in Vietnam, to the New York Times in 1971. (Ellsberg had been a duty officer at the Pentagon in August 1964 and knew that one of the ships that the then US defence secretary, Robert McNamara, described as the victim of an "unprovoked attack" had been engaged in electronic warfare.) By 1995, even McNamara would admit that at least one of the supposed attacks was a complete fabrication.

But in 1964, Congress gave the president the blank cheque he wanted. The vote in the House of Representatives was unanimous; in the Senate, there were just two dissenters, who were duly insulted by the New Yorker magazine.

The mainstream press was behind Johnson. Yet, even before Congress took a vote, Stone told his readers: "Everything is discussed, except the possibility that the attack[s] might have been provoked." Two weeks later, using nothing but his knowledge of history and the suspicious lack of damage to the ships that had supposedly been attacked, Stone blew the whistle on Johnson's deception.

And, in February 1965, when the state department published a white paper depicting the Vietnamese rebellion as an instance of Chinese-sponsored subversion, Stone used the Pentagon's own figures to demolish the government's claims. More than any other figure, he gave the anti-Vietnam war movement of the 1960s something the American left hadn't had for a long time: credibility.

So who was he? He was born Isidor Feinstein in Philadelphia in 1907, the eldest son of Russian-Jewish immigrants who later moved to the small rural town of Haddonfield, New Jersey, where Isidor got his first newspaper job covering high school sports. After dropping out of college, he worked on papers in Camden and Philadelphia before becoming chief editorial writer for the New York Post - in those days a liberal, pro-New Deal broadsheet - a few days shy of his 26th birthday.

As the Great Depression deepened, he turned from liberal to radical, writing for the Nation, investigating corruption in labour unions and successfully campaigning to get New York's red-baiting chief of police fired. But by the late 1930s, his politics were best summed up by the phrase "anti-fascism".

His support for the Spanish Republic got the Post banned by the Catholic archdiocese and his insistence that the US would have to go to war to stop Hitler eventually got him fired. By then, he'd changed his byline to "I F Stone", adopting the name legally as a concession to rising anti-Semitism, and moved to the left-wing tabloid PM - the paper that introduced America to Dr Spock and Dr Seuss - as columnist and star reporter. His exposés of how US corporations such as Alcoa put profits before preparedness on the eve of Pearl Harbor brought plaudits from Senator Harry Truman, who later, as president, also eagerly followed Stone's despatches from the underground railroad that took Jewish refugees from Europe to Pales­tine. (Stone, who covered Israel's war of independence from the front lines, became one of the most passionate advocates of justice for

the Palestinians.)

In the 1930s, his easy access to FDR's White House made him a consummate Washington insider. He was a radio pundit and a regular panellist on Meet the Press in its early years on television. But after Roosevelt's death, Stone's refusal to abandon his Popular Front belief in the need for liberals and radicals to work together made him an increasingly isolated figure. During the postwar red scare, his unstinting attacks on Senator Joseph McCarthy and the FBI chief J Edgar Hoover brought the wrath of the Bureau down on his head. By 1952, he was practically deaf, out of work and saddled with a manuscript of The Hidden History of the Korean War which even the New Statesman was afraid to publish. The US state department refused to renew his passport and the Nation wouldn't give him his old job back. "I feel for the moment like a ghost," he wrote.

And yet what followed was anything but elegiac. He struck out on his own, starting I F Stone's Weekly in 1953 with barely 4,000 subscribers. By the time he shut the Weekly down 18 years later, circulation had risen above 70,000 - helped in part by supporters such as Marilyn Monroe, who bought subscriptions for every member of Congress. Besides his prophetic opposition to the Vietnam war, he exposed the dangers of nuclear fallout and atmospheric testing, campaigned for civil rights for black Americans and travelled to Cuba, where he struck up a warm friendship with Che Guevara.

It isn't just the echoes of US imperial mis­adventures that make Stone required reading today. As Assange writes in his foreword, "His keen insight into how institutions carry their power and disperse it among individuals working within them is something he managed to apply even to ancient Greece." Stone's book The Trial of Socrates, published in 1988, near the end of his life, was a surprise bestseller.

His radicalism made him a pariah, an unperson who was kept off American television and radio for nearly 15 years. But, as Assange writes, "Stone never gave up, and he never foolishly compromised. That is his great lesson for British journalists."

When I started work on Stone's biography in 1991, the first thing I did was place a Freedom of Information request for his FBI file. By the time the FBI released the last of some 6,000 pages in late 2005, it was clear just what his determination had cost him: Hoover's men had tapped his phones, opened his mail and subjected him and his family to daily surveillance.

Stone was no believer in the myth of "objective" journalism. "What they call 'objectivity' usually is seeing things the same way everybody else sees them," he once said. Instead, the man who wrote that "all governments lie" remained determined to see for himself.

“Establishment reporters undoubtedly know a lot of things I don't," he often said. "But a lot of what they know isn't true." Which is one reason why, even today, Stone remains such a dangerous man.

D D Guttenplan's "American Radical: the Life and Times of I F Stone" (£25) will be published by Quartet/Charles Glass Books on 9 June