As movie stars go, he cuts an unlikely figure. He does not have the pulling power of Tom Cruise and he cannot open a movie like Tom Hanks. But there again, he is 87 and you have to defer to age and seniority. As he nears his 10th decade, Robert McNamara has just made his first film, of which he is the star and, in a sense, the subject. The Fog of War, which won Best Documentary Feature at this year's Oscars, is about the life of one of the most famous warmongers of the 20th century. As defence secretary for two American presidents, John F Kennedy and Lyndon B Johnson, in the 1960s, McNamara was one of the chief architects of the Vietnam war.

Now, from his headquarters in Washington, he is working for peace. He rarely gives interviews. But to publicise The Fog of War he has agreed to talk to me. As a journalist who had covered Vietnam, gone into combat with American boys, seen them bleed and die, meeting McNamara in the flesh, even all these years later, was intriguing and disquieting. But the man who ushered me to a sofa in his book-lined office overlooking a square off 14th Street was far from his previous self. There was no arrogance, no evasion, just self-reflection and a deep desire to explain.

I detected vulnerability in his sometimes tearful face: age perhaps, or was it the burden of the past and the pain of his rigorous self-examination? He harked back to his role in planning the firebombing of Japanese cities at the end of the Pacific war in 1945, in which hundreds of thousands died, saying he believed he had behaved as a war criminal who would have been prosecuted had the United States lost the war. "What is the moral foundation of being prosecuted as war criminals if you lose, but not if you win?" he asked.

What makes McNamara tick today is his blueprint for peace. "There have been times in the last year when I was utterly disgusted by the US position vis-a-vis the other nations of the world," he has pronounced elsewhere. "We're misusing our influence. It's morally wrong, it's politically wrong, it's economically wrong."

You might think this was the voice of Jacques Chirac, the French president, or of Tony Benn, the veteran anti-war campaigner and former Labour MP. But this is what Robert McNamara has to say today, and he is not talking about the Vietnam war, the war the US lost, which put him centre stage and made him into such a controversial figure. He is talking about Iraq. He sees a pattern. He believes that, in Iraq, America is making some of the mistakes of Vietnam all over again and is guilty of arrogance in the wielding of its power.

McNamara believes, like Kennedy, in the notion that American principles have imbued Americans with a special mission in the world. The nation, he thinks, is indeed "exceptional". But this "exceptionalism" is being interpreted by American leaders and the American people not just to mean that the US is different, but that it is better - more virtuous, honest, truthful, democratic and generous - than other nations.

This has given it a feeling of superiority. But its overwhelming power should not mean it is "omniscient" and can act unilaterally. He says it does not justify the way the US has demanded, on occasion after occasion, that the world should not just accept its leadership, but do as the US says it should do. The war in Iraq waged without UN approval is, he indicates, a prime example.

In his State of the Union address in January, President George W Bush declared that the US is serving God's will directly and does not need a "permission slip" from other nations, since the "cause we serve is right, because it is the cause of all mankind". McNamara disagrees. "Self-righteousness is a very poor foundation for establishing public policy."

He is speaking as a man who has seen the error of his ways. His views are the culmination of experience and the day-to-day handling of war. As defence secretary at the height of the cold war, between 1961 and 1968, he presided over the disastrous escalation of the Vietnam war, which killed 58,000 Americans and 3.5m Vietnamese, and which he now admits was "wrong, terribly wrong". He presided, too, over the 1962 Cuban missile crisis, when the US came close to a nuclear war with the Soviet Union. Now a new generation of Americans, and Britons too, are getting to know him as he reflects on the mistakes of the past through the The Fog of War. Its subtitle is Eleven Lessons from the Life of Robert S McNamara. The S stands for Strange.

Errol Morris, who directed this revelatory film on McNamara's life and times, has made other landmark documentaries, including Gates of Heaven, about two pet cemeteries in California, and The Thin Blue Line, which helped solve the murder of a Dallas policeman and free from prison a man who was wrongly convicted and facing execution. He demonstrated against the Vietnam war in his student days. The Fog of War is released in Britain on April 2. A compelling 106-minute documentary consisting largely of the former Pentagon chief talking to a fixed camera, intercut with hitherto-unseen historical footage and vivid re-creations, it takes the viewer on an intimate journey through key events of the past century as seen by one of its most important and controversial political players.

There are some extraordinary moments: McNamara's powerful denunciation of nuclear weapons, which he fears are bound to be used as long as they are allowed to exist; his warning that "military pre-emption and regime change are not going to take care of the problem"; his calls for a more collaborative foreign policy, and his tearful remorse at the firebombing of Japanese cities at the end of the second world war. He played a key part in this, until now little known: in 1945 he was attached to the planning staff of General Curtis LeMay's 21st Bomber Command stationed on the Pacific island of Guam. LeMay's B-29s showered 67 cities with incendiary bombs. McNamara describes the firebombing of Tokyo, a city largely built of wood: "In that single night," he says, "we burnt to death 100,000 Japanese civilians Ð men, women and children." The camera pans from the air across 50 square miles of ash.

After the war, LeMay said to McNamara: "If we'd lost the war, we'd all have been prosecuted as war criminals." "And I think he's right," McNamara says. "I'd say we were behaving as war criminals."

In the film, McNamara, both hero and villain, outlines the enormous lessons learnt from the conflicts and world-changing events, especially Vietnam, in the hope that similar errors won't be made in future. It is a subject he has honed over three books: In Retrospect, his Vietnam mea culpa, published in 1995, Argument without End (1999) and, most recently, Wilson's Ghost, named after President Woodrow Wilson, who pioneered the League of Nations. The lessons are his plan for peace. The parallels with Iraq jump out of the pages and from the screen. McNamara told me he did not feel it appropriate to comment on the war in Iraq. But it is easy to guess what he thinks.