This year's Reith lecturer John Keegan tells Harriet Swain why war, despite assorted Saddams and Serbs, is going out of fashion

When John Keegan was asked to give this year's Reith lectures he says he was so "flabbergasted" by the invitation he had to hear it twice. "I think it's a tremendous honour," he says. "When I was young, the Reith lectures were something of enormous importance and standing. They were only given by immensely distinguished people. I don't think I'm immensely distinguished at all. Not a bit."

But of course he is. Defence editor at The Daily Telegraph, where he has worked for more than 12 years, a lecturer at Sandhurst for 25 years before that and the author of ten books, he was asked for advice in the recent Gulf conflict, was invited to brief Bill Clinton on VE Day celebrations in 1995, and was awarded the OBE in 1991. Now 63, he was born a little over a decade before the start of the Reith lectures, which celebrate their 50th anniversary this year. Both have rapidly become part of the academic establishment. Keegan, whose speciality is war, marks the lecture series' return to more mainstream themes. Recent Reith lecturers have included American feminist Patricia Williams talking about race, Oxford academic Jean Aitchison on languages, architect Richard Rogers on cities and cultural historian Marina Warner on myths.

This year, partly to mark the anniversary, partly as a result of overall changes to Radio 4 and partly to ensure they remain an "event", the lectures will be delivered in front of invited audiences. For the first time, there will be questions at the end of each talk and some members of the audience will be given advance copies of the lectures to ensure questioning is well-informed and controversial. In a further effort to open up the Reith lectures to ordinary folk, the talks will be delivered at different spots around the country, starting at the Royal Institution in London, then on to the Royal Military Academy at Sandhurst, King's College, London and Glasgow University and ending at Broadcasting House in the capital.

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Keegan, who enjoys lecturing, looks forward to the debate this new system is likely to provoke on a subject he says is fundamentally contentious. His lectures begin with the premise that "war has been the scourge of this century", replacing famine and disease as the main threat to human life, emotional well-being, material comforts and cultural heritage. He describes the terror of women waiting in Britain during the first world war hearing the telegraph boy pedalling along the street and praying he would not stop at their house. He lists the buildings and art treasures destroyed throughout Europe during the second world war and the lives lost over the century.

After examining the origins of war, he discusses first war and the state and then war and the individual. States are ultimately the organisations that make war, since they have the money to do so, he says. But it is also important to recognise that certain individuals enjoy war and appear to have a particular talent for it. If it were not for such people, wars would not happen. Even so, circumstances have changed over the century. While there is such a thing as human aggression, it is controlled to a certain degree by cultural and moral systems. Until recently many societies were extremely violent but Keegan says that public tolerance of state violence in advanced societies has now dwindled. Also, he says, the development of nuclear weapons has meant that individuals would have to be "seriously unhinged" to instigate a large-scale conflict. War has now become too big for mankind. Its rewards are less, its costs far greater.

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This means that war will become less a tool for states, while remaining a route to power for mad and bad individuals. For that reason, says Keegan, it is vital that governments be prepared to be extremely tough on anyone who threatens peace, such as Saddam Hussein. He describes Saddam as the most dangerous man in the world at the moment. "If we keep our eye on Saddam, we warn other Saddams," he says. With that proviso, he is optimistic. "War is so dangerous now that there are very large numbers of states prepared to try all sorts of devices to avoid it," he says. "That makes for terrific optimism."

Sitting in the Telegraph conference room, radiating charm in a suit with thin pinstripes, blue shirt and matching hanky, Keegan exudes cheeriness and a certain bemusement at the path his career has taken. He never had any idea what he wanted to do, he says, never expected to get to Oxford, never had any ambition to be an academic or a journalist. After graduating in history from Balliol College in 1957 he "mucked about for a bit" - this turns out to be working as a political analyst at the US embassy - then found himself writing off to Sandhurst for a lecturer's job. His friend Max (Hastings) suggested he join the Telegraph and when he was later invited to take on the role of defence editor he "thought it would be chicken not to". All in all, distinction seems to have crept up on him in spite of himself.

Perhaps this is because at the very beginning, it appeared he was wildly off target. Thanks to a severe bout of TB, which has left him crippled, he spent most of his teens in hospital, missing out on huge chunks of school. "Somehow I caught up very quickly and went to Oxford, which was a great relief to my parents," he says. Once there, he felt as if he was on holiday, in spite of having to miss a year when his illness returned. "It was so wonderful to go to Oxford after having been out of circulation for so many years and not believing I would get educated," he says. He left in the postwar boom period when getting a job was not a worry. When he fetched up at Sandhurst he loved it so much - the beautiful campus, the library which contains "only the books you want to read", the "gentlemanly" attitude of the soldiers - that he stayed for a quarter of a century before he "broke out". He says he should not have been allowed to stay so long.

One of the reasons he did was that he liked the company of soldiers. Soldiers, says Keegan, are good for academics because they stop them taking themselves too seriously. He also admires them as particularly "moral" people. "They have taken a decision, which the rest of us couldn't take, which is that they are prepared to kill other people. Having decided that, they find a lot of other moral decisions much easier," he says. It is not a decision he would feel able to take himself. In any case, he says,flicking cigar ash into the Telegraph's paper bin, he is "too easily bored" and "too rebellious by nature" to be a soldier. He clearly felt extremely rebellious while he was at Sandhurst, although he admits, in retrospect, this was only because he was surrounded by extremely disciplined people. One legacy of those years is that he is now excessively punctual.

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None of these aspects of soldiery was what first attracted him to military history. Then, he says, it was "the superman stuff". As a child, he was precociously interested in current affairs and current affairs at the time were all about war. He would devour copies of Picture Post, which described heroic actions in battle and the people who performed them. "Now it is absolutely not the superman stuff," he says. "I think the idea that there can be supermen in war is rather pathetic. But I'm as interested as I ever was." For one thing, he now knows a lot about the subject and says it is hard not to be interested in something after spending nearly half a lifetime studying it. For another, he sees studying war as a crucial way of maintaining peace. Peace, he says, is something which needs to be preserved all the time. If we lose concentration and allow ourselves to slide into war it is hard to claw ourselves back. He describes himself as "very anti-war, almost a pacifist, but not quite because sometimes we have to have wars or at least threaten wars".

But he says studying the subject for so long has changed him. "It hasn't made me a cynic but I have a slight chronic depression about the way the world is. I have very few illusions about it." Wars "make people behave terribly badly. They give the worst human instincts free rein." He has just finished writing a history of the first world war, which he found particularly depressing. Now that it is over, he expects his spirits to lift. His lecture series picks up this mixture of gloom and optimism. After cataloguing the destruction caused by war over the century and its greater destructive potential over the next, he concludes that a serious war in the 21st century is unlikely mainly because reasonable people recognise its danger and because, he says, "Force gets you what you do not want."