You can take the man out of Newcastle upon Tyne, but taking Newcastle upon Tyne out of the man is much more difficult, and Paul Kennedy would not want it any other way.

It is not just that, more than 40 years after leaving his native city clutching the first-class degree that he had not expected to get, his speech retains the unmistakable rhythms of Tyneside, despite the effects of 25 years' teaching at Yale. Those origins also underpin the philosophy that enabled him to cope with the life-changing fame and fortune that came upon him 20 years ago with the publication of his book The Rise and Fall of the Great Powers - an anniversary marked this week by a session at the American Political Studies Association conference in Boston restating the question that made the book famous: "Is America in decline again?"

Success on that scale - sales running into millions, translation into 23 languages, becoming the focus for debate in an American presidential year and a "name" ever since - has serious traps for the unwary. Kennedy, though, had memories that inoculated him against being "dazzled by the apparatus of fame and the appearance of access to power".

"You have to keep a sense of proportion about it all," he explains. "I still remember as a child my Uncle Leo warning me not to become big-headed and my grandmother saying, 'Paul Kennedy, you weren't put on this earth to have it easy.' It was very salutary, and still means a lot to me."

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He has remained wary of the temptations of the lecture circuit. "It is very seductive to be asked to speak to the World Economic Forum at Davos, or to the AGM of the Asian Bankers Association in Bangkok," he admits. "Commercial agencies could have you lecturing every week in Palm Beach to conferences of realtors or investment bankers. But the danger is that you lose your bearings and fail to renew yourself intellectually. Your thesis becomes passe and you risk becoming boring, and bored with the same talk."

He offers the cautionary example of a Californian academic riding high on his book about the dot-com boom in the late 1990s who reasoned that, with apparently endless offers of lucrative lecture work, he would be better off resigning his academic post. When the boom went bust he was left desperately scrambling for work, his thesis no longer exciting and his academic base lost.

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So Kennedy continued to teach and research at Yale, where he holds a chair in history and is director of international security studies, and to choose his outside commitments carefully. "I do about three or four big commercial lectures a year, no more," he explains. "They tend to coincide with (times) when the car needs repairing or a child is going to college."

Another of the traps of eminence is ceasing to hear other voices. But when Kennedy lectures, he also listens. During the initial excitement over The Rise and Fall of the Great Powers, he spoke to more than 200 economists at the Brookings Institution. "One of the audience", he recalls, "said that he couldn't understand why people were so excited about it - it seemed to him a very basic and rather conventional Great Powers book and he thought I should have been looking at larger long-term global trends.

"They shouted the poor guy down, but when I was going back on the train I thought about what had been said and realised that he had a very good point. It was then that I started building up the files that would lead on to (a subsequent book) Preparing for the Twenty-First Century."

Similarly, awareness of what his students were saying - and perhaps missing - was part of the impetus for one of his two current projects, a study of the Second World War. "We know a huge amount about the war in terms of the people at the top," Kennedy points out, "and an increasing amount about the experience of the common man or woman. But there's a gap in the middle. At Yale I teach a course called 'Studies in grand strategy'. We talk about the Casablanca conference in January 1943 when Churchill, Roosevelt and the chiefs of staff met and outlined a series of objectives, all of which were fulfilled over the next 18 months.

"When I speak to my students, they understandably tend to think that Churchill and Roosevelt ordered these things and they just happened. But the war went very badly for most of 1943. Things had to be turned round and I'm interested in the people who weren't at the top or the bottom of the war effort but somewhere in the middle, who made that possible."

To make this concrete, Kennedy offers an example: "The war in the air was being lost for most of 1943 and, 12 months after Casablanca, the bombing of Germany had stopped. It was rescued by the introduction of a nimble, aerodynamically impossible long-range aircraft called the P51 Mustang that could fly to Berlin and back and shoot the German planes out of the sky.

"Its potential was spotted by a couple of test pilots from Rolls-Royce who flew it in April 1942, but the problem was that the American engine was underpowered. They came up with the solution of dropping in one of the new Merlin engines, and suddenly it worked.

"But it then took a huge battle to get the change of engine accepted. It is a story of how things got done that changed the course of the war, and there are a number of stories like it that are very little known."

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There is also a broader lesson to be drawn: "It is a lesson for the people who run organisations, military or otherwise, that you must allow room for experiments and for eccentrics, for the man in the middle of the organisation who solves things. When I tell this to scientists or business people, they get it straight away."

Working on two books at the same time is a habit that dates back to postgraduate days when Kennedy had simultaneously to manage his own doctoral studies and to act as research assistant to the military historian Basil Liddell-Hart, and found it stimulating rather than burdensome.

Kennedy's other project is a study of Rudyard Kipling. This might seem an odd choice for a historian renowned as a "big picture" man, but Kennedy has always been interested in individuals who illustrate his larger themes - another long-cherished, although as yet unfulfilled, project is a study of the German nationalist naval leader Admiral Alfred von Tirpitz.

"Kipling is hard to avoid if you're interested in imperial history," he argues, "and he has always fascinated me. He was a significant anticipator - even though he is known as the Bard of Empire he was very concerned about British imperial decline. He was also remarkable in writing in so many different genres - there are people you think of as poets or novelists, but Kipling was more akin to a musician such as Haydn, working across the whole range of his art."

Here, too, there is contemporary resonance. "Since Bush started his adventure in Iraq you'll find a vast increase in the number of Google hits on Kipling and all of his major works," Kennedy notes.

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"The big one for American conservatives is The White Man's Burden, which makes them very uneasy. It can seem triumphalist with its talk of child nations, but then you hit stanza five, which warns that they'll never like you, they'll soon want you out, and don't believe they'll ever be grateful. It is very contemporary."

The book will not be, he emphasises, either "Kipling for Iraq Dummies or a full-scale biography - there are already about six really fabulous biographies and there's no point in competing with them".

Kennedy sees it, instead, as "picking up the threads of intellectual interests that had buzzed in the back of my mind for years".

These books also represent a return to a more conventional academic trajectory, doing work chosen entirely by himself.

This is after two decades in which the success of The Rise and Fall of the Great Powers led to his being diverted first into Preparing for the Twenty-First Century (1993) and then, when he was planning a diplomatic history of the 20th century, into the consultancy work with the United Nations that fed into The Parliament of Man (2006).

He has no regrets about the way his life was changed by the book, and he may yet return to the magnum opus. The predictions about the world offered in the final chapter of Great Powers, a comparatively late addition to its historical narrative, looked towards 2010.

Accordingly, Kennedy says: "The possibility exists of doing a second edition in 2010, but I haven't decided exactly how to do it. If I rewrite that final chapter, will readers wonder what it was in the 1988 book that caused such a stir?"

Little in the past two decades, Kennedy believes, has undermined his underlying thesis that empires decline because they become stretched beyond the limits of their resources. He did not, as is sometimes reported, recant in a Financial Times article in 2001.

"It was the year when the World Economic Forum was held in New York," Kennedy recalls. "Everybody there read the first six paragraphs and was then interrupted by bumping into somebody they wanted to talk to."

Those opening paragraphs looked at the overwhelming military strength of the US, underpinned by an economy that was still growing fast at a time when its military rival, the Soviet Union, was imploding and its economic rival Japan had gone into a brutal and unpredicted recession.

"There is no point in denying that the 1990s worked very beneficially for the United States," Kennedy concedes, "but the underlying long-term point remained the same and has certainly not lost its force in the 2000s."

At the same time, he remains cheerful about the former empire he was born into. He has retained his British citizenship and has thoroughly enjoyed spending much of the past year as a visiting professor at the London School of Economics while living in Cambridge.

"One element in that (enjoyment) is indulging in passions - old churches, Ordnance Survey maps and good pubs. But there is also much to be positive about and I find the current mood difficult to understand, although it clearly takes something from the gloom of the London tabloids."

Kennedy's enthusiasm extends to Britain's universities: "I'm hugely impressed by the strength in depth of the British academic world. It extends well beyond traditional places such as Oxford, Cambridge and London.

"There are places such as York, Manchester, Edinburgh and Sussex that are stunningly good. There is too much bureaucracy and too few resources, but both the productivity and quality are astonishingly high."

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It seems a good bet - and Kennedy could probably offer you the odds, given that he worked as a racing tipster before being propelled into academic research - that Uncle Leo and Grandma would have been proud of him.