Few scholars have been on such intimate terms with that tricky duo, hubris and nemesis, as Hugh Trevor-Roper. It may appear absurdly inflated to invoke Greek tragedy to describe the life of someone who, rather than being a statesman or general or artist or other traditional "great man", was a historian and therefore a member of a tribe whose deities might seem to be scepticism and caution. Yet in reading his biography it is hard to escape a feeling of horrified fascination as, over and over again, the stakes rising at each new turn of the wheel, the overconfidence engendered (at least in part) by his prodigious talents led him to court, and eventually to encounter, disaster.

Nothing in his childhood, as the eldest son of a modestly successful country doctor, suggested what was to follow, except perhaps the combination of intellectual precociousness and a lack of love. Public school and Oxford accentuated both aspects of his upbringing; they also fostered in him an enduring susceptibility to the social, sporting and alcoholic tastes of the English upper classes. Precociousness became almost his trademark. In 1940, at the age of 26, he published an obviously clever if overly provocative first book, on Archbishop Laud. As a result of his role in wartime intelligence, he was called upon to investigate the circumstances surrounding Hitler's death, with the result that he wrote an international bestseller, and perhaps minor historical classic, The Last Days of Hitler, when he was 33. By the time he was 41, he was being paid a handsome retainer by the Sunday Times, just entering its great days, to write "special articles", which he did for more than 35 years. He married the daughter of an earl and was on visiting terms with heads of state. He was made regius professor of modern history at Oxford at 43. Was there anything that he couldn't do?

"Complete a major work of history," was the answer his contemporaries increasingly gave. Trevor-Roper failed to finish a truly remarkable number of books. His biography is studded with abandoned manuscripts and unfulfilled publishers' contracts. He tackled the big subjects, always in an original and combative way: the causes of the English civil war, the relations between Protestantism and capitalism, the European witch-craze, as well as Hitler and the origins of the second world war. He published some dazzling essays on these and other topics, but there were so many other things to do, so much money to be earned, so many duchesses to meet (he had a weakness for duchesses), so much journalism, so much travel . . . It says a lot about both his productivity and his underachievement that in the seven years since his death his executors have already brought out nearly as many books under his name as he himself saw through the press in his lifetime.

Something else he conspicuously failed to do was to win universal affection. He could be superior, sharp-tongued and downright dismissive. He made enemies with abandon: sometimes they were bores, sometimes they were fellow scholars (two categories he was prone to conflate), but sometimes they were whole social groups  he was pretty offensive to Christians, especially to Catholics (he couldn't be received at some of the best Catholic country houses, much to the chagrin of his snobbish wife), and for a while he came close to being public enemy number one in Scotland (he was very rude about Scottish history, and extremely rude about Scottish historians). Some of this was high spirits; some was love of the witty phrase (his own, anyway); some was the solipsism of the very clever; and some, it seems, may have been the expression of an emotional awkwardness, a deep self-protectiveness that few were able to penetrate. But when you have AL Rowse, of all people, asking, "Why are you so nasty to people?", you should realise all is not well with your character.

Nemesis lurked at Trevor-Roper's elbow in part because he was always so severe on other people's scholarly errors while being prone to commit a few of his own. He was a magnificent controversialist and pamphleteer. He had a notable, and intellectually fruitful, passage of arms with Lawrence Stone over the economic history of the 17th-century gentry in relation to the causes of the English civil war. He had a high-profile spat with AJP Taylor about the origins of the second world war. And he courted international controversy by challenging the Warren Commission's report on the assassination of President Kennedy (he always believed he was a better detective than the professionals).

Having been ennobled by Margaret Thatcher in 1979 (as Lord Dacre), he took the Tory whip in the Lords, but he was at heart a kind of maverick Whig  allergic to pieties of all kinds, cultivating Gibbonian irreverence. When he unexpectedly became master of Peterhouse, Cambridge in 1980 he soon found himself at odds with the more notoriously reactionary of its fellows, casting himself in the unwonted role of modernising reformer. (This chapter in Sisman's book should be skipped by anyone of a delicate disposition: the ugliness of the behaviour exhibited by his opponents was in inverse proportion to the laughable tinyness of the teacup.) It looked as though it was to be a sadly inglorious final stage of his career. But there was worse to come.

As a result of the worldwide success of The Last Days of Hitler, Trevor-Roper had frequently been asked to adjudicate on the authenticity of various documents allegedly written by the Führer or his inner circle. He enjoyed exercising his considerable forensic skills; he enjoyed being treated as an authority; and he enjoyed receiving large fees for relatively little labour. In 1983, the German magazine Stern claimed to have discovered Hitler's diaries, and the syndication rights were offered to the Times. If the diaries were genuine, this would be the scoop to end all scoops. Trevor-Roper was flown to Zurich to examine the documents held in the vaults of a Swiss bank. Circumstances dictated that he had to arrive at a decision quickly: he decided they were real, and the Times went ahead with the deal, with extracts to be serialised in the Sunday Times. The advance publicity was mountainous, and Trevor-Roper was very publicly staking his reputation on this one, rushed decision.

Others were more sceptical; the evidence started to look shakier; even Trevor-Roper's normally assured confidence began to waver. At a little after the last minute, he havered and made himself look foolish, even donnish, itself a defeat for someone who had always soared above such stereotypes. But it was too late for him or the Times to pull back. It was a case of publish and be damned  and boy, were they damned. When the "diaries" were revealed to be the work of a fraudster, Trevor-Roper was swept away in the mudslide of gloating. His reputation never properly recovered; the Times's own headline on the day of his death 20 years later was "Hitler diary hoax victim Lord Dacre dies at 89".

As Sisman reflects at the end of this hugely detailed but consistently engrossing biography, that was unfair and will surely not be how posterity rates him. Sisman has some of the partiality of the biographer who knew his subject and was given the run of his huge cache of personal papers (whatever else might be said about Trevor-Roper, he would have to be recognised as one of the great letter-writers, not out of place among the epistolary stylists of the 18th century). But he has done his subject a great service, both by putting together such a detailed narrative almost entirely from archival sources, and by giving us glimpses of a more ardent, more melancholy, even in his later years more lovable figure than the public image. Trevor-Roper's work receives rather scant attention in places, but otherwise the thoroughness, fairness and frankness of this biography are exemplary.

I had expected that reading a life of Trevor-Roper would stir a kind of envy in me  envy of his opportunities and of his moment, but also envy of his confidence and courage, and above all envy of his gifts as a historian and a writer. It certainly does that, but, quite unpredictably, one of my chief feelings on concluding this very long but interest-packed book is a kind of sympathy, almost of pity. There is the obvious tragedy of a man burnt by the flame of celebrity, but there is the deeper pathos of someone driven to shine, someone whose intellectual development in the first half of his life so outstripped his emotional development that he formed habits of insensitivity and egotism from which he never fully recovered. He was too clever and too knowing not to realise that his boundless ambition was at times sabotaged by the ease with which he could deploy his rich talents, including his talent to wound, and this knowledge is part of his pathos.

The peak of that ambition was to write a book that "someone, one day, will mention in the same breath as Gibbon". Anyone who confesses to such an ambition is obviously not shy about chatting up hubris and so has to accept that nemesis will come along and rough him up sooner or later. Trevor-Roper never completed his Decline and Fall, and perhaps that was a minor tragedy of sorts, but he did write a lot of long analytical essays that had enough ideas in them to keep several seminars' worth of lesser historians plodding in his tracks for years to come. And, as this biography reveals, he lived a life that was, for a scholar, unusually rich and varied in its achievements and gratifications. Anyway, nothing, I suspect, would have galled him more than the thought of lesser mortals feeling sorry for him. But then, perhaps nemesis never quite turns out to be what you expect.

Stefan Collini's books include Absent Minds: Intellectuals in Britain (Oxford).

Hugh Trevor-Roper, once a regular reviewer for this paper, was an English prose stylist with few rivals. He was also a historian who never lost sight of the function of history: to tell the truth. True, Trevor-Roper loved the minor squabbles of the common room, and the impenetrable feuds of the letters pages. He loved to score points off his enemies, and how those enemies rejoiced when he made the monumental blunder of momentarily authenticating the Hitler diaries for Rupert Murdoch's bully-boy editors at the Sunday Times in 1983. But the side of Trevor-Roper who loved the minor field sport of don-baiting was only one side of a much grander figure at home on the wider uplands of European humanism.

When, in his early 40s, he was rewarded with the regius chair of modern history at Oxford, he expressed to his old mentor, JC Masterman, his exasperation that Oxford's history faculty had become "a pitiful backwater" when compared with Chicago, Paris, Florence or Stockholm. When in old age he found himself the master of Peterhouse, Cambridge, he reviewed Religion and Public Doctrine in Modern England by Maurice Cowling, the history don who had secured him the mastership. Cowling was the guru to such rightwing journalistic luminaries as Peregrine Worsthorne and Colin Welch of the Telegraph, and to that extent he was a person of influence. "The subject is the intellectual history of our time and the great spiritual crisis in which we have found ourselves," Trevor-Roper wrote. "I find, on reading it, that this intellectual history has unfolded itself, and this crisis has been observed, and is to be resolved, almost entirely within the walls of Peterhouse."

Trevor-Roper's life, as this admirable book reveals, was lived on an altogether broader stage. After a brilliant academic start at Oxford, he joined the army at the beginning of the second world war and worked as an intelligence officer. His excellent German enabled him to interrogate many of the Nazi criminals, and he returned over and over again to the ruins of Hitler's bunker in Berlin. The result was what remains one of the best books ever written about the Third Reich  The Last Days of Hitler. Joachim Fest, the German historian whose book on the same theme was made into a successful film (Downfall), added almost nothing to Trevor-Roper's research. At the time, just after the war, Trevor-Roper had scotched the wild rumours about Hitler being alive; but more than that, he put the reader instantly in the right frame of mind to contemplate the National Socialists. His style was that of an enlightened, witty humane being; he made the last days in the bunker into a grotesque Gibbonian comedy.

Yet Trevor-Roper was unusual among historians of his generation in having the patience to take Hitler seriously as a thinker. His essay "The Mind of Hitler", an introduction to Hitler's Table Talk, is not only a masterpiece, but a useful corrective to those, such as Alan Bullock or Lewis Namier, who wanted to dismiss Hitler as a buffoon. It took a malign genius to rise from poverty in the slums of Vienna to carry through the conquest of western Europe, the invasion of the east, and the mass murder of the Jews.

So rich and varied was Trevor-Roper's life that it is worth considering the qualities his ideal biographer would need to possess. He would be a historian who understood the workings of history. He would also understand the world of journalism and have a sense of what Trevor-Roper himself liked to call the beau monde. He should be at home, as Trevor-Roper was, with prime ministers and duchesses. He would appreciate Trevor-Roper's malice but not share it, for that would be arch and tedious. But he would have a sense of the great generosity of Trevor-Roper's mind. He would also have a sense of Trevor-Roper's quirkiness, his love of the offbeat, the slightly naughty  witness his superb book The Hermit of Peking, a life of the fraudulent sinologist and homosexual pornographer Sir Edmund Backhouse. (Readers of the Backhouse book might have suspected, as did Trevor-Roper's own wife when she first met him, that he had a buried homoeroticism in his nature. Such readers will be astonished by the depth and passion of his relationship with that vague, boney wife, Lady Alexandra Howard-Johnston, as revealed in their extended correspondence.)

Finally, he would need to be someone with the delicacy to reveal that well-concealed organ, Trevor-Roper's heart: "I give my heart to you  rather a complicated object, you may say, like a sea-urchin, prickly outside and untempting within; but you asked for it," he wrote to Alexandra.

How lucky for Trevor-Roper, and for us, that the ideal biographer was here. It is impossible to praise Sisman's book too highly. It is sprinkled with the light comedy of academic malice (healthy minded readers will shout with approval during Trevor-Roper's triumphs over the deplorable fellows of Peterhouse). It will not disappoint those who reread Trevor-Roper's hilarious spoof, the Letters of Mercurius Oxoniensis  anonymous dispatches printed in Another Magazine about the student protesters of the 1970s and the antics of his fellow dons. But Sisman's book will also remind us all of why we value the life of the mind, and why style and intelligence are not superficial weapons against the forces of darkness.

Even the episode of the faked Hitler diaries, although it made a fool of Trevor-Roper (who had by now been awarded a life peerage, choosing the title Baron Dacre), does not really reflect all that badly upon him. As a director of the Sunday Times who had written about Hitler, he was the obvious expert to send to Switzerland to authenticate the diaries, when Murdoch decided he wanted to publish them. Trevor-Roper was initially sceptical, although, in common with another expert, Gerhard Weinberg of the University of North Carolina, he was impressed by their sheer bulk. Then he read the entry about Rudolf Hess flying to Scotland in May 1941, and he began to smell a rat. Doubts soon turned to outright disbelief. By then, though, it was too late. Murdoch wanted a splash, and when told of Trevor-Roper's doubts he sent back the now famous message  "Fuck Dacre"  which they proceeded to do. Trevor-Roper might well have been a habitual mischief-maker, but he was a perfect gent. He never made public the way that his proprietor or editors had behaved, and he never complained.

Bores liked to shake their heads about the Great Book that Trevor-Roper never wrote  too busy gossiping, dining out, writing journalism, pursuing very public feuds, and darting from one subject to the next: now the Elizabethan gentry, next Oliver Cromwell, then the pretensions of the Scots, then the mind of Erasmus. Certainly Trevor-Roper was acutely conscious that his own monumental work on the Puritan Revolution was being neglected. And yet, such is the skill of Sisman that we do not feel much of Trevor-Roper's life was wasted. Greatness can be revealed in an essay; and in an index-entry: as with "Aquinas, his Whig views". Trevor-Roper excelled in short forms, not because he had so little, but because he had so much to say. This great book confirms my sense that Trevor-Roper was not merely a clever, but also rather a great man.