BIOGRAPHY: Hugh-Trevor Roper: The Biography,by Adam Sisman, Weidenfeld and Nicolson, pp648, £25

MOST REMEMBER Hugh Trevor-Roper thanks to the debacle over the Hitler diaries. Under pressure from Rupert Murdoch's newspapers, Trevor-Roper, ennobled by Mrs Thatcher as Lord Dacre of Glanton, authenticated the newly discovered texts. All too quickly the diaries were exposed as forgeries. Trevor-Roper was ridiculed in Robert Harris's exposé Selling Hitlerand caricatured when Alan Bennett played him in the subsequent film. It was an unhappy episode upon which enemies – by then numerous – seized. The foibles and vanities which led to the misjudgement are laid bare in Adam Sisman's fine biography. So too are the reasons why Trevor-Roper, combative, imperious and provocative, had accumulated adversaries who were delighted to see him humbled.

Fortunately Sisman sets the contretemps in a longer perspective. He explains both why Trevor-Roper was turned to as a renowned expert on Hitler and the forces which went into the making of a complex personality. By power of intellect and assiduous application, Trevor-Roper established himself as one of the most original and incisive historians of his generation. At the same time, this son of a Northumberland doctor, reared in the shadow of a ducal castle at Alnwick, hauled himself up the social ladder. Educated and later a tutor at the grandest of Oxford colleges – Christ Church – he wove a dense filigree of connections within the ruling and cultural elites. Much of this was achieved through his own charm and intelligence, but it was consolidated and extended through marriage to a daughter of the first Earl Haig.

Sisman traces the assiduity with which Trevor-Roper advanced a variety of causes, including his own, but also the capacity for mischief and fun which made the young and irreverent gravitate towards him. A shy man, raised in a chilling emotional atmosphere worthy of an Ivy Compton-Burnett novel, he veered between introspection, self-doubt and gregariousness, typically in male company. He applied his formidable analytical skills to the matter of intelligence during the Second World War, contributing importantly to the decoding of German communications. His unique combination of forensic, historical and literary skills earned him the job of demonstrating to the world that Hitler was indeed dead, killing himself and Eva Braun in the Berlin bunker. The Last Days of Hitlerbrought fame and fortune to a writer hitherto known only for an iconoclastic study of a 17th-century Archbishop of Canterbury.

Thereafter, Trevor-Roper ranged boldly across the centuries and continents. Born a borderer, near the contested frontier between Scotland and England, with Welsh ancestry (the Trevors) and a mother who seemed to personify the grimmest stereotypes of northern Irish Protestantism, passionate about horses and fox-hunting, it was almost inevitable that he would not content himself with a narrow view of the history of England. He pioneered what has since been dubbed portentously "the new British history". When, in 1966, he agreed readily to supervise my researches into Cromwellian Ireland, I discovered how familiar he was with Ireland. Bit by bit – often after generous ingestions of the hock hauled up from his cellar – he recounted his Irish adventures. During the second World War, because the hunting season lasted longer in Ireland than England, he travelled over to Co Limerick. Frank Pakenham, later Lord Longford, a Christ Church colleague from whom he had cadged introductions, tipped off Frank Gallagher about the visit. As a result, gardaí burst into Hugh's bedroom in the Royal Hibernian Hotel, where, with pistols pointed at his head, his luggage was ransacked. He was suspected – like Elizabeth Bowen and his friend John Betjeman – of being a spy. Nothing more incriminating than a pair of black silk stockings was found. Undaunted by this reception, Hugh returned: sometimes to lecture; frequently on the social circuit that he and his wife relished. Survivors from the dwindling ascendancy became his pupils at Christ Church. Desmond Williams, a powerful presence in UCD, sharing Trevor-Roper's interest in Nazi Germany, was important in bringing Hugh regularly to Ireland to talk. Williams was a former fellow of the Cambridge college most attractive to the clever Irish: Peterhouse. In the 1980s, Trevor-Roper would become its Master. An influential intervention in a historical controversy was first delivered (in 1961) to a Galway audience, as Hugh phrased it, "powerfully reinforced by local monks and nuns". The listeners were adjudged unsympathetic and also uncritical. The auditors were polite but understandably restive since the speaker arrived long after the time scheduled for the lecture. Academic confrèreshad insisted on lunching him at a then notable hostelry and the lunch had proved so excellent that it was much prolonged. Later, at the height of the Troubles, he gave a series of four lectures in Belfast on the unpromising topic of the 17th- century ecumenical movement. Only one has ever been published. Part of the entertainment was sherry in the primatial palace at Armagh, then still occupied by the archbishop. The incumbent, George Otto Simms, delightedly reminded Trevor-Roper of the Christ Church connections of the builder of the palace, Archbishop Robinson. Latterly, Trevor-Roper, returning with relief from Cambridge to the familiar and friendly world of Oxford, became an enthusiastic supporter of the newly established Carroll chair of Irish history. At one celebratory event in the Irish College in Oxford, Xandra, his wife, revealed an unexpected taste for quaffing black velvet.

One of the numerous merits in Sisman's life is that it captures the ebullience of its subject, the qualities which so endeared this undoubted grandee to his graduate pupils. Ever the gadfly, he delighted to sting, usually discriminatingly. Gleefully, even gratuitously, anti-clerical and anti-Catholic, he modelled himself on Edward Gibbon. Sometimes, late into the night and invigorated by the hock, it seemed that one was in the company of the contemporary Voltaire. In time, the furore over the bogus diaries will fade and instead The Last Days of Hitlerwill be read and re-read, sending off admirers to search out the dazzlingly written essays. That process of rediscovery is mightily encouraged by Sisman's account, itself compulsive reading.

Toby Barnard teaches history at Oxford University. His first book, Cromwellian Ireland, was reprinted in 2000