Siegfried Sassoon: a biography, by Max Egremont

Plenty of other people were on hand during Sassoon's long middle-to-old age - he died in 1967, a few days short of his 81st birthday - to confirm his apparent detachment from the life that burned on around him. Anthony Powell, visiting the old man's estate in the early 1960s, noted that "for Captain Sassoon, though no longer involved in it, the first war was still in progress". Powell thought that the vague, dreamy squire, wandering around his sombre manor house, "looked like a ghost from the fields of Passchendaele or Bapaume".

As Max Egremont's new biography - unmistakably the best thing anybody has ever written about Sassoon - demonstrates, the Great War gave the aspiring poet his vocation. In its aftermath, nothing - with the exception of one bizarre relationship with a man 20 years his junior - would ever be the same again.

In strict literary terms, the author of "The General" and "Everyone Suddenly Burst Out Singing" is a textbook example of the writer as man of action whom no subsequent prize can ever quite compensate for the lost physical excitements of his youth. Sassoon, a less moneyed offshoot of the Jewish manufacturing clan (although family resources still allowed him an income of £500 a year), had uniformly dim pre-1914 prospects. A diffident Marlborough public schoolboy and an unsuccessful Cambridge undergraduate, he drifted through into early twenties publishing his poems - at this point sedate and pastoral - at his own expense and fretting about his homosexuality. His literary earnings in these early years were below £5.

The war did two things for Sassoon. On the one hand his service in the trenches, where his acts of bravery frequently shaded into outright recklessness, authenticated a sense of manhood that had previously only been nurtured by the hunting field. On the other, it provided the raw material for a series of poems on war's futility, whose importance was incalculably strengthened by the public profile of the poet.

As a serving soldier with an MC on his lapel Sassoon was, to quote Waugh again, "the perfect person" to argue the case for other serving soldiers who suspected the war was being unduly prolonged by people who were doing well out of it. The meteoric success brought by his collections The Old Huntsman (1917) and Counter-attack (1918) was stoked by controversy. Back in England, recovering from wounds, Sassoon declined to obey further orders, issued a pacifist statement, and was eventually removed by the authorities to a mental institution in Scotland. Returning to Flanders in 1918 he was wounded again - mistakenly, by one of his own troops - and went back to Blighty for good.

The war had presented Sassoon with a mental dilemma - how to reconcile his terror of the trenches with an urge to "prove himself". Post-Armistice, he was caught in an aesthetic trap: the difficulty, now the guns had fallen silent, of finding a subject. Why couldn't he create something, he wondered irritably in 1921? The question clanged on endlessly for four-and-a-half decades. His entire post-Great War life, Egremont implies, was a struggle to fit in, to square his temperament (passive, reflective, "English" in a practically antediluvian sense) and his sexuality with a world and a literary climate that he regarded with deep mistrust.

Nowhere was the strain of accommodating these different sides of his nature more apparent than in his relationship, begun in the mid-1920s, with Stephen Tennant. Egremont's account of the six-year affair between the craggy war veteran of fixed tastes and humours and this effeminate ornament of the Mayfair drawing rooms ("You despicable pieces of filth," a woman once shouted, seeing them together in the street) is perhaps the best thing in the book.

It shows on the one hand the passionate attraction between the two ("I ask only to be near him always," Sassoon once wrote) and on the other the hulking temperamental fractures - Sassoon's disapproval of Tennant's flibbertigibbet friends, "Steenie"'s habitual flightiness - that would eventually drive them apart. In the end, the narcissistic and consumptive Tennant seems to have decided that having "Sieg" on the premises, where he antagonised the nursing staff and barred the door to callers, was making him worse.

Meanwhile, the poems had given way to the two autobiographical prose sequences begun in 1928 with Memoirs of a Fox-hunting Man - commercially successful, but progressively more staid and, the critics pronounced, disconnected from the pulse of interwar Europe. "Back to the jingle of hansom cabs in the recollected calm of 1907," as Egremont characterises one of the later volumes. On the rebound from Stephen, he married a much younger woman, Hester Gatty, with high hopes of uxorious serenity, but the marriage foundered on locked bedroom doors and a writerly need for quiet. There was a single son, George, a divorce, and a great deal of ill-natured bickering.

Sensitive to his subject's ever-changing moods, in which pique, hauteur and amiable generosity routinely combined, Egremont is adept at uncovering the odd mixture of idealism and complacency that coloured Sassoon's late-period outlook on life. Had up at Salisbury magistrate's court on a motoring offence - he was a famously erratic driver - he complained "How many of them suspected that I have written poems in the last 2 months which will be glorious long after Salisbury Town hall has been pulled down and carted away?" Towards the end - a rather foreseeable destiny for this brand of visionary English mysticism - came Mgr Ronald Knox, Dame Felicitas Corrigan and the consolations of the Catholic Church. As he was dying of stomach cancer, the stink of the Flanders mud still rose in his nostrils. "This is going to be the final test of my endurance," he told his son, "and I intend to put up a really good show."

D J Taylor's 'Orwell: the life' is published by Vintage