Few write about Winston Churchill without attempting to enlarge the meaning of his state funeral on January 30, 1965.

The denouement amid pomp and nostalgia on a grey London day is almost too perfect, the sense of an ending too resonant to overlook. Hindsight's broad brush needs a big canvas. For most, the tableau staged that day remains irresistible.

So it was that James Morris ended 1978's Farewell The Trumpets, the last in his Pax Brittanica "triptych", with 100 nations represented, the bugle's Last Post sounding, and a funeral train travelling through the gathering dusk "to the green country heart of England". "Everyone knew what was happening, even the enemies of Empire," wrote Morris, as though imperialism's foes were new upon the scene in the middle of the 1960s.

In the final pages of his 2001 Churchill biography, meanwhile, Roy Jenkins invoked Wellington and Gladstone for pallbearers. Of the state funeral, the languid scholar observed that here was "the last in the British tradition of imperial ceremony". That January they came from around the world to bury empire, not to praise it, but in his last words the biographer still nominated his subject as "the greatest human being ever to occupy 10 Downing Street".

Jenkins, like all of his political generation, knew the plain truth well enough. Britain's last claim to deference had disappeared finally in the humiliation of the squalid Suez adventure in 1956. Churchill's hopes of an empire renewed by the Hitler war had been exposed as utterly bankrupt. Nevertheless, like Morris, like so many others, Jenkins allowed a touch of la gloire in his summation. It seemed fitting. It felt unavoidable.

Lawrence James is no exception to this general rule, but his attitude - brisk, undeceived and politically non-aligned - is typical of his book. Often enough, he renders his twin portrait of man and empire as Churchill would have had it rendered - vivid, dramatic, even lurid - but the student is never spellbound by his subject. "The trouble was," James concludes dryly of the great funeral, "that Churchill had outlived the imperial ideals he had cherished."

We live still with the truth of that judgment. Winston Churchill was made for empire. Britain's empire, in its turn, might have been designed and created as a stage for the last leader from these islands capable of thinking and speaking, unblushingly and honestly, of national purpose and destiny. You can, without too much of a stretch, judge one ramshackle, self-created monument in terms of the other.

They were alike in their flaws and their virtues, in their hypocrisies and their decencies, their courage and their self-aggrandising pride. They each depended on unlikely quantities of sheer luck. They were each ruthless and conscience-stricken by turns. Each relied on a gargantuan capacity for self-deceit and a profound misunderstanding of the world and what it contained. Yet, for an unfeasible length of time, they got away with it.

One question was left unasked at Churchill's funeral. It is still asked too rarely in the endless arguments over colonialism and its legacy. What had been the point of it all? Amid their grand schemes the British imperialists, brilliant or dim, did not often pause to wonder about the worth of all their efforts, their sacrifices and their cruelties. Churchill could furnish the usual excuses with the best of them - morality, civilising purpose, the "inherent" superiority of his "English-speaking peoples" - but he did not pause for long over the contradictions.

He gloried in war. As James describes it, Churchill couldn't help himself, whether thrilling as a young man to the "stirring anachronism" of the cavalry charge at Omdurman, or imposing Britain's will on Iraq in the 1920s through "air control", he was invigorated by conflict. He consoled himself at every stage of his career with the unexamined conviction that a victory for the empire was synonymous always with "another stride forward for civilisation".

The truth of that belief was rarely self-evident. Churchill was therefore as indulgent of his taste for guilt as he was of his appetite for sentimentality. A striking aspect of the James portrait is the frequency with which his subject disowns the acts of his beloved empire. Omdurman was disgraced by "the inhuman slaughter of the wounded". The strafing of women and children in Iraq was "a disgraceful act". The treatment of Boer prisoners, the Amritsar massacre in India, the Black and Tan outrages in Ireland: this civilising mission was a bloody business.

And for what? Churchill's career, like his rhetoric, provided a script for an empire founded on nothing better, most of the time, than profit and prestige. When the former became hard to come by, the latter became an end in itself. So India had to be held at all costs. Therefore Suez and Egypt had to be held. Therefore Russia had to be forestalled, Germany countered, native peoples brought to understand - forced to understand - their place within the imperial family.

In an envoi to his narrative, James depicts Churchill celebrating his last day as Prime Minister in April 1955. After all the blood, toil, tears and sweat - most of it wrung from the bodies and lives of people given no choice in the matter - the old man had still not resolved "the quintessential dilemma of Empire". As James writes: "Was the process of bringing progress and civilisation to those who lacked them compatible with their coercion?"

A supplementary question could be added. Britain is afflicted still by memories of empire. "Churchillian" is still the state to which all puny party hacks aspire. The defeat of Nazism remains the single unblemished passage in the long story of Churchill and his empire. But what of the rest? Country upon country has been returned to its peoples: so much is obvious. Yet what of the legacy of the cause for which Britain's fleets and armies were summoned time and again?

Should Iraq be grateful to the empire? Egypt, South Africa, Afghanistan? Those states in which the rule of law and parliamentary democracy just about survive are held up still as imperial bequests, therefore as Churchill's bequests. But India, the best example ever adduced, was the jewel he would never have relinquished, given the choice. Around the world, dozens of other countries are still recovering, if they are recovering, from their meetings with the British.

In the James portrait Churchill emerges as something less than the devil incarnate of anti-colonialists. Contrary to legend, he did not cause the RAF to drop poison gas on Iraqi tribes in the 1920s. But he did consent to the hellish scheme, one that was only prevented by logistical difficulties. He did have a sense of honour and purpose, meanwhile, but both were sacrificed, time and again, to expediency and "racial dogma".

James has devised a fine portrait of eccentric genius in pursuit of a lost cause, of a blind devotion to an empty idea, and of an arrogance that never faltered. Churchill was as remarkable as he was tragic when, one by one, his beliefs in the enduring white man's empire were chipped away. But his final defeat was no loss for the wider world. What ended in the obsequies of 1965 was not a grand national pageant but a fantasy.