“I have not become the King’s first minister,” Winston Churchill famously remarked in 1942, “in order to preside over the liquidation of the British Empire.” It was an entirely predictable thing for Churchill to have said, steeped as he was in imperial glory and martial endeavor. Not only had he fought on India’s northwest frontier, in Sudan and on the Western Front, but he had stood for much of his political career as Britain’s leading imperialist. Yet in the six huge volumes of his war memoirs, replete with quotations from other speeches, these words do not appear once. They had become an embarrassment to Churchill, because, for all his bravado in 1942, presiding over the end of the British Empire turned out to be precisely what he would do.

One of the achievements of “The Last Thousand Days of the British Empire” (Bloomsbury, 592 pages, $35), Peter Clarke’s learned and elegant new character-driven history, is to remind us how sudden Britain’s fall from empire truly was. At the official celebrations of victory over fascism in June 1946, the massed bands of the United States and Soviet Union were invited to lead the parade, but the rest of the procession was a glittering testament to the unprecedented scope of Britain’s global imperium: bands from Canada, Australia, and New Zealand; engineers and artillery from India; the Gurkha Rifles; the 10 West African corps from Nigeria, the Gold Coast, Sierra Leone, and Gambia; 26 more corps from East and Central Africa, and then yet more troops from Aden and Bermuda, Cyprus and Ceylon, Fiji and Tonga and Malaya and Jamaica, to name but a few. Little Malta, its people awarded a collective George Cross for bravery, was given the honor of bringing up the rear.

Yet just over a year later, in August 1947, India, the jewel in the imperial crown, was gone, and King George had to get used to signing himself, instead of “RI” (Rex Imperator), simply “R.” There is an argument that the Empire enjoyed a comeback in the 1950s, with British politicians talking earnestly of their new “partnership” with the African and Asian colonies. But, as Mr. Clarke argues, with India gone, the game was really up. As he puts it, his book tells the story of how the British suddenly came to find themselves, after the war, “as much victims as victors” — victims not of their Nazi enemies, but of their American allies.

Indeed, among the greatest strengths of Mr. Clarke’s book is its refreshingly clear-eyed approach to what in 1946 Churchill romantically called the “special relationship.” Even during World War II, many Americans frankly loathed Britain and the British, and Mr. Clarke teases out a thread of Anglophobic isolationism that did not suddenly stop at Pearl Harbor. “When the Prime Minister said that he was not selected to be the King’s Minister to liquidate the Empire,” remarked Harry Hopkins, “every isolationist in America cheered him” — the reason being that, in their eyes, Churchill had laid bare the grasping imperialism at the heart of British diplomacy. American and British generals often got along extremely badly: The case of “Vinegar” Joe Stilwell, who called his British colleagues “bastardly hypocrites” and “pig f—ers,” was not unrepresentative. And neither Roosevelt nor Truman harbored much love for British imperialism; from the very start, FDR made little attempt to hide his belief that Britain must get out of India as soon as possible.

For Mr. Clarke, Britain essentially had no choice but to cooperate, since the threat to its political and commercial empire from American goods and American fleets mattered little in comparison with the threat to its very survival from Nazi bombers and Nazi bullets. What becomes clear from this account, though, is that for all the self-deluding imperial propaganda of the British press, epitomized by the cult of Field Marshal Montgomery, the victor of El Alamein, most British politicians and senior officials recognized very quickly that there was no point resisting the rise of the Pax Americana. As Harold Macmillan, Churchill’s man in North Africa, famously put it, the aim now was to become “Greeks in this new [Roman] empire.”

Whether the prime minister himself shared this view, however, is another matter. Churchill dominates this book, a far more substantial figure than more elusive presences such as Gandhi and Jinnah, or even Roosevelt, whose presence in its subtitle seems more a marketing ploy than an indication of the book’s content. (The British edition, published last year, did not mention Roosevelt on the cover at all.) In Mr. Clarke’s eyes, Churchill clung to romantic notions of imperial glory far longer than most of his aides, which explains why at the hour of victory, he cut a less than ebullient figure, many of his illusions about the postwar world having been shattered. A more “well-briefed and prudently calculating leader,” Mr. Clarke says, would have realized Britain’s weakness earlier, and would therefore not have been so badly disappointed.

And yet this book’s portrait of Churchill is hardly unflattering. In many ways, the narrative is a tribute to his sheer presence. Just as we think the story is about to turn to India or Palestine, neither of which gets quite the attention it deserves, the great man reappears, demanding yet another turn in the limelight. Mr. Clarke tries to give us a more “fully plausible, fallible and inimitable” figure, illustrated with copious quotations from the diaries of Churchill’s colleagues, and he triumphantly succeeds.

We see Churchill’s weaknesses — his preposterously childish and selfish posturing, his refusal to prepare properly for meetings, his ignorance of the shortest and simplest briefs — but we also see his strengths, above all the spirit and drive that impressed even his fiercest critics. And he remains, of course, a supremely funny figure, whether annoying his aides by staying up all night playing cards on the flight to Yalta, or ostentatiously relieving himself in the Rhine in the wake of his armies. As Mr. Clarke puts it, if he had not been Churchill, he would have been better prepared for the loss of the empire. But then, if he had not been Churchill, he would not have offered “the British people his simple policy: victory at all costs.” And for that alone, he deserves his place at history’s highest table.

Mr. Sandbrook is a member of the Oxford University history faculty and the author of “Eugene McCarthy and the Rise and Fall of Postwar American Liberalism,” “Never Had It So Good: A History of Britain from Suez to the Beatles,” and “White Heat: A History of Britain in the Swinging Sixties.”