We do not usually think of Winston Churchill except as statesman — one of the greatest, indeed, of the 20th or any other century. But he would still be remembered today for his books even if he had never become the savior of his country, or entered politics at all. He belongs in the select company of Julius Caesar and Alfred the Great, among those rulers who were also writers.

Churchill wrote all his life: His first book appeared in his early 20s, his last in his mid-80s. He wrote so much mainly because he needed to earn a great deal of money to finance his lifestyle, though it would be churlish to call him extravagant: He was, after all, born in Blenheim Palace, the grandson of a duke.

He was a fluent and versatile author who could turn his hand to almost any genre, including fiction, but it is chiefly for his work as an historian that he received the Nobel Prize for Literature, and rightly so. The award of the highest literary accolade to an amateur occasioned a good deal of envious or patronizing comment from men of letters at the time and since. Churchill, however, knew more about writing than any writers know about politics — and he was a lot less pretentious, too.

“A History of the English-Speaking Peoples” was the octogenarian’s last book: Begun before the war, Churchill could finish it only after retiring as prime minister in 1955, by which time he had suffered a serious stroke and was beset by many other ailments. Yet this chronicle of what was not yet known as the “Anglosphere” is in many ways his epitaph. It expresses all that this child of an Anglo-American marriage stood for — his vision of a future in which the terrible conflicts that had divided humanity in his lifetime would be reconciled by the benign but unrelenting determination of the Anglo-Saxon nations to transplant the secret of their liberty to every corner of the earth.

Despite his extraordinary political career, Churchill wrote on a grand scale: four volumes on his ancestor Marlborough, five on World War I, six on World War II. The “English-Speaking Peoples” required a mere four, but they stop in 1900 when the story is just reaching its climax. So it was a good idea to bring the story up to date, and a capital one to entrust the task to Mr. Roberts, who has taken on the Churchillian mantle, if not yet as a politician, then certainly as a historian.

From every point of view, Mr. Roberts stands in comparison with the giant on whose shoulders he sits. His prose is vivacious, even mischievous, and pugnacious without ever becoming tediously polemical. He has mastered a prodigious quantity of material, drawn from wide knowledge of the archival sources and the copious reading of a genuinely cultivated mind.

Above all, Mr. Roberts has a cracking good tale to tell, and he tells it very well. Whatever else the last century has been about, the inexorable rise of the English-speaking peoples is impossible to ignore. Just as the 19th century had been dominated by British politics, economics, and culture, so the 20th proved to be the American century, and the Anglophone ascendancy bids fair to shape the 21st, too. The distinctive way of life of an insular nation became first a European, then a trans-Atlantic, and ultimately a global civilization.

Mr. Roberts writes books not for specialists but for the general public, and he is successful enough to be able to subsist as a private rather than a tenured scholar. Consequently, he is free not only of academic jargon but also of the prejudices against Judaeo-Christian values that predominate in the universities of the West. Mr. Roberts has his own prejudices, of course: in favour of patriotism, and even imperialism, of the Anglo-American varieties, and against “the politics of the preemptive cringe.”

Mr. Roberts is well-known throughout the English-speaking world as a conservative, and this book will delight all who are tired of liberal narratives. In this sense, he is a revisionist — as are all conservative historians who, like him, challenge the assumption that history is a matter of impersonal abstractions — globalization, secularization, decolonization, urbanization, or any number of others — rather than of individuals and peoples.

A vast undertaking such as this requires an organizing principle, and Mr. Roberts finds his in the succession of “assaults” on the primacy of the English-speaking peoples. He summarizes the rivals thus: “Tirpitz’s proto-fascism, Hitler’s fascism, Soviet Red fascism, today’s Islamo-fascism: all are profoundly antagonistic world philosophies from that which actuates the English-speaking peoples.”

The first of these challenges came from Prussian militarism, which posed a threat that we tend to underestimate by comparison with the Nazis. The First World War — a phrase that shocked contemporaries, because it implied there would be a second — is now usually dismissed as a futile bloodbath. Yet when in 1935 the poet and classicist A.E. Housman recalled “the most formidable assault which has ever been delivered upon the safety and freedom of these realms,” he spoke for all his contemporaries.

The Great War, as it was generally known, enabled America to make its presence felt on the world stage for the first time. But though American intervention was decisive, the Treaty of Versailles that followed proved to be merely, in the words of Marshal Foch, “an armistice for 20 years.”

The conventional wisdom — which derives from John Maynard Keynes — is that Versailles was too harsh and is thus blamed for the rise of Hitler. On the contrary, argues Mr. Roberts: The problem was not the treaty but the failure (especially by the Americans) to enforce its terms. The injustice was not Germany’s “war guilt” but the Allies’ “peace guilt,” which paved the way for isolationism and appeasement.

Mr. Roberts is scathing about claims of moral equivalence between Soviet communism and English-speaking capitalism. His short, sharp comparison of their records in power is a useful corrective to the academics who write books with titles such as “American Gulag” and “Britain’s Gulag.” He expertly debunks all the myths of the left, from Suez and Vietnam to Chile and Iraq.

Over the course of a century, a pattern emerges: “the essentially pacific English-speaking peoples and their allies coming under sudden, unprovoked and usually lethal attack from an aggressive foe whose assaults must be militarily avenged if honour and prestige are to be secured.” The English-speaking peoples did not invent the ideas that made them great, but they have been uniquely successful in using them for their own good and that of humanity. Ideas now dubbed neoconservative, such as the export of liberal democracy out of hard-headed self-interest or pre-emptive warfare in self-defense, are really long-standing Anglo-Saxon traditions that have been constantly adapted to new circumstances.

If Mr. Roberts’s splendid and thoroughly Churchillian sequel has one overriding message, it is that the English-speaking peoples are invincible for as long as — and only for as long as — they are united in a common purpose. Hitler and Stalin both came close to world domination by exploiting trans-Atlantic divisions, and now the Islamo-fascists and their apologists in the West are playing the same game. The Anglo-Saxons are not infallible — Mr. Roberts lists “a long and at times shameful catalogue of myopia and failed statesmanship” — yet they are overwhelmingly the force for good in our world. He gives the last word to Churchill, in a speech at Harvard: “As long as we are together, nothing is impossible.” We ignore the greatest Anglo-American’s parting shot at our peril.