How far back in history do you have to go before it stops making sense to take sides? Even today, it is impossible for an American to read about the Civil War or the American Revolution without engaging in silent partisanship. Hundreds of years after the fact we feel personally implicated in those struggles. In some sense that is not quite logical, but also far from discreditable, we root for the North to maintain the Union for the 13 Colonies to throw off the British yoke, as though our own fortunes were at stake. And of course they are: If those contests had gone the other way, we ourselves would not be the same people, and America would not be the country we know. To affirm our past is to affirm our present.

Likewise, to contest the past — to read history wishing that things had come out differently — is always a way of contesting the present. That is certainly the case in David Levering Lewis’s new book, “God’s Crucible” (WW Norton, 476 pages, $29.95). In telling the story of the rise of Islam and its conflict with Christianity between the sixth and 13th centuries CE, Mr. Lewis is driven by a 21st-century agenda. He means to strike a blow against what he perceives as Western arrogance and condescension towards the Islamic world. In fact, he argues, we should regard the Battle of Poitiers, where in 732 the Franks stopped the Muslim advance into Europe, not as a triumph but as a catastrophe. “The economic, scientific, and cultural levels that Europeans attained in the 13th century could almost certainly have been achieved more than three centuries earlier had they been included in the Muslim world empire,” he writes in his preface.

Such a claim, with its simple value judgment on events that took place almost 1,300 years ago, raises the question of historical partisanship in an acute form. Mr. Lewis asks us to contemplate a past where the armies of Islam, which had already conquered the Middle East, North Africa, and Spain, went on to sweep through France and Italy, trapped Constantinople in a continental pincer movement, and turned the Mediterranean into an Islamic lake. Yet if this had come to pass, the world today would be so different from the one we know that it is actually impossible to say whether it would be better or worse — not just because all of history since the eighth century would be unknowable, but because we ourselves, living in an Islamic West, would judge our fates by unknowably different standards. Mr. Lewis drives home his point by quoting Edward Gibbon’s famous verdict on the Battle of Poitiers. If the Arabs had won, he shuddered, “the interpretation of the Koran would now be taught in the schools at Oxford, and her pulpits might demonstrate to a circumcised people the sanctity and truth of the revelation of Mahomet.” For Gibbon this was a horror show; for Mr. Lewis it is a missed opportunity. But Mr. Lewis’s mischievous reversal of Gibbon only highlights how unhistorical the great historian was being when he conjured this upside-down world. For in his hypothetical Muslim England, of course, there would be no Oxford University, just as there would be no “Decline and Fall of the Roman Empire.” The counterfactual is so counter to fact that it is simply incommensurable.

Indeed, one feels that a historian with an intimate grasp of early Muslim history would be too alert to its foreignness to venture a paradox like Mr. Lewis’s. But Mr. Lewis is not such a historian. He is best known as the Pulitzer Prize-winning biographer of W. E. B. DuBois, and “God’s Crucible” is not the book anyone would have expected him to write. Even he seems a bit surprised at the way it turned out, to judge by his acknowledgments: “Some of those thanked below anticipated a different book; others probably never knew quite what to expect,” he admits. The reason for this uncertainty, it seems clear, is that Mr. Lewis’s subject belongs to the distant past, but his motives were entirely present-minded. “In this long, fraught saga of cultural roles reversed and hegemonies upended,” he writes, “we can discern many of the causes for the troubled history being made in the twenty-first century.”

Yet despite his revisionist intentions, the book Mr. Lewis has actually produced is remarkably old-fashioned. It is kings-and-battles history, crammed to the margins with names and dates, hurried along by Mr. Lewis’s expansive curiosity and his need to corral a huge volume of unfamiliar material. Starting in 570 CE, the year of Muhammad’s birth, Mr. Lewis energetically retells the story of Islam’s founding and its shockingly fast spread. This is by far the most useful part of “God’s Crucible,” since Mr. Lewis rightly judges that the classic stories of Islam — so important a part of world culture, and so vividly present to the minds of millions of Muslims — are largely unknown to most non-Muslim readers. He offers a useful primer on the career of Muhammad in Mecca and Medina, and the achievements of his successors down to Ali, whose martyrdom initiated the still-murderous split between Sunnis and Shiites.

Once the armies of the Prophet have conquered the Middle East and North Africa, the story shifts to the western end of the Mediterranean, as Mr. Lewis narrates the Muslim conquest of Spain and the subsequent centuries of political evolution in painstaking, at times painful, detail. Then Mr. Lewis turns to the Christian side of the Pyrenees, leading the reader through an equally minute chronicle of the fortunes of Charles Martel and Charlemagne. The clash of these faiths and dynasties, at Poitiers and elsewhere, forms the fulcrum of the book. The four subsequent centuries are dealt with much more hastily, in order to bring the story down to the terminal date of 1215, when Pope Innocent III preached a crusade to expel the Muslims from Spain.

So occupied is “God’s Crucible” with every twist and turn of military and political history, in fact, that Mr. Lewis’s would-be controversial interpretation, and his lessons for the present, are mostly forgotten. They surface only in the form of occasional valentines to the Spain of the Umayyads — whose “ethos of storied tolerance and mutuality…might have served as a model for the continent” — and corresponding insults to Carolingian Christendom — “an economically retarded, balkanized, fratricidal Europe that, in defining itself in opposition to Islam, made virtues out of religious persecution, cultural particularism, and hereditary aristocracy.”

The problem with such verdicts is not just that they are unconvincingly reductive, but that they are clichés. If Western readers know anything about Muslim history, it is that Golden Age Spain was a golden age. That this moment of relative tolerance and prosperity coincided with the Dark Ages in Western Europe helps to make it what Saul Bellow called a “contrast gainer.”

But to take one brief moment out of a millennia-long story, and use it to represent the essences of Christianity and Islam, is completely ahistorical. The relative merits of the two cultures in the 10th century tell us nothing about them in the 21st. Indeed, to the extent that Mr. Lewis means to imply that, because Islam was a force for enlightenment under Abd al-Rahman and is still a force for tolerance under Ayatollah Khamenei, he is positively misleading. Civilizations do not have essences, either good or bad; what they have instead are histories.

akirsch@nysun.com