When the British historian Steven Runciman published his classic three-volume “History of the Crusades” in the early 1950s, the subject must have seemed as securely antique as the Pharaohs. After all, the nightmares of the Cold War world were ideological, not religious; its geopolitical fault line ran along the Elbe, not the Euphrates. For today’s readers, of course, the Crusades are even more distant in time than they were for Runciman. It has been 911 years since Pope Urban II launched the First Crusade in an open-air sermon at Clermont, urging the faithful to take up the cross and liberate Jerusalem. But in one of the Mobius twists that make nonsense of chronology, the world is certainly much closer to the Crusades in 2006 than it was in 1956. With a Western army occupying Baghdad and Osama bin Laden vowing to restore the caliphate, the Crusades — with their bizarre mixture of idealism and ambition, devotion and cruelty — seem terribly relevant once again.

That makes the moment ripe for a book such as “God’s War: A New History of the Crusades” (Harvard University Press, 922 pages, $35), by the Oxford historian Christopher Tyerman; and it makes the book Mr. Tyerman has actually written something of a disappointment. Such a reaction is not really fair to Mr. Tyerman, who clearly did not set out to write imaginative, narrative history in the Runciman tradition. “The judgmental confidence of a Macaulay — or a Runciman — is warranted neither by modern fashion nor by the discipline of the subject,” he warns in his preface. Instead, his book displays the virtues of the academic historian: massive erudition and patient synthesis. The juice of thousands of articles and monographs has been squeezed for “God’s War,” and it surely reflects the state of historical knowledge about the Crusades better than any other book.

Unfortunately, the result is still pretty dry. It is perhaps a compliment to say that Mr. Tyerman has no literary instinct — that he continually disrupts the momentum of his narrative, refuses to highlight dramatic episodes and characters, and writes a dogged prose whose only flamboyant quality is an odd fondness for the word “fissiparous.”All this represents a kind of integrity, a refusal to dress up a complicated story for mass consumption. At the same time, it also undermines Mr. Tyerman’s claim to be offering a decisively new account of the Crusades. The novelty, which is all but invisible to lay readers, resides not in bold concepts, but in a running Pyrrhic combat with received interpretations, which Mr. Tyerman disputes without replacing.

The result is that “God’s War” offers a good deal less pleasure than information; but the information is still well worth having. For the Crusades are, at the same time, one of the most dramatic episodes in European history and one of the least clearly remembered. For 200 years, starting with the conquest of Jerusalem in 1099 and ending with the fall of Acre in 1291, repeated waves of European warriors crossed a continent to fight against hopeless odds, all for the sake of a city and a land they only knew about from books. That they achieved even minor and equivocal success is fantastic — so much so that Richard the Lionheart and Saladin seem to occupy the same compartment of the imagination as purely invented figures like King Arthur and Prester John.

In fact, the triumph of the First Crusade, which achieved Pope Urban II’s goal of liberating the Holy Sepulchre in Jerusalem, looks in retrospect like a freak accident, and not necessarily a lucky one. The warrior aristocracy of France took the lead in responding to Urban’s call, motivated by the allure of conquest and booty; the pope’s promise of remission of sins; and an ardent romantic love for Jerusalem, the scene of Christ’s Passion. In a startlingly short time — less than two years after the Crusade was launched — an army of some 60,000 Franks, as the Latin invaders were known, had made their way to the Near East. On July 15, 1099, the army finished an arduous campaign by storming the walls of Jerusalem, massacring the Jews and Muslims it found there, and establishing a new Kingdom of Jerusalem, with Duke Godfrey of Bouillon on the throne. In some form or another, this state — which never controlled much territory, and left virtually no mark on the landscape of the Middle East except the ruins of its castles — survived for almost two centuries.

Given that every subsequent Crusade ended in dismal failure, the real mystery is how this First Crusade managed to accomplish so much. The answer, Mr. Tyerman shows, is that it probably shouldn’t have. The turning point in the campaign came on June 3, 1098, when the Frankish army finally took the important city of Antioch after a siege that lasted almost eight months. The very next day, a Muslim relief army appeared before the city, trapping the Christians inside until they broke out a month later. Had the siege been prolonged by one more day, the Muslims would have found the Franks completely exposed, and the First Crusade would have ended in disaster. It was enough to make you believe that God was on the Crusaders’ side, if you were so inclined.

In fact, a defeat at Antioch might have been a blessing in disguise. For once Jerusalem was won, the West was committed to defending it, even though the Frankish kingdom made no sense geographically or economically. Each subsequent Crusade — the Second in 1147, the Third in 1188, the Fourth in 1204, the Fifth in 1218 — was meant to reinforce or recapture territory that the First had conquered. But while these later Crusaders were more numerous, better supplied, and led in person by the greatest kings of Europe, they met with disaster after disaster. The Fourth Crusade, infamously, didn’t even get to the Holy Land, choosing instead to sack the Christian city of Constantinople.

In the end, Mr. Tyerman convincingly argues, the major legacy of the Crusades was not to be found in Jerusalem at all, but in the religious and political development of Europe. By inventing a theology of crusading, the papacy advanced its claim to leadership of the Christian world. By levying Crusader armies, the kings of England and France helped to centralize and consolidate state power. At the same time, the validation of holy war encouraged the Christian West in its brutal violence against pagans in Eastern Europe, Muslims in Spain, and Jews everywhere. The first victims of the First Crusade, Mr. Tyerman reminds us, were the Jews of Germany, attacked by mobs who reasoned that it was not necessary to travel thousands of miles to kill the enemies of God. Today, it is this fanatical and bloodthirsty face of the Crusades that we recognize best. We can be proud that the word “crusade” has dropped out of the world’s political vocabulary. If only “jihad” would follow it, the legacy of the Crusades might finally be put to rest.

akirsch@nysun.com