HOLY SMOKE.

In its original meaning, a crusade was a Christian holy war, and in that sense it was a contradiction in terms. Christ's whole teaching was to love thy neighbor, not kill him. But, like everyone else, the early Christians had enemies, whom they needed to fight on occasion. So the Church fathers went to work on the doctrine, and by the eleventh century it was agreed that in certain circumstances God might not only condone war but demand it. Of course, there had to be an important cause. The Church claimed that it had such a cause: Jerusalem had fallen into the hands of infidels. Actually, that had happened more than four hundred years earlier, and in the ensuing period Christians were generally treated far better in the holy city than non-Christians were in Europe. But there was another call to arms: Alexius I Comnenus, the emperor of Byzantium--that is, of Catholic Europe's Eastern brother--had asked the Pope for help against Muslim forces threatening his borders. Again, however, this was something less than an emergency. Byzantium and Islam did fight, but no more frequently than most neighboring powers of the time.

According to many modern historians, what triggered the Crusades was not an external cause but an internal one: a campaign, beginning with Pope Gregory VII, in the late eleventh century, to reform the Church. This was a two-pronged effort. One goal was to stamp out immorality: get the priests to stop marrying, stop selling ecclesiastical offices, live by their vows. A second, and probably more important, objective was to strengthen the Papacy. In religion as in politics, Europeans of that period had little respect for centralized authority. The Pope's sovereignty was disputed not just by secular rulers but within the Church. When Urban II, Gregory's successor, was elected, in 1088, it took him six years to get a rival, German candidate out of the Lateran Palace. (He finally had to bribe him.) This is not to speak of the fact that the Pope had no control over the Eastern churches, the dioceses of the Balkan peninsula, Asia Minor, Syria, and Palestine. Most of these territories were under the jurisdiction of the Byzantine Empire and hence of the Greek Orthodox Church, which, to Rome's abiding fury, had broken with the Western Church in 1054. The Vatican wanted to get mightier and holier, and Urban II took on the job.

In 1095, he went on a tour of France, and one afternoon in Clermont he gave a sermon calling on Christians to journey to the East and reclaim the Holy Land. "A race absolutely alien to God," he said, was defiling Christian altars, raping Christian women, tying Christian men to posts and using them for archery practice. None of this was true, but it had the desired effect. First, as the postcolonial theorists would say, it "otherized" the Muslims. Second, it gave the European nobles a cause that could distract them from warring with their neighbors--a more or less daily occupation of knights in that period--and unite them, for a holy purpose. In the months that followed, at convocations across Europe, between sixty thousand and a hundred thousand people came forward and knelt to "take the Cross."

Thus was launched the crusading movement, whose high tide lasted for two centuries. As time went on, a "crusade" no longer meant just a march against the Muslim infidel. Any perceived enemy of the Church--the Wends, in Germany (pagans); the Cathars, in southern France (heretics)--could be the target of a crusade. But the Crusades against Islam were the model, and the two most interesting were the first (1095-99) and the fourth (1202-04). The First Crusade is important because, apart from being first, it was successful, at least in the Church's terms: the men recruited by Urban did capture Jerusalem, together with other rich territories in the East, and in consequence--because those lands had to be defended--they made the later Crusades to the East necessary. The Fourth Crusade is famous for the opposite reason. In Christian terms, it was the least successful--indeed, a scandal. The Crusaders never got to Jerusalem; instead, they attacked Christian cities, notably Constantinople, the capital of the Byzantine Empire, which they effectively destroyed. They thus shifted the center of Christian civilization from East to West, and permanently altered the history of the world. These two expeditions are the subject of a pair of recent books, "The First Crusade: A New History" (Oxford; $35), by Thomas Asbridge, and "The Fourth ...