In 610 A.D., Muhammad ibn Abdallah, a forty-year-old man from a prosperous merchant family in Mecca, repaired to a cave on nearby Mt. Hira to meditate—a retreat he had made many times. That year, though, his experience was different. An angel appeared and seized him, speaking to him the words of God. Muhammad fell to his knees and crawled home to his wife. “Wrap me up!” he cried. He feared for his sanity. But, as the voice revisited him, he came to believe that it truly issued from God. It called on him to reform his society. Poor people were to be given charity; slaves were to be treated justly; usury was to be outlawed. Muhammad’s tribesmen, the Quraysh, were polytheists, like most people in the Arabian Peninsula at that time, but this God, Allah, proclaimed that he was the only God. He was the same deity that the Jews and the Christians worshipped. Jesus Christ wasn’t his son, though. Christ was just a prophet, like the prophets of the Old Testament. Their word was now superseded by Muhammad’s, as their creeds were supplanted by this new one, Islam.

When Muhammad started preaching in Mecca, people saw him as a harmless crank, but as he gained followers he began to be regarded as a menace. Mecca was an important trading hub, with rich merchants. Muhammad’s God forbade all ostentation. Furthermore, if, as he instructed, the pagan idols were to be discarded, that would mean no more revenue from their shrines. In 622, Muhammad and his followers were driven out of Mecca. They fled to Yathrib, which became known as Medina, and from there they warred with their native city. In the beginning, Muhammad’s treatment of his fellow-monotheists the Jews and the Christians was conciliatory, but new religions do not normally establish themselves with the help of older religions. The local Jewish tribes conspired against him. After a decisive battle in 627, Muhammad had seven hundred Jews beheaded in Medina’s central market. In 630, he and his men took Mecca. Muhammad ordered the destruction of the three hundred and sixty idols around the city’s great temple, the Kabah. He proclaimed the supremacy of Islam, and reportedly sent messengers to the rulers of Persia, Byzantium, Yemen, and Ethiopia bidding them to convert. According to his biographer Karen Armstrong, he spent his few remaining years trying to establish peace, sometimes over the objections of his lieutenants.

Soon after Muhammad’s death, in 632, the record of what God had said to him was collected in the Koran, and his contemporaries’ testimonies about his life were gathered in the Hadith. At the same time, Islam expanded, with a speed unique in history. One of the obligations imposed on the faithful by the Koran was jihad, or struggle. This has been translated as “holy war,” and there are passages in the Koran to support such a reading, notably the recommendation that Muslims kill enemies of the faith: “Fight against them until idolatry is no more and Allah’s religion reigns supreme.” But just a few paragraphs later the Koran makes the opposite decree: “There shall be no compulsion in religion.” Some interpreters of the Koran—particularly in recent years, when holy war has become a matter of public alarm—have argued that “jihad” actually means spiritual combat, every Muslim’s fight within himself against the temptations of evil. I don’t know why a book that was collected, rather than composed, should have to be internally consistent, or why a religious document that originated within a nomadic society in the seventh century and includes such things as the moon splitting in two should be asked to conform to post-Enlightenment thought. The Bible also contradicts itself, and has water turning into wine. Such matters are a problem only for literalists. As for slaying one’s enemies, this is enthusiastically recommended in Psalms (“Happy shall he be, that taketh and dasheth thy little ones against the stones”), as it is in many premodern writings.

In any case, however much Muhammad’s immediate successors may have struggled with their souls, they also, in the eighty-some years following his death, conquered Syria, Egypt, North Africa, Anatolia, Iraq, and Persia. By the beginning of the eighth century, Muslim forces stood at the northwest corner of Africa. There, only the Strait of Gibraltar, nine miles wide, separated them from the Iberian Peninsula. Iberia at that time was ruled by the Visigoths, a Christian people who did their best to wipe out other religions within their territory—Judaism, for example. There is some evidence that the Iberian Jews invited the Muslims to invade. In 711, they did so. The state that they established in Iberia, and maintained for almost four centuries, is the subject of David Levering Lewis’s “God’s Crucible: Islam and the Making of Europe, 570 to 1215” (Norton; $29.95).

This book has to be understood in context, or, actually, two contexts. The first is post-colonialism, the effort on the part of scholars from the nineteen-seventies onward to correct the biases that accompanied and justified the colonization of eighty-five per cent of the earth by European powers between the sixteenth and the twentieth centuries. In that period, according to Edward Said’s 1978 “Orientalism”—the founding document of post-colonial thought—history-writing about the Near East and the Middle East was an arm of empire. Its goal was to make non-Western peoples seem uncivilized, so that European control would appear a boon. Since Said, much writing on Europe’s former colonies has been an effort to redress that injustice.

The other context in which Lewis’s book must be read is, of course, the history of terrorism, since the late nineteen-seventies, on the part of people claiming to be instructed by the Koran. When this started, most Westerners had little idea of what the Muslim world was. Harems, hookahs, carpets—that was about it. Nor, after the terrorist attacks, was it easy to catch up in any proper way, for, while there has been an outpouring of books on Islam in the past two decades, many of them were for or against it. A number of prominent intellectuals have denounced Islam. Other people have protested that the vast majority of Muslims do not support terrorism. Some historians have condemned not just the demonization of Islam but the West’s ignorance of the Muslim world—a failure now seen as political folly, not to speak of arrogance. Scholars went to their desks to testify to the glories of Islamic cultures. Salma Khadra Jayyusi, in the foreword to her magnificent anthology “The Legacy of Muslim Spain” (1992)—a collection of forty-nine essays describing not just the politics and the religion of Muslim Iberia but its cities, architecture, music, poetry, calligraphy, and cooking—calls the omission of Islam from the West’s story of civilization a “historical crime.”

Lewis’s book is part of that revision. The Muslims came to Europe, he writes, as “the forward wave of civilization that was, by comparison with that of its enemies, an organic marvel of coordinated kingdoms, cultures, and technologies in service of a politico-cultural agenda incomparably superior” to that of the primitive people they encountered there. They did Europe a favor by invading. This is not a new idea, but Lewis takes it further: he clearly regrets that the Arabs did not go on to conquer the rest of Europe. The halting of their advance was instrumental, he writes, in creating “an economically retarded, balkanized, and fratricidal Europe that . . . made virtues out of hereditary aristocracy, persecutory religious intolerance, cultural particularism, and perpetual war.” It was “one of the most significant losses in world history and certainly the most consequential since the fall of the Roman Empire.” This is a bold hypothesis.

The Muslims took most of Spain in a little over three years. The Visigoths had more men, but the Arabs were very skillful warriors, seeming to dance in front of the enemy, attacking, retreating, attacking again. Here and elsewhere, Lewis appears to see them as clever underdogs, David to Goliath, Muhammad Ali to George Foreman. He inventories the great sacks of gold and silver and precious stones that, together with vast numbers of slaves and young women (harem-bound), they sent back to their caliph in Damascus, the capital of the empire. Included in the shipments were the heads, pickled in brine, that they had removed from Visigoth grandees. In 714, they were just short of the Pyrenees, the northern border of Iberia. The peninsula was now theirs. They renamed it Al Andalus.

As news of the conquest spread, Arabs from the East streamed into Iberia, and they brought with them the conflicts brewing among them—above all, a nasty feud between northern and southern Arabs. The first twenty-two emirs (governors) of Al Andalus had an average reign of two years apiece. Stability, or as much of it as Muslim Spain ever had, began with the reign of Abd al-Rahman I, a Syrian-born prince who took over in 756 and managed to stay in power until his death, thirty-two years later. Rahman is the hero of “God’s Crucible.” Lewis loves him, and calls him by his sobriquet, the Falcon. Rahman was a firm ruler—he had taken his throne by force—but he was also a man of the people. Lewis describes him strolling about the capital city, Córdoba, in a white djellabah, without bodyguards, and preaching at Friday services in the mosque. Raised in a palace, he was an arts lover. It was he who built the Great Mosque of Córdoba, the most spectacular extant example of Muslim Spain’s architectural achievements. He also botanized, and imported to Spain its first date palms, its first lemons, limes, and grapefruit, as well as almonds, apricots, saffron, and henna. This of course was good for trade, which flourished during his time.

Accordingly, so did Spain’s cities, led by Córdoba, which under Rahman had an estimated population of about a hundred thousand. Lewis describes the metropolis that the emir left to his successors: “The _qasr”—_palace—“was new, completed just as the Falcon ordered the foundations laid for the Friday (Great) Mosque. Not many steps away were the public baths. Nearby was the central market, where basic commodities of bread, vegetables, fruit, oil, and lamb at regulated prices were upstaged by Persian carpets, Damascus metalware, China silks, fine leather and jewelry, slaves, and much else supplied on demand by the Muslim world economy. . . . The capital’s streets, following no particular pattern from the long wall beside the gray Guadalquivir River, linked neighborhoods where Jews, Berbers, Catholics and Orthodox, Arabs and _muwalladun”—_non-Arab converts to Islam—“lived as though in their own separate worlds. Sephardic apothecaries, Visigoth blacksmiths, and Greek surgeons offered services in these long, narrow arteries.” Orange and lemon trees in the public gardens perfumed the air. Outside the city, “the long Guadalquivir plain, abundantly irrigated by waterwheels, was carpeted with cereal plantations of wheat, rye, and barley, and olive trees forever.” You want to move there.

Rahman was the founder of Muslim Spain’s famous convivencia. Translated literally, the word means “living together,” in spite of differences, and this idea is the burning center of “God’s Crucible.” I think it is the reason that Lewis chose to write about Muslim Spain. He is not an Arabist. He is best known for a two-volume biography of W. E. B. Du Bois (1993 and 2000), which won two Pulitzer Prizes, one for each volume. But that book, if it is not about Arabs, is about racial justice, and it is for the furtherance of such justice that Lewis so admires Rahman. Nevertheless, as he points out, the convivencia had its limits. It was not just a humane policy—an act of obedience to the Koran (“There shall be no compulsion in religion”) and a way of being civilized—but also a matter of Realpolitik. Iberia was a ragbag of religious and ethnic groups. Tolerance, what we would now call multiculturalism, was more likely to hold them together than forced conversion. Furthermore, the convivencia never involved complete equality. In the early years, a number of restrictions were placed on Jews and Christians. They had to wear identification badges. They could not proselytize, and they were required to pray quietly. Their houses could not be taller than Muslims’ houses. Most important, they had to pay a heavy tax, called the jizya. In time, many of these rules (not including the tax) fell away. Jews, especially, were allowed to enter public service, as scribes, clerks, advisers. They taught the Muslims how to run a government, Lewis writes. The golden age of Al Andalus, he says, was also the golden age of Sefarad, the Sephardic Jews. But even those who did not have brilliant careers no doubt found badges and taxes preferable to forced conversion or death. Eventually, many Jews and Christians did convert—probably, in many cases, to avoid the tax. At the end of the eighth century, the vast majority of people in Iberia were Christians. Two hundred years later, the majority were Muslims.

The Muslims never ruled all of Iberia. Certain regions, particularly in the north, remained independent, or were only loosely allied with the emirate, and they repeatedly rebelled. Then there was the strife within the Muslim populace. Christians and Jews were not the only ones who received unequal treatment—any non-Arab did. That included the descendants of the North African Berbers whom the Arabs had added to their empire at the beginning of the eighth century and who came with them to Spain. The Iberian Berbers were Muslims, and the emirate’s best warriors. (Tariq ibn Zayid, who led the invasion of Iberia, was a Berber, and so was his cavalry.) They also outnumbered the Arabs in Spain. Therefore, they resented their second-class status. For much of the history of Al Andalus, the emirs had to deal with Berber revolts. Another reason the Iberian Arabs had to go back to war was that the conversions to Islam, in freeing people from the jizya, were depleting the treasury. The emirs had to find more infidels to tax. Finally, there was the command of jihad. The Arabs had never meant to stop at the Pyrenees, and, in 732, only twenty-one years after they entered Iberia, they scaled the great mountains and went down the other side.

The kingdom they were invading was Frankland—roughly, France, Belgium, and parts of western Germany—under the rule of Charles the Hammer, or, in French, Charles Martel. The Muslims lost some engagements and won others, but because of a Berber revolt at home they were soon forced to withdraw. In 778, the Franks—now under the leadership of Charles Martel’s grandson, Charles I, the Great, or Charlemagne—retaliated. Second to Rahman, Charlemagne is a hero in Lewis’s eyes: not just a military genius but a good man and a dashing fellow. Lewis tells us three times that the Frankish king stood six feet three inches tall. He pictures him, his blond hair flowing, his blue cape billowing behind him, astride a huge white stallion. If the Franks were primitive, Lewis says, the peoples whom Charlemagne had conquered—the Saxons, the Lombards, the Avars—were more so. Their new king tried to improve them. He imposed chastity on the priests, to their distress; he forbade incest, apparently a popular practice among the people. To elevate the culture of Frankland, he founded the Palace School, a combination think tank and college, and imported learned men to staff it, but his efforts were limited by the fact that he was constantly in the field. Charlemagne’s neighbors did not submit easily to his rule—especially since, unlike the Muslims, he required them to convert. So when he wasn’t conquering he was reconquering. That is what ended his Spanish campaign. After a few months, he received news that the Saxons were in revolt, and he had to go home.