There are many ways to describe Pope Benedict XVI’s remarks at the University of Regensburg last September concerning the “evil and inhuman” nature of Muhammad’s teachings. Impolitic is one way, maladroit another. They can also be described as deeply ignorant concerning Islam’s role in preserving, transmitting and enhancing classical culture at a time when most Europeans were still snuggling up to their oxen for warmth. But the most striking thing about the comments was the way they reflected a view that was alarmingly one-sided and unreflective. The religious zeal of the proto-Muslim forces that erupted out of Arabia in the seventh century has been much exaggerated on both sides of the Muslim-Christian divide. Far from forcing subjects to adopt their faith (which at that point was no more than embryonic), they discouraged conversion if only because it would have undermined the poll tax on Jews and Christians on which their revenue depended. But the religious zeal of Catholic armies that have periodically erupted out of Western Europe has not been exaggerated. Beginning with Charlemagne, who is said to have beheaded 4,500 Saxons for the crime of lapsing into paganism, they repeatedly engaged in conversion by the sword along with mass murder of all those who refused. If forced conversion is “contrary to God’s nature,” as the Pope asserted at Regensburg, then no institution has behaved in a more ungodly fashion than his own church.

On the other hand, considering that Exodus 15:3 plainly states that “Yahweh is a warrior” and that slaughtering unbelievers is one of the things that warrior-god worshipers do, perhaps no institution has behaved in a more godly fashion either. Were it not for the fact that he has been working on it since at least 1998, Christopher Tyerman’s massive new history of the Crusades could be read as a rejoinder to the Benedictine doctrine that forced conversion is something the other side practices, never our own. Rather than beginning with Charlemagne, God’s War opens a couple of centuries later, when, following an extended period of punishment at the hands of Viking, Magyar and Saracen raiders, the Catholic West turned the tables on its tormenters and began striking back with startling vigor. In 1015-16, Pisan and Genoese raiders were attacking Arab pirate bases in Sardinia. A few decades after that, Christian rulers in the north of Spain were hitting back at Muslim rulers to the south. By 1060 Norman adventurers were campaigning to take Sicily from the Arabs, who had taken it two centuries earlier from the Byzantines. Then, in 1095, came the climax, an immense human tide that poured out of France, Flanders and other provinces and began making its way east with the purpose of liberating, so to speak, the Holy Land from the Seljuk Turks.

This was the opening salvo in the Crusades, a century and a quarter of invasions and assaults not only against Palestine but Egypt, Lebanon, Syria and even Constantinople, the Byzantine capital that was the Latin West’s supposed ally. Tyerman does not limit his account to those expeditions, however, but also throws in the Albigensian crusade of 1209-29 against a heretical group of “dualists” in the South of France known as the Cathars; the campaigns by Teutonic Knights against pagan tribes in the eastern Baltic; and the Christian reconquest of Spain, which was largely complete with the fall of Seville in 1248 even though a Muslim outpost in Granada managed to hold on until 1492.

All were of a piece, which is to say that all flowed from a Western resurgence that had its first stirrings in the tenth century and had reached high gear by the late eleventh. Two things about this resurgence stand out. One is its multidimensionality. Weak, depopulated and culturally impoverished, Western Europe was on everyone’s list as the most unlikely candidate for resurgence. Yet it was soon racing ahead not just militarily but religiously, politically, economically and even intellectually. The Parisian student of dialectics was one aspect of the Western European takeoff, the powerful growth of shipping and trade was another, while the crusading knight and the fiery cleric proclaiming, “Kill them. The Lord knows who are his own”–as the Abbot Arnaud Aimery reportedly did in urging on the indiscriminate slaughter of the people of Béziers, a Cathari stronghold–were a third. All were products of an increasingly aggressive and energetic society bent on strengthening politico-religious authority at home and extending its reach abroad.

The other thing that stands out about this small-r renaissance is its roughness and brutality. The elegant Byzantine court was appalled by the barbaric hordes that landed on its doorstep at the tail end of the eleventh century. Anna Comnena, daughter of Emperor Alexius I, was contemptuous of Norman soldiers who “ravaged the outskirts of Nicaea”–located across the Sea of Marmara from Constantinople–“acting with horrible cruelty to the whole population.” Not only were these Catholic brutes greedy, arrogant and hotheaded, she wrote, but even their battlefield skills were overrated. They were “indomitable in the opening cavalry charge,” but “afterwards, because of the weight of their armor and their own passionate nature and recklessness, it is actually very easy to beat them.”

Still, barbaric enthusiasm carried the Crusaders a long way. Although (or perhaps because) they were headstrong and impulsive, their military exploits, at the outset at least, were little short of astounding. Traveling some 2,000 miles from their home base, they quickly adapted to the unfamiliar terrain and novel tactics they encountered on crossing into Seljuk territory. They mastered the fighting march and learned to counter the rapid attack, feint and ambush that were standard in the Middle East. Anna Comnena was right about at least one thing, moreover: Provided he kept his seat, the Western mounted knight was well-nigh invincible. He was the armored tank of the day, “maneuverable [and] impervious to most of the fire power available to the opposing infantry,” as Tyerman puts it. “Mounted on increasingly well-bred, specially trained and larger horses, protected by armor and wielding heavy lances, maces and swords, a few knights could hold their own against scores of infantry. The repeated accounts of seemingly miraculous victories or escapes by hopelessly outnumbered bands of knights, while likely to be exaggerated, preserved a truth.”

Staggering from the heat and thirst, the chevaliers and their followers fought their way across Anatolia and then somehow marshaled the resources to mount an eight-month siege of the city of Antioch in what is now southern Turkey. Buoyed by the supposed discovery of the Holy Lance that had pierced Christ’s side–in reality a piece of metal dug up from beneath the floor of an Antiochene cathedral–the exhausted Crusaders then turned around and defeated a far larger Turkish force sent to relieve the city in June 1098. They celebrated their triumph in good Christian fashion by slaughtering every Muslim male within reach and driving lances into the bellies of the women as well.

But Antioch was merely a warm-up. Making their way down the coast of present-day Syria, Lebanon and Israel, the Crusaders turned inland at the town of Arsuf, a few miles north of Jaffa, and headed for Jerusalem. After setting up camp, they proclaimed a three-day fast and, led by a priest, marched barefoot around the city in imitation of Joshua at the battle of Jericho. Once the fighting began on June 13, 1099, they used mobile siege towers constructed by Genoese engineers to edge closer and closer to the city walls. After a month of dodging arrows and bolts, they finally got near enough to throw down planks and leap across. Swarming down the ramparts, they overwhelmed the city’s Jewish and Muslim defenders. “The scale of the slaughter,” writes Tyerman, “impressed even hardened veterans of the campaign, who recalled the area [around the Temple Mount] ‘streaming with blood’ that reached to the killers’ ankles.” Jews who fled into a synagogue were set ablaze, while “Muslims were indiscriminately cut to pieces, decapitated or slowly tortured by fire.” A few days later, the Crusaders ordered Muslim survivors to remove the dead bodies clogging the narrow streets and carry them outside to be burned. When the Muslims had done as they were told, the Christians massacred them as well. On July 15, 1099, the victors gathered in a local church to give thanks for their triumph. “With the fall of the city,” one participant noted, “it was rewarding to see the worship of the pilgrims at the Holy Sepulcher, the clapping of hands, the rejoicing and singing of a new song to the Lord. Their souls offered to the victorious and triumphant God prayers of praise which they could not express in words.”

Turning the other cheek this was not. But, contrary to a long tradition of romanticizing the Other, Tyerman argues that the slaughter in Jerusalem was consistent with the prevailing regional standard. “The recent Turkish conquests in the Near East had been accompanied by carnage and enslavement on a grand scale,” he observes. “When it suited, Muslim victors could behave as bestially as any Christian.” A Jewish observer noted that, unlike Muslims, Christians at least did not rape enemy women before killing them. On the other hand, there was nothing in the Islamic camp that quite compared with what God’s War describes as “a daredevil but starving group” of Christian warriors known as the Tafurs, who not only killed their Muslim opponents but reportedly ate them, too.

One side engaged in mass rape, while the other preferred a dash of cannibalism to liven things up. Despite wading through blood, both others’ and their own, the Crusaders were able to maintain a grip on Jerusalem a scant eighty-eight years before being dislodged by a Kurdish warlord named Saladin (who, according to Tyerman, does not quite merit the accolades heaped upon him by Western xenophiles). Decades of economic decline followed in the Crusaders’ remaining territories up and down the coast. With it came deepening disorder marked by, among other things, a vicious civil war that erupted in 1256 between Venetians, Pisans, Provençals, Templars and Teutonic Knights on one side and Genoese, Hospitallers and Catalans on the other. Peace was mostly restored in 1258, but then Crusader possessions began toppling under growing Muslim pressure–Jaffa in 1268, followed by Tripoli in 1289 and then, in one fell swoop, Acre, Tyre, Sidon, Beirut, Tortosa and Athlit in 1291. Western knights continued to gain ground in Spain and Portugal. They created a short-lived Latin republic in Constantinople in 1204, while participating a century later in lavish campaigns against Prussian, Livonian and Lithuanian pagans complete, according to Tyerman, with “special feasts, displays of heraldry, souvenirs and even prizes.” Yet in the eastern Mediterranean, they clearly overshot themselves, traveling too far to wrest too little from an enemy that ultimately proved too strong.

What did it all add up to? Not much, according to Francis Bacon, who jeered at the Crusades as a “rendezvous of cracked brains that wore their feather in their head instead of their hat.” David Hume agreed, calling the campaign to take back the Holy Land “the most signal and most durable monument of human folly that has yet appeared in any age or nation.” Despite efforts by economic determinists to describe the Crusades as a trade war in religious guise, the French medievalist Jacques Le Goff once wisecracked that the sole economic benefit to derive from them was the introduction of the apricot. In fact, according to Tyerman, Westerners did not need the Crusades to insinuate themselves into the economic life of the eastern Mediterranean; the region, he writes, was “crawling” with Western pilgrims, mercenaries, merchants and pirates before they even began. There is simply no evidence that the Western economic position was any stronger than it would otherwise have been.

So what is the explanation? The Crusades should be understood not as a clash between two systems but as the product of a Christian-Islamic system from the Atlantic to the Persian Gulf that was by turns competitive and symbiotic. The two halves of this system thrusted and parried, advanced and withdrew, but also borrowed from each other, traded and stole. Inter alia, they used the extended warfare between them to reorganize themselves politically. Both underwent a process of convergence as those at the top tightened the controls and marshaled social resources. European society grew increasingly uniform from one end of the continent to the other with the result, according to Fernand Braudel, that a traveler would feel “as much at home in Lübeck as in Paris, in London as in Bruges, in Cologne as in Burgos, Milan, or Venice. Moral, religious and cultural values, and the rules of war, love, life and death were the same everywhere, from one tier to another, whatever their quarrels, their revolts or their conflicts.” Similarly, growing Turkish hegemony in the Middle East led to the imposition of a Sunni orthodoxy that was just as uniform. In the West, the popes had tried to solve the problem of endemic warfare among the nobility by “externalizing” it onto the Muslims. In the East, Islamic rulers also had to put a stop to endemic infighting before dealing with Turkish freebooters known as the Khwarazmians, and they checked the Mongols at the Battle of Homs in 1281 before finishing off the remaining Crusaders. By the fourteenth century, they were ready to turn the table on the Christians and begin marching up the Balkans under Ottoman leadership.

God’s War is a long but highly readable account of this extensive back-and-forth struggle. It is an impressive achievement, a work that manages to tie together an extraordinary number of threads across nearly half a millennium of European history. Although it can be taken as a response to Pope Benedict XVI’s comments at Regensburg, it is more properly read as an extended rejoinder to Steven Runciman’s classic three-volume History of the Crusades, published in the early 1950s, a long and colorful account that is nonetheless studded with judgments that now seem prejudiced and amateurish. Tyerman, by contrast, is never amateurish. His knowledge of the period is encyclopedic, and his judgments are sharp, astute and fair–which is to say unsparing–to both camps. He neither vilifies Islam nor engages in the easy Euro-bashing that is the obverse of Islamophobia. With so many people succumbing to subjectivism these days, it is bracing to come across a historian who remains resolutely above the fray, who insists on viewing the conflict as a whole and who always has the broader context in mind.

Not that God’s War is entirely free of faults. The writing is clear, although on one or two occasions it gets bogged down in details. More significant is the near absence of anything by way of a social or economic dimension. Tyerman tells us little about what it was about Western Europe that enabled it to project so much force at so great a distance. The Third Crusade, launched in 1189, for instance, was heavily dependent on sea transport, and although Tyerman informs us that “the variety of vessels available, the certainty of planning and routes, the awareness of naval logistics, the distances covered and the accurate predictions of timing reflected the exponential growth in maritime activity and exchange around the coasts of Europe in the twelfth century,” he doesn’t say where this exponential growth came from or what socioeconomic innovations allowed it to occur. He is silent about the uniquely European blend of fragmentation and cohesion that led to endless political infighting but also to the formation of highly effective fighting orders such as the Templars and Hospitallers. Thanks to an emerging body of corporate law that granted a high degree of autonomy and self-governance to such groups, Europe was able to create new sources of economic and political power that other societies over the long haul could not match.

The same goes for economic productivity, the ultimate reason for all those bigger knights, bigger horses and weightier armor–Tyerman tells us nothing about that either. His discussion of the evolution of the Christian doctrine of holy war similarly stops short. The problem, admittedly, is a difficult one. Despite the pacific tone of the Gospels, the Christian society that emerged in the Latin West, as historian William McNeill once noted, was the most heavily militarized on earth, with the possible exception of shogun Japan. Indeed, God’s War recounts the tale of the abbot of St. Germain during the Viking siege of Paris in 885-86 who, rather than loving his enemies, was said to be “capable of piercing seven men with a single arrow [and] in jest he commanded some of them to be taken to the kitchen.” Tyerman trots out the usual suspects in explaining where this sort of Christian bloodlust came from–not only the compromises that church fathers made with state power following the conversion of Constantine in the early fourth century but also the sacred texts that seem to urge more and more, such as the Book of Revelation, with its vision of a world drowning in blood, and the Old Testament tales of murder and mayhem that make the Koran seem positively Gandhian. He is quite correct in all this, although he might have pointed out that Christ had his militant side as well, one he revealed when denouncing the rich, or proclaiming that he had “not come to bring peace but a sword” or, in an especially curious incident outside Jerusalem, blasting a fig tree merely because he was hungry and it was bare of fruit.

But even if Christ hadn’t caused that fig tree to wither, it wouldn’t have mattered. Even at its purest, Christian pacifism reflects a kind of inward-directed violence that can all too easily be turned in the opposite direction. Given Christ’s command that “if your right hand causes you to sin, cut it off,” what is a good Christian to do with a sinner causing an entire community to go astray? Excommunicate him or burn him at the stake? Even more fundamental is the issue of revelation itself. Revelation in the Western monotheistic tradition is a form of divine intervention in which the supreme being makes himself known to ordinary mortals. Because such visitations are miraculous and hence outside the realm of nature, they can’t be proven according to the normal rules of evidence. Instead, they must be taken on faith. What’s more, because they constitute the greatest events in history, belief in them becomes the greatest of virtues while disbelief becomes the basest of sins. For that reason, the people in the next valley who believe in a different revelation can’t simply be ignored or laughed off–they must be ruthlessly suppressed, if not exterminated. Revelation, no matter how benign, thus becomes the basis for ceaseless warfare.

This was the ideology at the heart of the Crusades. Reason and evidence are forces for convergence since they lead to conclusions on which most thoughtful people can agree–that the earth is round, for instance, that it circles the sun, that E=mc² and so on. Faith, by contrast, is a force for divergence–or, more precisely, a force for convergence and divergence, as entire societies are forced to believe in one “truth” and then to launch a holy war against those believing in another that is equally unverifiable. In his talk at Regensburg, Pope Benedict XVI complained about “reason which is deaf to the divine.” God’s War is about what happens when people are all too alert to voices that no one else can hear and are all too determined to carry out what they believe to be those voices’ instructions. The results were appalling during the Crusades. It is even more appalling that the same habits are now making a comeback.