When Pope Benedict XVI recently quoted a 14th-century emperor who claimed that Islam’s singular contribution to the world was the doctrine of spreading religion by violence, Muslim reaction was swift and angry. Amid the uproar, the influential Oxford scholar of Islam Tariq Ramadan observed that embedded in Benedict’s speech was a more subtle and revealing concept, a notion of what it means to be European: “In his speech at Regensburg, the pope attempted to set out a European identity that is Christian by faith and Greek by philosophical reason. But Benedict’s speech implicitly suggested that he believes that Islam . . . is excluded from being European.”

If Ramadan is correct that European has come to mean “not Muslim,” then the origins of this identity may well rest in the Crusades. Such is the fascinating argument of Ramadan’s colleague, Oxford historian Christopher Tyerman, in his ambitious new history God’s War.

Over the course of a thousand pages, Tyerman introduces the reader to the tangled, messy and habitually misunderstood period that began with Pope Urban II’s call to crusade in 1095; that extended over four centuries through military forays by Western Christianity into not merely the Holy Land but Spain, Egypt, Eastern Europe and Byzantium; and that died a slow and unceremonious death upon the dawning of the nation-state. Among the more intriguing developments of this period, Tyerman contends, were the invention (or reinvention) of Christendom, the emergence of a rhetoric of public violence (later to be borrowed by and adapted to the secular state) and the development of a “European identity”—an identity grounded in a common moral cause, dedicated to a sacralized vision of expansion and conquest, and able to unify, at least at times, previously divided cultural groups. In creating a “them”—namely, Muslims—the Crusades led Europeans for the first time to begin to define an “us.”

The journey toward this conclusion—both in history and in Tyerman’s sprawling book—is a protracted one. First, a monumental challenge confronts any scholar who would attempt to write a history of the Crusades: the writer must first answer the question of what counts as a crusade. Historians and laypeople alike have long disagreed.

As Tyerman observes, the Muslims who first battled the crusaders saw no novelty in their encounter with the Christians; to them, the battle was just another in a continuous string of military skirmishes with peoples from the West. Soon Christians themselves were deeply divided on what constitutes a true crusade. Many of the Christian entities that had supported the crucesignati in their attempts to win back the Holy Land from Islam were far less supportive of military expeditions against Egyptians, Albigensians (French Christian dualists), Ottomans and Eastern Christians. Were the latter undertakings true crusades in the spirit of Pope Urban’s initial call?

Tyerman’s response to the question is to be inclusive. While recognizing that this book, like all history, is an “exercise in historical selectivity,” he considers the category of crusade to be a broad one, encompassing all of these historical moments and more.

Tyerman also holds that to “extract the thread of crusade from the weave of the middle ages distorts both.” He thus gives us a rich account of the social, economic, religious and political life of the time. There were multiple factors that led hundreds of thousands of Europeans to leave their homes and families and to trek, often on foot and woefully unprepared, thousands of miles to fight the foes of “true” Christianity. Part of the impetus was spiritual fervor (and Tyerman carefully provides convincing accounts of the sincere spiritual motivations of many of the crusaders); other causes ranged from tensions between the religious and secular realms to economic pressures and the emergence of public taxation. Extended movements such as the Crusades, Tyerman suggests, do not simply arise from the call of a single individual but are developed and sustained by a cultural shift that is both widespread and immensely complex.

Tyerman has produced what is in many ways the first comprehensive history of the Crusades since Steven Runciman’s three-volume History of the Crusades appeared 50 years ago, and he correctly points out that “scholarship and the world have moved since 1954.” But at times the reader of God’s War might crave a little more complexity in the story. When Tyerman explains at the outset that his perspective is that of a Western European scholar and that he will not explore in much detail the too-long-ignored Muslim history of the Crusades, one cannot help being a little disappointed. And when, toward the end of the book, Tyerman refers to “the stories of the battered wives of absent crusaders,” one wishes that this were more than a passing reference and that Tyerman had told a few more tales about everyday people. These are the very perspectives on the Crusades that have yet to be fully explored by historians.

Still, what Tyerman does accomplish in God’s War is impressive. Writing with clarity and in command of an incredible amount of historical detail, he takes the reader on a journey that, like the treks of the crusaders themselves, is filled with curious sights and unexpected twists. The reader learns that as the Crusades began, the Muslim cities of Baghdad and Cairo boasted populations of half a million each while the populations of Rome, Paris and London hovered around 30,000. So much for Christian hegemony. The reader also discovers that Francis Bacon’s 17th-century taxonomy of Europeans was based almost entirely upon characters—and characteristics—emerging from the Crusades: thoughtful theologians, fiery religious zealots, pragmatic soldiers, calculating courtiers and temporizing politicians (categories that come close to defining our present political landscape). But perhaps most important, the reader who joins Tyerman on his extended journey through more than four centuries of Crusades glimpses again and again the origins of who “we” in the West are and how we came to be.