Excerpted from "Why We Fight: Moral Clarity and the War on Terrorism" by William J. Bennett, with permission of the publisher.

According to the eminent scholar Bernard Lewis, whose work I rely upon heavily here (and whose essay in Commentary, "The Return of Islam," a prescient analysis of things to come, appeared as long ago as 1976), the Islam of the September 11 killers "is not the true Islam. But it comes out of Islamic history and culture." This makes it incumbent upon us to familiarize ourselves with pertinent elements of that history and culture.

To begin at the beginning, does the word Islam mean "Peace," as one of former President Clinton's speechwriters put it? In fact it means "submission"-in particular, submission to the will of Allah. Having been vouchsafed a revelation of that will, the Prophet Muhammad founded a religion inspirited by the zeal to succeed and propelled by a militant confidence in the rightness of his cause. And indeed, for the first few centuries of its existence, Islam was an almost unstoppable force-and force, including the murder or conversion of masses of people at swordpoint, was regarded as the wholly legitimate tool of its ascendancy. In rapid succession its armies conquered the lands of Persia, Egypt, and Syria, eventually sweeping across the North African littoral, spreading into Spain, and moving inexorably northward until halted by Charles Martel at the Battle of Tours in 732.

Over the next several centuries, Islam and Christianity traded victories and defeats, with Crusader thrusts into the Holy Land and elsewhere being more than offset by Muslim reconquests and the takeover of India and the eastern parts of the old Roman Empire. During this period Islam reached peaks of intellectual and economic accomplishment, anticipating, rivaling, or surpassing the achievements of Christian Europe. But decay had set in even before the decisive defeat of advancing Muslim forces at the gates of Vienna in 1683. That event heralded an even longer period of decline, colonization, and loss, highlighted in the early twentieth century by the traumatic breaking-apart of the Ottoman Empire at the hands of the Western powers after World War I.

It would be hard to overestimate the significance in Muslim consciousness of this series of defeats: When Osama bin Laden alluded cryptically in an October speech to the dissolution of the Ottoman Empire, he could count on the ominous resonance of his words among his Arabic-speaking listeners, for in the classical understanding, the world is divided into the "realm of Islam" and the "realm of the sword"-that is, the realm not yet conquered by Islam-and no piece of land, once fallen to the former, can be renounced or alienated or surrendered to the latter. (Although President Bush was sharply criticized for offending Muslim sensibilities when he used the word crusade to characterize our war against terrorism, it is plain that we harbor no plans to conquer anyone, or to separate anyone from his religion; the same cannot be said for Islamist radicals.) Similarly, a penalty of death awaits the convert to Islam who reverts to his previous religion.

Note, in the previous paragraph, the almost reflexive joining of political considerations (land, power) with spiritual ones (adherence to a religious faith). I have referred once already to this conjunction in discussing comparative attitudes toward pacifism among the monotheistic religions, but it bears emphasis again. In theory, if not in practice, Christianity as a faith was always distinct from Christendom as a political reality; by contrast, the "world of Islam" comprises and combines temporal and spiritual concerns. "From the lifetime of its founder," writes Bernard Lewis, "Islam was the state, and the identity of religion and government is indelibly stamped on the memories and awareness of the faithful."

To these faithful there is but one Muslim world or empire, artificially divided into nations and ethnic groupings but united in essential loyalty and identification. It is not a question of being an Arab or a Turk--indeed the fragmentation of the Islamic empire into national groupings with their own separate interests is one of the many alleged disasters perpetrated by infidels that Osama bin Laden meant to correct. It is, rather, a question of being a Muslim or being an infidel. Between the two there exists a state of permanent war.

To be sure, the nature of that war is governed by strict codes, adumbrated in the Koran itself and refined over centuries of reflection and interpretation. Killing innocents, for example, is forbidden, and an offensive jihad come under tighter limits than a defensive one in which "they [the unbelievers] attack you [the faithful] first." Terrorism is ruled out, as is suicide, suggesting in Lewis's words that "there is no precedent and no authority in Islam" for the assaults of September 11. On the other hand, martyrdom in the service of Allah has always been honored among Muslims, and the category of the martyr in Islamic thought, unlike in Christian thought, routinely includes one "killed in battle."