Once you see Muhammad in this light—as a political leader who deftly launched an empire—the parts of the Koran that bear on war make perfect sense. They are just Imperialism 101. Like the Byzantine and Persian Empires that the Islamic Empire would largely displace, Muhammad used a combination of war and diplomacy to expand his turf. All-out jihad—attack the infidels wherever you find them—wouldn’t have made sense for an incipient military power, and that is why you don’t find it in the Koran.

You do find something like it in the decades after Muhammad’s death. Now a true doctrine of jihad takes shape: Muslims, it is said, have a duty to engage in ongoing struggle—military when necessary—to expand Islam’s bounds. In the strong version of the doctrine, which crystallized more than a century after Muhammad’s death, the world is divided between the “House of Islam” and the “House of War.” The House of War is the part of the world still laboring under unbelief even though Islamic doctrine has reached it. It is called the House of War because the duty of Islam’s leader is to fight there.

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It’s in one sense surprising that jihad, a doctrine taken seriously by Muslim thinkers over the years, has no solid grounding in what they consider the most reliable record of Muhammad’s, and God’s, utterances [Koran]. In another sense, it’s par for the course. The consistent moral of the story of the Abrahamic religions is that any given book of scripture can be put to a wide variety of uses.

But if the bad news is how malleable scripture is, that’s also the good news. Yes, when you see your interests opposed to those of another group, you can find scriptural validation of animosity. But when your interests seem to lie in cooperation with another group, you may find your God counseling restraint.

This second edge of the sword was illustrated by Muhammad’s successors as the Islamic Empire grew and some of its borders stabilized. By the early 800s, only a few decades after Muslim thinkers had divided the world between a “House of Islam” and a “House of War,” a seminal Islamic jurist had declared that there was actually a third house: the “House of Truce or Treaty.” And by the late 800s, another Islamic thinker had labeled war in the name of Islam the “lesser jihad” and said, “the greater jihad is the struggle against the self.” As we’ve seen, this idea of two kinds of jihad is consistent with the different uses of the term in the Koran. But on what basis would anyone say which was greater and which was lesser? The hadith to the rescue! By one account, Muhammad had himself told Muslims returning from war, “You have returned from the lesser jihad to the greater jihad.” This account was late to surface, but better late than never.

An especially important doctrine was fard kifaya—the idea that jihad, though a duty, was a communal duty, not an individual duty. So if war seemed inappropriate in your part of the empire, you could live a peaceful yet devout life, secure in the knowledge that somewhere, some Muslim was fighting on behalf of Islam.

But all such moderating influences were hostages to fortune. They might dominate when cooperation with neighbors, or at least peaceful coexistence, seemed auspicious, but things could always change. When Muslims were being attacked, the definition of jihad changed from fard kifaya to fard aynl—a duty incumbent on each Muslim. When Christian crusaders reached Syria, for example, a treatise published in Damascus announced the shift to fard aynl. As ever, swings between the non-zero-sum and the zero-sum could change the mood of a religion.

The Price of Tolerance

The malleability of the doctrine of jihad was as evident within Islam’s borders as along them. Though making all the world the “House of Islam” would seem to imply turning everyone you subjugate into a Muslim, that goal, if it was ever part of jihad, didn’t stay that way for long. The more unbelievers you subjugate, the clearer it becomes that their ongoing antagonism won’t be an asset, and the less attractive is the prospect of incurring their wrath by coercing them into conversion. Once you’ve got an empire to run, the less friction within it, the better.

Here again, useful guidance could be found in scripture so long as you looked hard enough. The Koranic verse that comes closest to calling for jihad on a global scale also has a crucial loophole. It begins, “Make war upon such of those to whom the Scriptures have been given as believe not in God, or in the last day, and who forbid not that which God and His Apostle have forbidden,” but then ends, “until they pay tribute out of hand, and they be humbled.” In the end, money would substitute for theological fidelity.

There was nothing new about this. Ancient empires expanded as far as was feasible and demanded tribute of their vassal states. That, after all, was half the point of being an empire. The Roman Empire had done it, and so had the two empires that Islam was now taking land from—the Persian Empire and the eastern heir of the Roman Empire, the Byzantine Empire. So subjects of the emerging Islamic Empire shouldn’t have found the taxes imposed on them disorienting.

In fact, some Christians preferred the new Muslim overlords to the old Christian ones. The Byzantine Empire had fought heretical Christian sects, whereas to the Muslims, a Christian was a Christian; so long as they paid their taxes, heretics could worship as they wished. It was win-win: formerly suppressed Christians got their freedom for a price they considered a bargain, and Muslim rulers got peace within their empire and, to boot, a steady source of revenue. In fact, around 700, Muslim rulers banned conversion to Islam lest revenue fall.

It was a deft maneuver that Muhammad’s successors pulled off: declare war on a people because of their religion and then, shortly after the conquest, feel tolerance welling up. Fortunately, Islamic rulers had the ambiguity of the Koran to back them up. They cited the Koranic injunction against “compulsion in religion”—a passage that, perhaps, had receded to the margins of their awareness during the conquest itself, back when more pungent verses sprang to mind.

And then there was the ever flexible hadith. When ruling over unbelievers, Muslims recalled that Muhammad had said, “If they convert to Islam it is well; if not, they remain (in their previous religion); indeed Islam is wide.” This from the same man who supposedly had said, “I was ordered to fight all men until they say ‘There is no god but God.’ ”

Sometimes there was no contradiction between these statements. Subjects who were Jewish did, in fact, believe that there was no god but God even though they hadn’t converted to Islam. So too with Christians (even if their monotheism was a bit suspect in light of Jesus’s divinity). Conquest of the largely Christian lands of Syria and Egypt, then, entailed little doctrinal amendment. “People of the Book,” it was said, were allowed to keep their religion.

But what about the conquest of Persian lands? Here tolerance of the native faith, Zoroastrianism, took creativity. After all, Zoroastrians didn’t have scriptures devoted to the Abrahamic god—and so weren’t in any clear sense “People of the Book.” But, hey, the Zoroastrians did have a book of scripture—the Avesta—so they were in some sense People of the Book, or at least, People of a Book. Conclusion: they could be tolerated, too! And later, as Muslim conquests spread deep into Asia, it turned out that there was a way to extend this basic idea—taxes in exchange for toleration—to Buddhists and Hindus. And Muslim rulers in Africa decided that there, too, polytheists could be tolerated.

In the end, the basic modus operandi of the Islamic Empire was the basic m.o. for ancient empires: conquer and then tax. And an easily collected tax requires empire-wide pax, whether it be Pax Romana or Pax Islamica.

Over the centuries, Islamic tolerance of Christians and Jews (like Christian tolerance of Muslims and Jews) would fluctuate. As voluntary conversion to Islam set in—sometimes with the goal of escaping the tax, sometimes with the goal of easing career advancement—the population of Christians dwindled to a point where Muslims found it less crucial to stay on good terms with them. This change in attitude presumably heightened the incentive to convert to Islam. Jews, more averse to conversion, stayed intact and sometimes faced persecution. But on balance, as the scholar Claude Cahen has observed, Islam showed more tolerance toward Jews over the centuries than did Christian Europe.

Meanwhile, the reinterpretation of jihad went on and on, swinging between truculence and reserve as circumstances warranted. By the early twentieth century, many mainstream Muslim thinkers had stripped the doctrine of its offensive connotations: Islamic “holy war” was justified only in self-defense. This convergence with western views on just war is, of course, what led Sayyid Qutb to complain in the mid-twentieth century about “the sorry state of the present Muslim generation.” Qutb’s complaint foreshadowed a resurgence of militant interpretations of jihad. And here we are.