The Islamic State, also known as the Islamic State of Iraq and al-Sham (ISIS), follows a distinctive variety of Islam whose beliefs about the path to the Day of Judgment matter to its strategy, and can help the West know its enemy and predict its behavior. Its rise to power is [like] the realization of a dystopian alternate reality in which David Koresh or Jim Jones survived to wield absolute power over not just a few hundred people, but some 8 million [...]

There is a temptation to rehearse this observation—that jihadists are modern secular people, with modern political concerns, wearing medieval religious disguise—and make it fit the Islamic State. In fact, much of what the group does looks nonsensical except in light of a sincere, carefully considered commitment to returning civilization to a seventh-century legal environment, and ultimately to bringing about the apocalypse [...]

To take one example: In September, Sheikh Abu Muhammad al-Adnani, the Islamic State’s chief spokesman, called on Muslims in Western countries such as France and Canada to find an infidel and “smash his head with a rock,” poison him, run him over with a car, or “destroy his crops.” To Western ears, the biblical-sounding punishments—the stoning and crop destruction—juxtaposed strangely with his more modern-sounding call to vehicular homicide. (As if to show that he could terrorize by imagery alone, Adnani also referred to Secretary of State John Kerry as an “uncircumcised geezer.”)

But Adnani was not merely talking trash. His speech was laced with theological and legal discussion, and his exhortation to attack crops directly echoed orders from Muhammad to leave well water and crops alone—unless the armies of Islam were in a defensive position, in which case Muslims in the lands of kuffar, or infidels, should be unmerciful, and poison away.

The reality is that the Islamic State is Islamic. Very Islamic. Yes, it has attracted psychopaths and adventure seekers, drawn largely from the disaffected populations of the Middle East and Europe. But the religion preached by its most ardent followers derives from coherent and even learned interpretations of Islam.

Virtually every major decision and law promulgated by the Islamic State adheres to what it calls, in its press and pronouncements, and on its billboards, license plates, stationery, and coins, “the Prophetic methodology,” which means following the prophecy and example of Muhammad, in punctilious detail. Muslims can reject the Islamic State; nearly all do. But pretending that it isn’t actually a religious, millenarian group, with theology that must be understood to be combatted, has already led the United States to underestimate it and back foolish schemes to counter it.

Following takfiri doctrine, the Islamic State is committed to purifying the world by killing vast numbers of people. The lack of objective reporting from its territory makes the true extent of the slaughter unknowable, but social-media posts from the region suggest that individual executions happen more or less continually, and mass executions every few weeks. Muslim “apostates” are the most common victims [...]

According to Haykel, the ranks of the Islamic State are deeply infused with religious vigor. Koranic quotations are ubiquitous. “Even the foot soldiers spout this stuff constantly,” Haykel said. “They mug for their cameras and repeat their basic doctrines in formulaic fashion, and they do it all the time.” He regards the claim that the Islamic State has distorted the texts of Islam as preposterous, sustainable only through willful ignorance. “People want to absolve Islam,” he said. “It’s this ‘Islam is a religion of peace’ mantra. As if there is such a thing as ‘Islam’! It’s what Muslims do, and how they interpret their texts.” Those texts are shared by all Sunni Muslims, not just the Islamic State. “And these guys have just as much legitimacy as anyone else.” [..]

In Haykel’s estimation, the fighters of the Islamic State are authentic throwbacks to early Islam and are faithfully reproducing its norms of war. This behavior includes a number of practices that modern Muslims tend to prefer not to acknowledge as integral to their sacred texts. “Slavery, crucifixion, and beheadings are not something that freakish [jihadists] are cherry-picking from the medieval tradition,” Haykel said. Islamic State fighters “are smack in the middle of the medieval tradition and are bringing it wholesale into the present day.” [...]

“What’s striking about them is not just the literalism, but also the seriousness with which they read these texts,” Haykel said. “There is an assiduous, obsessive seriousness that Muslims don’t normally have.”

[Supporters of the Islamic State don't consider the old Ottoman Caliphate] legitimate, because it didn’t fully enforce Islamic law, which requires stonings and slavery and amputations, and because its caliphs were not descended from the tribe of the Prophet, the Quraysh. [...]

Baghdadi spoke at length of the importance of the caliphate in his Mosul sermon. He said that to revive the institution of the caliphate—which had not functioned except in name for about 1,000 years—was a communal obligation. He and his loyalists had “hastened to declare the caliphate and place an imam” at its head, he said. “This is a duty upon the Muslims—a duty that has been lost for centuries … The Muslims sin by losing it, and they must always seek to establish it.” Like bin Laden before him, Baghdadi spoke floridly, with frequent scriptural allusion and command of classical rhetoric. Unlike bin Laden, and unlike those false caliphs of the Ottoman empire, he is Qurayshi. [...]

Before the caliphate, “maybe 85 percent of the Sharia was absent from our lives,” Choudary told me. “These laws are in abeyance until we havekhilafa”—a caliphate—“and now we have one.” Without a caliphate, for example, individual vigilantes are not obliged to amputate the hands of thieves they catch in the act. But create a caliphate, and this law, along with a huge body of other jurisprudence, suddenly awakens. [...]

Choudary said Sharia has been misunderstood because of its incomplete application by regimes such as Saudi Arabia, which does behead murderers and cut off thieves’ hands. “The problem,” he explained, “is that when places like Saudi Arabia just implement the penal code, and don’t provide the social and economic justice of the Sharia—the whole package—they simply engender hatred toward the Sharia.” That whole package, he said, would include free housing, food, and clothing for all, though of course anyone who wished to enrich himself with work could do so. [...]

In broad strokes, al-Qaeda acts like an underground political movement, with worldly goals in sight at all times—the expulsion of non-Muslims from the Arabian peninsula, the abolishment of the state of Israel, the end of support for dictatorships in Muslim lands. The Islamic State has its share of worldly concerns (including, in the places it controls, collecting garbage and keeping the water running), but the End of Days is a leitmotif of its propaganda. Bin Laden rarely mentioned the apocalypse, and when he did, he seemed to presume that he would be long dead when the glorious moment of divine comeuppance finally arrived. “Bin Laden and Zawahiri are from elite Sunni families who look down on this kind of speculation and think it’s something the masses engage in,” says Will McCants of the Brookings Institution, who is writing a book about the Islamic State’s apocalyptic thought. [...]

They were anticipating, within a year, the arrival of the Mahdi—a messianic figure destined to lead the Muslims to victory before the end of the world. McCants says a prominent Islamist in Iraq approached bin Laden in 2008 to warn him that the group was being led by millenarians who were “talking all the time about the Mahdi and making strategic decisions” based on when they thought the Mahdi was going to arrive. “Al-Qaeda had to write to [these leaders] to say ‘Cut it out.’ ” [...]

These include the belief that there will be only 12 legitimate caliphs, and Baghdadi is the eighth; that the armies of Rome will mass to meet the armies of Islam in northern Syria; and that Islam’s final showdown with an anti-Messiah will occur in Jerusalem after a period of renewed Islamic conquest [...]

The Islamic State has attached great importance to the Syrian city of Dabiq, near Aleppo. It named its propaganda magazine after the town, and celebrated madly when (at great cost) it conquered Dabiq’s strategically unimportant plains. It is here, the Prophet reportedly said, that the armies of Rome will set up their camp. The armies of Islam will meet them, and Dabiq will be Rome’s Waterloo or its Antietam. [...]

Now that it has taken Dabiq, the Islamic State awaits the arrival of an enemy army there, whose defeat will initiate the countdown to the apocalypse. Western media frequently miss references to Dabiq in the Islamic State’s videos, and focus instead on lurid scenes of beheading. “Here we are, burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive,” said a masked executioner in a November video, showing the severed head of Peter (Abdul Rahman) Kassig, the aid worker who’d been held captive for more than a year. [...]

The Prophetic narration that foretells the Dabiq battle refers to the enemy as Rome. Who “Rome” is, now that the pope has no army, remains a matter of debate. But Cerantonio makes a case that Rome meant the Eastern Roman empire, which had its capital in what is now Istanbul. We should think of Rome as the Republic of Turkey—the same republic that ended the last self-identified caliphate, 90 years ago. Other Islamic State sources suggest that Rome might mean any infidel army, and the Americans will do nicely [...]

It’s hard to overstate how hamstrung the Islamic State will be by its radicalism. The modern international system, born of the 1648 Peace of Westphalia, relies on each state’s willingness to recognize borders, however grudgingly. For the Islamic State, that recognition is ideological suicide [...]

Chastened by our earlier indifference, we are now meeting the Islamic State via Kurdish and Iraqi proxy on the battlefield, and with regular air assaults. Those strategies haven’t dislodged the Islamic State from any of its major territorial possessions, although they’ve kept it from directly assaulting Baghdad and Erbil and slaughtering Shia and Kurds there.

Some observers have called for escalation, including several predictable voices from the interventionist right (Max Boot, Frederick Kagan), who have urged the deployment of tens of thousands of American soldiers. These calls should not be dismissed too quickly: an avowedly genocidal organization is on its potential victims’ front lawn, and it is committing daily atrocities in the territory it already controls.

One way to un-cast the Islamic State’s spell over its adherents would be to overpower it militarily and occupy the parts of Syria and Iraq now under caliphate rule. Al‑Qaeda is ineradicable because it can survive, cockroach-like, by going underground. The Islamic State cannot. If it loses its grip on its territory in Syria and Iraq, it will cease to be a caliphate. Caliphates cannot exist as underground movements, because territorial authority is a requirement: take away its command of territory, and all those oaths of allegiance are no longer binding. Former pledges could of course continue to attack the West and behead their enemies, as freelancers. But the propaganda value of the caliphate would disappear, and with it the supposed religious duty to immigrate and serve it. If the United States were to invade, the Islamic State’s obsession with battle at Dabiq suggests that it might send vast resources there, as if in a conventional battle. If the state musters at Dabiq in full force, only to be routed, it might never recover.

And yet the risks of escalation are enormous. The biggest proponent of an American invasion is the Islamic State itself. The provocative videos, in which a black-hooded executioner addresses President Obama by name, are clearly made to draw America into the fight. An invasion would be a huge propaganda victory for jihadists worldwide: irrespective of whether they have given baya’a to the caliph, they all believe that the United States wants to embark on a modern-day Crusade and kill Muslims.

Given everything we know about the Islamic State, continuing to slowly bleed it, through air strikes and proxy warfare, appears the best of bad military options [...] And with every month that it fails to expand, it resembles less the conquering state of the Prophet Muhammad than yet another Middle Eastern government failing to bring prosperity to its people. [...]

Even so, the death of the Islamic State is unlikely to be quick, and things could still go badly wrong: if the Islamic State obtained the allegiance of al‑Qaeda—increasing, in one swoop, the unity of its base—it could wax into a worse foe than we’ve yet seen. The rift between the Islamic State and al-Qaeda has, if anything, grown in the past few months; the December issue of Dabiq featured a long account of an al‑Qaeda defector who described his old group as corrupt and ineffectual, and Zawahiri as a distant and unfit leader. But we should watch carefully for a rapprochement.

Without a catastrophe such as this, however, or perhaps the threat of the Islamic State’s storming Erbil, a vast ground invasion would certainly make the situation worse.