Graeme Wood’s 10,000-word feature article on ISIS, written for The Atlantic, is worth reading from beginning to end. But if you don’t have the time, here are most of the key passages:

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 less like the triumph of the Muslim Brotherhood in Egypt (a group whose leaders the Islamic State considers apostates) than 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.

We have misunderstood the nature of the Islamic State in at least two ways. First, we tend to see jihadism as monolithic, and to apply the logic of al‑Qaeda to an organization that has decisively eclipsed it. The Islamic State supporters I spoke with still refer to Osama bin Laden as “Sheikh Osama,” a title of honor. But jihadism has evolved since al-Qaeda’s heyday, from about 1998 to 2003, and many jihadists disdain the group’s priorities and current leadership.

Bin Laden viewed his terrorism as a prologue to a caliphate he did not expect to see in his lifetime. His organization was flexible, operating as a geographically diffuse network of autonomous cells. The Islamic State, by contrast, requires territory to remain legitimate, and a top-down structure to rule it. (Its bureaucracy is divided into civil and military arms, and its territory into provinces.)

We are misled in a second way, by a well-intentioned but dishonest campaign to deny the Islamic State’s medieval religious nature.
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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. We’ll need to get acquainted with the Islamic State’s intellectual genealogy if we are to react in a way that will not strengthen it, but instead help it self-immolate in its own excessive zeal.
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Denying the holiness of the Koran or the prophecies of Muhammad is straightforward apostasy. But Zarqawi and the state he spawned take the position that many other acts can remove a Muslim from Islam. These include, in certain cases, selling alcohol or drugs, wearing Western clothes or shaving one’s beard, voting in an election — even for a Muslim candidate — and being lax about calling other people apostates. Being a Shiite, as most Iraqi Arabs are, meets the standard as well, because the Islamic State regards Shiism as innovation, and to innovate on the Koran is to deny its initial perfection. (The Islamic State claims that common Shiite practices, such as worship at the graves of imams and public self-flagellation, have no basis in the Koran or in the example of the Prophet.) That means roughly 200 million Shia are marked for death. So too are the heads of state of every Muslim country, who have elevated man-made law above Sharia by running for office or enforcing laws not made by God.

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.
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In the past, Westerners who accused Muslims of blindly following ancient scriptures came to deserved grief from academics — notably the late Edward Said — who pointed out that calling Muslims “ancient” was usually just another way to denigrate them. Look instead, these scholars urged, to the conditions in which these ideologies arose — the bad governance, the shifting social mores, the humiliation of living in lands valued only for their oil.

Without acknowledgment of these factors, no explanation of the rise of the Islamic State could be complete. But focusing on them to the exclusion of ideology reflects another kind of Western bias: that if religious ideology doesn’t matter much in Washington or Berlin, surely it must be equally irrelevant in Raqqa or Mosul.
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Before the rise of the Islamic State, no group in the past few centuries had attempted more-radical fidelity to the Prophetic model than the Wahhabis of 18th‑century Arabia. They conquered most of what is now Saudi Arabia, and their strict practices survive in a diluted version of Sharia there. [Princeton scholar Bernard] Haykel sees an important distinction between the groups, though: “The Wahhabis were not wanton in their violence.” They were surrounded by Muslims, and they conquered lands that were already Islamic; this stayed their hand. “ISIS, by contrast, is really reliving the early period.” Early Muslims were surrounded by non-Muslims, and the Islamic State, because of its takfiri tendencies, considers itself to be in the same situation.
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For certain true believers — the kind who long for epic good-versus-evil battles — visions of apocalyptic bloodbaths fulfill a deep psychological need. Of the Islamic State supporters I met, Musa Cerantonio, the Australian [preacher], expressed the deepest interest in the apocalypse and how the remaining days of the Islamic State — and the world — might look. Parts of that prediction are original to him, and do not yet have the status of doctrine. But other parts are based on mainstream Sunni sources and appear all over the Islamic State’s propaganda. 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.

“Dabiq is basically all farmland,” one Islamic State supporter recently tweeted. “You could imagine large battles taking place there.” The Islamic State’s propagandists drool with anticipation of this event, and constantly imply that it will come soon. The state’s magazine quotes Zarqawi as saying, “The spark has been lit here in Iraq, and its heat will continue to intensify … until it burns the crusader armies in Dabiq.” A recent propaganda video shows clips from Hollywood war movies set in medieval times—perhaps because many of the prophecies specify that the armies will be on horseback or carrying ancient weapons.

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.
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An anti-Messiah, known in Muslim apocalyptic literature as Dajjal, will come from the Khorasan region of eastern Iran and kill a vast number of the caliphate’s fighters, until just 5,000 remain, cornered in Jerusalem. Just as Dajjal prepares to finish them off, Jesus — the second-most-revered prophet in Islam — will return to Earth, spear Dajjal, and lead the Muslims to victory.
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In London, [Anjem] Choudary [former leader of a banned Islamist group called Al Muhajiroun] and his students provided detailed descriptions of how the Islamic State must conduct its foreign policy, now that it is a caliphate. It has already taken up what Islamic law refers to as “offensive jihad,” the forcible expansion into countries that are ruled by non-Muslims. “Hitherto, we were just defending ourselves,” Choudary said; without a caliphate, offensive jihad is an inapplicable concept. But the waging of war to expand the caliphate is an essential duty of the caliph.

Choudary took pains to present the laws of war under which the Islamic State operates as policies of mercy rather than of brutality. He told me the state has an obligation to terrorize its enemies — a holy order to scare the shit out of them with beheadings and crucifixions and enslavement of women and children, because doing so hastens victory and avoids prolonged conflict.
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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. Other Islamist groups, such as the Muslim Brotherhood and Hamas, have succumbed to the blandishments of democracy and the potential for an invitation to the community of nations, complete with a UN seat. Negotiation and accommodation have worked, at times, for the Taliban as well. (Under Taliban rule, Afghanistan exchanged ambassadors with Saudi Arabia, Pakistan, and the United Arab Emirates, an act that invalidated the Taliban’s authority in the Islamic State’s eyes.) To the Islamic State these are not options, but acts of apostasy.
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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. Yet another invasion and occupation would confirm that suspicion, and bolster recruitment. Add the incompetence of our previous efforts as occupiers, and we have reason for reluctance. The rise of ISIS, after all, happened only because our previous occupation created space for Zarqawi and his followers. Who knows the consequences of another botched job?

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.
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It would be facile, even exculpatory, to call the problem of the Islamic State “a problem with Islam.” The religion allows many interpretations, and Islamic State supporters are morally on the hook for the one they choose. And yet simply denouncing the Islamic State as un-Islamic can be counterproductive, especially if those who hear the message have read the holy texts and seen the endorsement of many of the caliphate’s practices written plainly within them.
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The Islamic State’s ideology exerts powerful sway over a certain subset of the population. Life’s hypocrisies and inconsistencies vanish in its face. Musa Cerantonio and the Salafis I met in London are unstumpable: no question I posed left them stuttering. They lectured me garrulously and, if one accepts their premises, convincingly. To call them un-Islamic appears, to me, to invite them into an argument that they would win. If they had been froth-spewing maniacs, I might be able to predict that their movement would burn out as the psychopaths detonated themselves or became drone-splats, one by one. But these men spoke with an academic precision that put me in mind of a good graduate seminar. I even enjoyed their company, and that frightened me as much as anything else.