(Warning: the linked piece is long, and takes roughly half an hour to read.)
If you don't have a half hour to spare, here are a few of the highlights:
-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.
-All Muslims acknowledge that Muhammad’s earliest conquests were not tidy affairs, and that the laws of war passed down in the Koran and in the narrations of the Prophet’s rule were calibrated to fit a turbulent and violent time. 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."
-The waging of war to expand the caliphate is an essential duty of the caliph.
-Islamic law permits only temporary peace treaties, lasting no longer than a decade. Similarly, accepting any border is anathema, as stated by the Prophet and echoed in the Islamic State’s propaganda videos. If the caliph consents to a longer-term peace or permanent border, he will be in error. Temporary peace treaties are renewable, but may not be applied to all enemies at once: the caliph must wage jihad at least once a year. He may not rest, or he will fall into a state of sin.
-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.
-Muslims can say that slavery is not legitimate now, and that crucifixion is wrong at this historical juncture. Many say precisely this. But they cannot condemn slavery or crucifixion outright without contradicting the Koran and the example of the Prophet. “The only principled ground that the Islamic State’s opponents could take is to say that certain core texts and traditional teachings of Islam are no longer valid,” Bernard Haykel says. "That really would be an act of apostasy.