There’s an undeniable attraction in this horror for a number of young people around the Middle East, North Africa, and even Europe and America, who want to leave behind the comfort and safety of normal life for the exaltation of the caliphate. The level of its violence hasn’t discouraged new recruits—the numbers keep growing, because extreme violence is part of what makes ISIS so compelling. Last year, Vice News shot a documentary in the Islamic State’s de facto capital of Raqqa, Syria, and what was striking in the footage was the happiness on the faces of ISIS followers. They revelled in the solidarity of a common cause undertaken at great personal risk. They are idealists—that’s what makes them so dangerous.

In this sense, ISIS is less like a conventional authoritarian or totalitarian state than like a mass death cult. Most such cults attract few followers and pose limited threats; the danger is mostly to themselves. But there are examples in modern history of whole societies falling under the influence and control of a mechanism whose aim is to dictate every aspect of life after an image of absolute virtue, and in doing so to produce a mountain of corpses. ISIS doesn’t behave like a regional insurgency or a global terrorist network, though it has elements of both. It joins the death cult to an army and a rudimentary state. It presents itself as the avant-garde of a mass movement, like the Khmer Rouge. The Islamic State resembles certain modern regimes driven by murderous ideologies, but it is also something new—as new as YouTube—and this makes it even harder to understand.