The number of recruits is declining, the former ISIS intelligence officer and trainer had told me here, on the shores of the Bosporus. But, at least in those indoctrination classes, there’s no want of young men looking for a quick trip to Paradise. “They keep volunteering,” said Abu Khaled.

In the wide world outside al-Dawla al-Islamiya, the Islamic State, we have caught occasional glimpses of these incendiary young zealots. There was, for instance, Jake Bilardi, a disaffected Australian 18-year-old, who, judging by the blog he left while still in Melbourne, made a rather seamless transition from Chomskyism to takfirism, before detonating himself at a checkpoint in Iraq.

Abu Abdullah al-Australi, as he went to his death in Ramadi, was convinced that he was carrying out a noble act of self-sacrifice, turning kamikaze for the caliphate. For him, jihad began at home. “The turning point in my ideological development,” he’d written, coincided with the “beginning of my complete hatred and opposition to the entire system Australia and the majority of the world was based upon. It was also the moment I realised that violent global revolution was necessary to eliminate this system of governance and that I would likely be killed in this struggle.” He was right about that last part, if not quite about how his fellow revolutionaries determined his use-value.

For pragmatic reasons, ISIS has tolerated homogeneity within the ranks of its katibas, or military battalions, much as the republicans did with their international brigades during the Spanish Civil War. One of the best-trained and best-equipped katibas, or battalions, is named for Anwar al-Awlaki, the American-born al Qaeda cleric who was killed by a U.S. drone strike in Yemen in 2011. “Everything is in English for this katiba,” Abu Khaled said. “And we have another one with a lot of Americans called Abu Mohammed al-Amiriki. It’s named for a guy from New Jersey. He got killed in Kobani. This katiba also has a lot of foreigners.”

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