A few months after the release of the Eid video, another ISIS production focused on the ISIS’s substantial foreign-fighter contingent in an entirely different way.

In a procession were a long line of at least 17 foreign fighters, many of them white-skinned Europeans, each guiding with his left hand a prisoner identified as a Syrian soldier. Only one wore a mask, the British fighter known as “Jihadi John,” who had executed James Foley and other American and European hostages.

After the jihadists had hacked through the necks of their victims, the camera played over the faces of the executioners, ensuring that they were clearly visible and sparking a rush to identify them. Media reports identified the perpetrators as French, German, British, Danish, and Australian citizens, although some of these claims were tentative.

ISIS propaganda and messaging is disproportionately slanted toward foreign fighters, both in its content and its target audience. Important ISIS messages are commonly released simultaneously in English, French, and German, then later translated into other languages, such as Russian, Indonesian, and Urdu.

“Foreign fighters are overrepresented, it seems, among the perpetrators of the Islamic State’s worst acts,” said Thomas Hegghammer, a leading scholar of jihadist history, in an interview with BillMoyers.com. “So they help kind of radicalize the conflict—make it more brutal. They probably also make the conflict more intractable, because the people who come as foreign fighters are, on average, more ideological than the typical Syrian rebel.”

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One of the most important questions about the threat presented by ISIS, and the conflict in Syria and Iraq in general, is numerical: How many foreign fighters are there, where do they come from, and what will they do after fighting?

The question is nearly impossible to answer with any kind of specificity, due to the dangers that ISIS presents for journalists and intelligence operatives on the ground. In the open-source world, there are only estimates, and the situation does not appear to be much better in the world of secret intelligence. According to one 2013 tally, from Radio Free Europe/Radio Liberty, there were between 17,000 and 19,000 fighters in Iraq and Syria, though this count is likely too low.

The majority of those fighters originated in the Middle East and North Africa, especially Tunisia and Saudi Arabia. The remainder came from other places around the world, including former Soviet republics, the Americas, and Australia. But numbers were unavailable for several countries known to have provided fighters, including Azerbaijan, Indonesia, the Philippines, and Somalia. In general, moreover, foreign-fighter estimates are often unclear as to whether fighters are affiliated with ISIS and whether they pertain only to Syria or to Iraq and Syria.