Thomas Hegghammer
November 27, 2013

Since 2011, large numbers of European Muslims have gone to Syria to fight with the rebels. But exactly how many are they, and which countries are providing most of the fighters? The question matters because some of these foreign fighters may return to perpetrate attacks in the West, and Western governments are now grappling with the question of how to design and calibrate countermeasures.

Assessing the terrorist threat to Europe from the foreign fighters in Syria is tricky. On the one hand, as I showed in an earlier study summarized here on the Monkey Cage, foreign fighters are much more likely to engage in international terrorism than the general Muslim population, and they produce more lethal attacks than do plotters without foreign fighting experience. On the other hand, only a small proportion of Western foreign fighters tend to come home to attack. Moreover, the return rate varies considerably between destinations; for example, Western foreign fighters in Pakistan have tended to return for plots more frequently than their counterparts in Somalia.

However, a prerequisite for any threat assessment is a decent estimate of the gross number of departing fighters. Knowing the return rate doesn’t help if we don’t know how many people left in the first place. One estimation strategy consists of collecting all conceivable types of open-source reports of foreign fighter flows, from individual martyrdom notices to aggregate estimates from the United Nations. Aaron Zelin and I have been doing this for the past 15 months, and this work-in-progress has yielded a collection of over 800 data points. This approach has many advantages – which we will highlight in future publications – but it is not ideal for producing comparable country-level estimates, because it includes observations of very different types.

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