The Washington PostDemocracy Dies in Darkness

Number of foreign fighters from Europe in Syria is historically unprecedented. Who should be worried?

Islamist fighters carry their flag during a funeral outside Aleppo. (Zain Karam/Reuters)
Placeholder while article actions load

[Joshua Tucker: The following is a guest post from political scientist Thomas Hegghammer of the Norwegian Defense Research Establishment.]


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.

Another strategy, which is simpler and more reliable – albeit more limited in scope – is to consider only a particular type of observation, namely, estimates provided by individual countries’ own intelligence services. Every now and then, government officials will mention such estimates in public, be it in unclassified reports, interviews with journalists or parliamentary hearings. Of course, these estimates are not unproblematic: They may vary in their accuracy and may have been generated in slightly different ways. Still, they are probably the best estimates available because security services have at their disposal a much wider range of collection tools than do academics, and because European agencies appear to have been tracking their respective foreign fighters closely since the start of the Syrian uprising.

To get a sense of the scale and distribution of foreign fighters from Europe, I collected national estimates provided by European intelligence services over the past 6-7 months. With the help of my assistant Kaja Blattmann, I found such numbers for 12 European countries. These are listed below in Table 1. For the countries that provided range estimates, I have used the minimum estimate and put the maximum estimate in parentheses.

The first thing to note here is the total number. The low estimates add up to over 1,100, the high ones to over 1,700. Although these 12 countries are probably the main suppliers of foreign fighters to Syria, there is ample evidence that other European countries have people there too. We can thus say with high confidence that at least 1,200 European Muslims have gone to Syria since the start of the war. This is a remarkable figure; we are talking about the largest European Muslim foreign fighter contingent to any conflict in modern history. In fact, the number of European fighters in Syria may exceed the total number of Muslim foreign fighters from all Western countries to all conflicts between 1990 and 2010 (which my abovementioned study estimated to just under a thousand). And we are only 2 1/2 years into the Syrian war.

The problem is not equally serious for all countries, of course. The estimates above show substantial variation, and the absolute numbers indicate that four countries – France, Germany, Britain and Belgium – have particularly large contingents to worry about.  However, the absolute size of the contingent is not necessarily the best indicator of the scale of the threat, because larger countries have more resources to throw at the problem. Ten prospective terrorists is a greater problem for Luxembourg than for the United States. When we control for the overall size of the population (using data from Wikipedia) and count the number of foreign fighters per million inhabitants, we get Table 2 below:

If we assume that the proportion of foreign fighters who return to attack is the same for each contingent, and that, within the European Union, population size is a reasonable proxy for peacetime policing capacity, then these numbers say something about the relative effort each country will need to exert to handle the returnee problem. Denmark, for example, will have to spend relatively more of its resources to deal with the returnee problem than will Italy. Incidentally, it is worth noting, for perspective, that the Danish Syria contingent of 65 people is the population-adjusted equivalent of 3,600 Americans.

What do these numbers tell us about the relative scale of the radicalization problem within the various Muslim populations in Europe? Thus far, not much, because the relative size of the Muslim population varies between countries. For example, Britain and France have the same overall population, but the French Muslim population is about 65 percent larger than that of the U.K. If we measure contingent size as a proportion of the Muslim population in each country (using Pew data from 2010) we get a somewhat different picture. Table 3 below ranks the twelve countries according the number of foreign fighters per million Muslims:

As a measure of overall radicalization levels, these numbers should be taken with a large pinch of salt. Not all foreign fighters are radicalized Muslims, and not all radicalized Muslims are interested in foreign fighting. There are also constraint factors that may make it easier to travel to Syria from certain countries than from others. Rather than indicating radicalization levels, high scores may suggest the presence of effective recruitment networks with critical mass. Still, policymakers in Copenhagen, Oslo, Brussels and Vienna should take note of these numbers.

It is too early to speculate what may account for the variation in relative recruitment rates. To answer this question, we probably need to look in more detail at the composition of the various contingents, the structure of the recruitment networks, and the local history of the Islamist community in each country.

In the meantime, we can conclude from this simple exercise that the number of European foreign fighters in Syria is alarmingly high and historically unprecedented.  Moreover, France, Germany and the U.K. may have the largest foreign fighter contingents in Syria, but Denmark, Norway, Belgium and Austria have contributed a much higher proportion of their population. Given that police resources are limited, these countries may have a larger problem on their hands than do their bigger European neighbors.