Many Western countries are worried about the high number of Western Muslim foreign fighters currently fighting in Syria. The concern is that these fighters will return radicalised, battle-hardened and determined to mount a terrorist attack in their home country. While it is very difficult to analyse the current threat posed by Western Muslim foreign fighters – and this should remain the domain of organisations that possess both the expertise and the means to do so – it is possible to look at foreign fighters in the past.

From a historical approach, one can investigate to what extent these foreign fighters have indeed been directly involved in jihadist-inspired terrorist plots and attacks in Europe. Rather than presenting a threat assessment of the risk posed by current fighters, it is possible to investigate historical examples of Western Muslims fighting in conflicts abroad.

Two complementary approaches exist: the first approach examines the presence of foreign fighters in the most serious terrorist attacks and plots in Europe between 1994 and 2013 by using a database; the second approach focuses on foreign fighters in Afghanistan, Bosnia and Somalia. From this, the result is a typology of foreign fighters based on their post-conflict behaviour, consisting of five types: the martyr, the veteran, the recruiter, the reintegrated fighter and the terrorist.

Together, these approaches illustrate that the threat posed by Western Muslim foreign fighters in terms of direct involvement in terrorist attacks and plots in Europe has been rather low and certainly lower than what is now being portrayed in the media with regard to the current fighters in Syria. In fact, this research finds that the ratio of former foreign fighters in twenty-six terrorist plots and attacks in Europe since 1994 is 1 out of 11 to 17 (depending on the plot selection) rather than the widely-accepted ratio of 1 out of 4 that is presented by the terrorism scholar Thomas Hegghammer. This difference is largely caused by the definition of foreign fighters being employed. This shows there is a need to differentiate between foreign fighters (those who fight in an insurgency) and foreign ‘trainees’ (those who train in a terrorist training camp), although this distinction must be applied with extreme caution.

When looking at the post-conflict behaviour of foreign fighters, only a very small proportion of foreign fighters can be categorised as being involved in terrorist activity. It is not possible to quantify the distribution of foreign fighters among the five categories due to a lack of empirical data, but it is clear that the scenario of ‘foreign fighters turning into terrorists’ occurs very rarely. Unfortunately, it only takes one foreign fighter to launch a deadly attack and such important exceptions do exist.

More on this research, see The Hague Governance Quarterly or contact the author.