On 24 May 2014, Mehdi Nemmouche opened fire and killed four people at the Jewish Museum of Belgium in Brussels. Nemmouche was a French citizen who had fought in Syria, and his attack appeared to fulfill the biggest fear of U.S. and European counterterrorism officials: that the Syrian war would prove to be an incubator of terrorism on the scale of Afghanistan before September 11 or worse. A few months later, Moner Mohammad Abusalha raised alarms in the United States by becoming the first American to conduct a suicide attack in Syria. Most dramatic and deadly of all, on 13 November 2015, teams of attackers linked to the Islamic State killed 130 people in Paris, and several foreign fighters—including the probable mastermind—were among their ranks. Yet not every war in the Muslim world that draws Western recruits produces international terrorism when the foreign fighters return home. Indeed, some, such as those in Somalia and Mali, led to little terrorism “bleedout”—in contrast to Afghanistan, which spawned numerous terrorist attacks.
For Western security officials, the risk seems overwhelming. The Netherlands' national coordinator for security and counterterrorism warned that “these people are not only coming back with radical ideas; they are also traumatised and fully prepared to use violence.”2 American officials have similar fears. Federal Bureau of Investigation director James Comey stated that “all of us with a memory of the '80s and '90s saw the line drawn from Afghanistan in the '80s and '90s to Sept. 11.” He then warned, “We see Syria as that, but an order of magnitude worse in a couple of respects. Far more people going there. Far easier to travel to and back from. So, there's going to be a diaspora out of Syria at some point and we are determined not to let lines be drawn from Syria today to a future 9/11.”
U.S. policy has responded to this potential risk. In June 2014, the United States (again) intervened in Iraq, striking the forces of the Islamic State there and in Syria. Fear that the Islamic State was, like al Qaeda in the past, building a mini-army that would attack the West was one of the reasons for the intervention. President Barack Obama warned that “if left unchecked, these terrorists could pose a growing threat beyond that region, including to the United States.”4 After the intervention, skeptics grew concerned that the U.S. strikes would further turn the Islamic State against the West, making terrorist attacks more likely.
How dangerous is the threat of terrorism from foreign fighters? Why do so many foreign fighters not join terrorist groups? How can effective policy further reduce the threat? These are the questions this article seeks to answer.
Officials' fears and policy responses are not unjustified; they draw on a logical (although often unstated) model of how foreign fighters become international terrorists and fears specific to the latest conflict in Syria and Iraq. As examples, officials point to bloody attacks such as the 7 July 2005, transportation bombings in London as vivid illustrations of the risks that returning fighters can pose. Using Afghanistan as the model, they point out that wars in the Muslim world serve as breeding grounds for terrorists: the causes attract Westerners who, when they go abroad, come into contact with international terrorist groups and are indoctrinated into their anti-Western agenda. They also gain skills and networks that make this new mind-set far more dangerous. If anything, Syria seems even more deadly, attracting far more Western recruits than any past war and imbuing the fighters with bloody purpose.
These fears are not so much wrong as they are overstated. The potential threat is tremendous, but the actual threat—particularly when counterterrorism efforts are incorporated into the overall assessment—is lower, particularly with regard to attacks on the U.S. homeland. Many important jihads, such as the fight against the United States in Iraq after 2003, produced little terrorism in the West. Of those who go to fight, many do not return: some die, while others stay on in the war zone or become professional jihadists elsewhere in the Muslim world. Of those who return, many come back with no desire to attack their home countries, and others are disillusioned or even traumatized. The skill set gained is more suitable for insurgency or civil war than for clandestine terrorism. In contrast to al Qaeda in Afghanistan when the Taliban ruled, most of the groups in Syria, including the Islamic State, are focused first and foremost on the war zone, not on attacking the West. Perhaps most important, security services play an important role and regularly disrupt those who are dangerous: the potential threat is real, but so is the response. Terrorist use of social media radicalizes potential recruits but plays a positive role as well, revealing valuable intelligence on foreign fighters that makes detection more likely.
Effective policy can further reduce the danger. Deradicalization programs can dissuade some potential fighters from ever leaving, and community engagement can identify at-risk individuals. Efforts to disrupt travel through intelligence coordination and better border security in Turkey can prevent fighters from getting to the war zone or returning undetected. In the war zone, offensive counterintelligence can sow suspicion of foreign fighters. Upon their return, deradicalization programs can again play a role, and effectively triaging returnees can help security services determine which ones should simply be monitored and which deserve arrest. Finally, an effective security service response can stop the most dangerous from conducting attacks—but it must be well resourced to do so.
In making these arguments, this article draws on two sources. First, it looks at the fighters going to Iraq and Syria in the context of past foreign fighter flows such as Iraq after the U.S. invasion in 2003, Mali, and Somalia, as well as Afghanistan. Second, it draws on interviews on both the threat and the response. The author conducted interviews with several dozen officials in the United States and Europe. Most of these interviews were conducted with government officials, often in intelligence services, on the condition of anonymity. Several other interviews involved leading academic and think tank experts. The officials' knowledge is often impressionistic and limited to their own countries, but taken together it gives at least some sense of the scope of the problem and the effectiveness of the response.
The remainder of this article has three sections. The first section looks at the reasons for fearing the influx of foreign fighters to Iraq and Syria as they relate to terrorism in their home countries. Drawing in part on the superb work of the scholar Thomas Hegghammer, the article looks at the role foreign fighters have played in important terrorist attacks. With this in mind, it presents a model from initial radicalization to final attack. The second section then takes a step back, examining why foreign fighter violence is usually overpredicted. Using the same model as a framework, it shows a range of mitigating factors that individually make violence less likely and collectively can have a decisive impact. In particular, the attitude of the group training the fighters and the role of security services can be decisive. Finally, the article presents a range of policy steps that can further reduce the danger but argues that the risk level can only be reduced, not brought to zero.