In an interview with the Washington Post in May, FBI Director James B. Comey, who also served as President George W. Bush’s deputy attorney general, compared the wave of militants pouring into Syria and Iraq to the rush to join Osama bin Laden in Afghanistan as the Taliban swept that country. “We see Syria as that, but an order of magnitude worse in a couple of respects,” he said. “Far more people going there. Far easier to travel to and back from.”

But not everybody agrees that the United States should be alarmed. Writing in the New Yorker last month, the journalist Steve Coll pointed to “some terrorism specialists,” who argue that Islamic State of Iraq and al-Sham (ISIS) is fighting a sectarian war and is more concerned with killing other Muslims than Westerners; that it “has shown no intent to launch attacks in the West, or any ability to do so.” In a widely cited article in the American Political Science Review, Thomas Hegghammer, a senior research fellow at the Norwegian Defense Establishment, argued that there is an essential philosophical difference between those who carry out attacks at home and those who go abroad to fight on behalf of al Qaeda and its affiliates. Many of the Westerners who have gone to Syria and Iraq, he wrote, are unlikely to want to attack targets at home.

Yet fighters returning from Syria have already attempted to carry out violent attacks in the West, and, in one instance, they succeeded. In October 2013, the London Metropolitan Police stopped a car traveling near the Tower of London carrying two men who were reportedly on their way to execute an attack. The two men, both London residents but not British citizens, had recently returned from Syria. In March this year, the French police unraveled a terrorist cell in Nice that was allegedly planning to use improvised explosive devices on the French Riviera. The perpetrators had also recently returned from Syria and were linked to a cell that was held responsible for an attack on a Paris kosher shop in September 2012. Then, at the end of May, Mehdi Nemmouche, a French citizen linked to a militant group known as Forsane Alizza (Knights of Pride), killed three people in front of the Jewish museum in Brussels. He also apparently took part in holding a group of journalists hostage in Syria between July 2013 and December 2013. In late July this year, the Norwegian government put the country on alert that four terrorists from ISIS were on their way to carry out a bombing in the country. The plan, it seems, was for the terrorists to kidnap a family, record themselves decapitating its members, and then post the video to YouTube. And finally, this week, the Australian police carried out the largest counterterrorism operation in the country’s history against an ISIS-linked group based in Sydney. The group allegedly planned to carry out random beheadings of one or more pedestrians taken off the street.

This doesn’t sound like a group of people who has no interest in attacking on Western soil.


According to official estimates, about 3,000 Westerners have joined ISIS or Jabhat al-Nusra, the al Qaeda­–affiliated group in Syria. In addition, hundreds of women from Europe and Australia (and a few Americans) have followed the men, marrying them online before they leave home or linking up with fighters after they arrive for training. They are already pushing against traditional jihadist gender boundaries by setting up female-only fighter groups and taking a prominent role on social media networks -- including posting pictures of themselves with mutilated corpses. They could very well end up becoming violent themselves.

If allowed back into their Western countries of origin, how many of these fighters -- both the men and women -- pose a serious threat to the West?

Here, historical data may provide a baseline estimate. For some years, I have worked with my students to track Westerners who have committed terrorist acts on behalf of al Qaeda and other jihadist groups in its mold. Between 2012 and May 2014, we identified -- by name or fighter alias -- 600 who have left to fight in Syria and Iraq since June 2014. We also identified about 900 individuals who, between 1993 and until about 2012, fought in previous jihadist insurgencies or attempted to link up with terrorist groups and training camps abroad associated with al Qaeda, not including the jihadist groups in Syria and Iraq. Many of these veteran fighters fought in multiple insurgencies. Some of them are now back at work in Syria or Iraq, or have died fighting there. That makes for a data set of nearly 1,500 Western foreign fighters about whom we have basic demographic information, such as age and national and ethnic origin. We know which insurgency they participated in, and what they did after that.

The data show that, in fact, we should be very afraid of the “backflow” from Syria and Iraq. The experience of fighting in a foreign conflict zone, or receiving military-style training from a terrorist organization abroad, often primes Western militants to perpetrate a violent attack at home.

By our count, there have been approximately 279 violent terrorist plots on Western soil since 1993 that were unrelated to the ongoing mobilization in Syria and Iraq. (Here, “plot” might more accurately be described as an arrest related to plans or attempts to do something illegal related to terrorism.) Of the 279 plots, 114 (or 41 percent) included foreign fighters. We identified 275 foreign fighters overall who participated in these plots. Taking a baseline number of nearly 900 foreign fighters (all pre-Syria), in other words, approximately one-in-three Western fighters or veterans of training camps participated in a violent domestic plot. They also helped in fundraising, recruitment, and other schemes, but non-violent activities are not included in this risk assessment.  

Of course, producing clean metrics is tricky. “Doing something” abroad and at home are closely related events, and the sequence does not necessarily go “training abroad and then violent action at home.” (Of the 275 identified participants, 235, about 85 percent, participated in a Western plot after returning.) On occasion, perpetrators became foreign fighters after attempting violence in the West. In those cases, they were often fleeing to join a terrorist group abroad to avoid presenting themselves in court for trial. Others launched attacks after going abroad and failing to obtain sponsorship from an al Qaeda affiliate. Tarek Mehanna, a pudgy pharmacy student from Sudbury, Massachusetts, made no less than three unsuccessful attempts to join an al Qaeda affiliate abroad. After one of his unsuccessful trips, he and his friends played with the idea of shooting up a local mall.

We can drill down still further to look at so-called homegrown conspiracies following 9/11 that have posed a significant risk of large-scale civilian casualties -- for example, the Boston Marathon Bombings, the failed 2009 plot by three school friends from Queens to bomb the New York City subway, and Faisal Shahzad’s failed 2010 Times Square car bomb. We identified 24 such extremely violent plots on Western soil. Of these, 79 percent -- four out of five -- involved returning foreign fighters or individuals who had received training abroad. Of all returning foreign fighters, about one in 12 attempted something along these lines.

In short, not all Westerners return home from jihad abroad to take part in a violent attack. But many do, and they tend to become involved with extremely dangerous plans. Of course, alarming as these numbers are, the ratio of disrupted violent incidents to actualized ones is high -- about four-to-one -- and has increased since the early years of homegrown terrorism following the 9/11 attacks. The odds of disruption for the plots that posed a significant risk of mass casualties are about 50–50.


Assuming that past behavior contains some insights into future behavior, the historical data can help policymakers assess the risk posed to domestic safety by Western returnees from the battle in Syria and Iraq. Combat zone death rates are high among the Western volunteers in Syria and Iraq, about one-in-three, by our count. Generally, insurgent casualty rates are high, but the Westerners are also often used as suicide bombers. (As one former ISIS fighter put it: “I saw many foreign recruits who were put in the suicide squads not because they were ‘great and God wanted it’ as [ISIS] commanders praised them in front of us, but basically because they were useless for ISIS, they spoke no Arabic, they weren’t good fighters and had no professional skills.”) Accepting the estimate that there are (or have been) about 3,000 Western fighters in the theater, we would expect that about a thousand will die. Of those who don’t, most return home or travel to another Western country. Using the one-in-three ratio of returnees from previous conflicts who have come back to do something violent, we would expect over 600 returning fighters from Syria and Iraq to attempt to carry out a violent attack in a Western country within the next few years. This number does not include the essentially unknowable risk stemming from the women who have become radicalized during the time spent with their husbands in Iraq and Syria.

This is not to say that the current wave of jihadists is the same as previous waves. First, the demographics of Westerners in Syria and Iraq today are very different from those in previous jihadist insurgencies. For one, Western fighters in Syria are generally younger (with a mean age of 24) than in previous conflicts. In Bosnia, the average age was 30. In Pakistan and Afghanistan, before and after 9/11, the average was around 27. In the first jihadist insurgency in Iraq (2004–07), the fighters were nearly 28 years old. Recruitment through social media is often held responsible for the age shift, but ISIS has also deliberately recruited very young fighters, even teenagers. Further, the fighters in Syria and Iraq are far more diverse in terms of ancestral origin and race, with white Europeans comprising about 20 percent. No clear socio-economic profile exists either, with gang members from Europe’s ethnic enclaves and drop-outs from universities and prestigious private schools joining up in equal measure. And, finally, there are more women because of militant groups’ policy of getting young jihadists married very early. Some of these factors would seem to indicate a heightened risk -- for example, the increased involvement of women would arguably expand the pool of possible attackers. Other factors are more ambiguous -- young people on a jihadist “gap year” may return home regretting what they have done. Or they might want to go elsewhere and do something more when, if they survive, they leave or are expelled from Iraq and Syria.

Second, unlike before 9/11, when recruitment was a product of direct contact with exiled preachers based in the West, today, the recruitment of Westerners to fight in Syria and Iraq comes from extensive jihadist organizations in the West with deep roots and long histories of perpetrating violent attacks. The fighters in Syria and Iraq are thus deeply enmeshed in networks that were already responsible for violent incidents in West before the Syrian conflict captured their attention. That will increase the likelihood that returnees will be redirected to plots in the West or dispatched to other insurgencies abroad.

Finally, the jihadist ideology has changed from previous conflicts. In Afghanistan, the enemy was the Soviet Union. In Bosnia, it was the Serbs. In Somalia, it was the Ethiopians. In Syria and Iraq, the fight is primarily against other Muslims. And the jihadist insurgents in Syria and Iraq --irrespective of their factional differences -- share a strategic interest in expanding the conflict to the whole of the Middle East so that they can undo the much-hated Sykes–Picot borders that effectively divided the collapsing Ottoman Empire into British and French protectorates. These terrorists recognize no borders or territorial limits to their fight. And that, too, increases the risk that the returnees may become a significant security risk at home.


So what can the West do? Above all, it cannot discount the threat of Western fighters in foreign conflicts. There are simply too many, and their ability and willingness to launch major attacks on the West is too great, to ignore.

Preventing people from leaving to fight with a terrorist organization in the first place is an urgent priority. All, or nearly all, of the newly minted Western-based jihadists from the Syrian and Iraqi conflicts hold Western passports. Current administrative controls target suspected terrorists and individuals who are known to have committed terrorist acts but who, for one reason or another, cannot be charged with criminal offenses. Measures range from impounding passports to imposing house arrests and curfews on individuals who are considered a risk to public safety.

To tackle the migration of a growing number of Western citizens to the frontlines of terrorist campaigns, though, preventive restrictions would have to be extended to hundreds if not thousands of people. Governments are already impounding passports in bulk, but ad hoc measures imposed in the absence of public debate will spawn a backlash against counterterrorism efforts down the road. Restricting the rights of citizens to travel is contrary to core Western values and, within the European Union, runs counter to years of efforts to promote mobility.

In the meantime, the West will have to calibrate its policing strategies to allow non-combatants and those with regrets to come home, while sorting out dangerous individuals for prosecution and detention. For pragmatic reasons -- ranging from problems with obtaining evidence that meets the exacting standards of war crimes prosecutions to cost considerations -- the authorities are likely to opt for prosecutions on lesser charges for which the evidence may be obtained closer to home. That is, they might focus on crimes committed in the West during the preparation for terrorist acts committed abroad. On the positive side, such a strategy could get dangerous terrorists off the street. On the negative, it will bring little comfort to the victims in Iraq and Syria, and Western states may look unwilling to punish their own citizens for crimes against non-Westerners.

A strategy for rehabilitation of post-conflict returnees who cannot be charged with criminal offenses is a must. As the conflict intensifies and casualties grow in Syria and Iraq, many of the less-experienced fighters, the teenagers and the women, will want to come home. Some will be traumatized. A few may even express regrets. Good counter-radicalization and rehabilitation strategies draw on the experience of dealing with gangs: Teams of law enforcement agents and social workers collaborate to provide mentors to former members. They also implement various types of direct supervision -- bans on access to computers, for example, and limitations on the right to communicate with particular individuals. An advantage of the rehabilitation approach is that local authorities are able to keep a close watch on specific individuals and involve families.

Finally, there is the matter of providing justice to the victims. Westerners have participated in executions and crucifixions and raped and plundered in Syria and Iraq. Anticipating war crimes prosecutions of returnees, a number of countries (Sweden, France, Spain, and Canada) have recently enshrined crimes against humanity in their own countries’ penal code, allowing domestic courts to prosecute severe crimes committed abroad. But such prosecutions require custody of the accused.

In other words, bringing the most hardened Western foreign fighters to justice would require their capture and rendition on a large scale. No precedent exists for legal renditions and judicial cooperation of this scale. In the past, European courts have spent years fighting over extraditions of terrorists wanted for trial elsewhere. Khaled al-Fawwaz, bin Laden’s secretary, went about his business in London for more than a decade before he was extradited to stand trial in the United States on charges in connection with his role in the planning of the 1998 bombings of the U.S. embassies in Kenya and Tanzania. (The trial is set for November 3 in the Southern District of New York.) 

The West is now faced with a foreign fighter problem of an unprecedented scale. It can expect that Westerners currently in Iraq and Syria will continue to commit atrocities abroad and will come home and attempt some kind of terrorist plot. It can expect most of the plots on Western soil to be thwarted and the perpetrators rounded up. That means, however, that the Western legal systems will have to finally adjust to dealing with unprecedented numbers of very dangerous people committing crimes for which the evidence is largely foreign, photographic, or found online. The risk of not doing so is already evident: a decade of terrorist recruitment in the West that drew thousands of young people into a violent revolutionary movement -- all without provoking much of a response.

In the United States, the panic that Americans will soon be slaughtered in their beds by returning jihadists is barely concealed. That will not happen, but a realistic assessment of the scale of the threat nonetheless calls for extraordinary measures and international collaboration on the prevention, discovery, apprehension, and detention of the operatives that are responsible for funneling Western recruits into jihad campaigns abroad.

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  • JYTTE KLAUSEN is Lawrence A. Wien Professor of International Cooperation at Brandeis University.
  • Research for this paper was supported by a grant from the U.S. Department of Justice, Office of Justice Program, The National Institute of Justice. (Award 2012 ZA-BX-0006.) Opinions or points of view expressed in this paper are those of the author and do not necessarily reflect the official position or policies of the U.S. Department of Justice.
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