On May 24, 2014, a man opened fire inside the Jewish Museum in Brussels, quickly killing three people and fatally wounding a fourth before disappearing into the city’s streets. The alleged perpetrator, a French citizen named Mehdi Nemmouche, who has since been arrested and charged with murder, had spent the previous year fighting with jihadist opposition groups in Syria. His attack appeared to mark the first time that the Syrian civil war had spilled over into the European Union. Many security officials in Europe and the United States fear that this strike foreshadowed a spate of terrorist attacks that the chaos in Syria—and now Iraq—could trigger.

The Syrian conflict has captured the imaginations and inflamed the passions of Muslims around the world, spurring thousands to join the mostly Sunni rebels resisting the Assad regime. The influx of volunteers has bolstered jihadist groups such as the Islamic State of Iraq and al-Sham (ISIS), also known as the Islamic State, a militant organization that swept across Syria’s border into Iraq this past summer and proclaimed an Islamic caliphate.

Although most foreign fighters in Syria and Iraq come from the Arab world, a sizable contingent hails from the West’s large Muslim communities; 19 million Muslims live in the EU, and more than two million call the United States home. Since the beginning of the Syrian civil war, about 2,500 people from those places (as well as Australia, Canada, and New Zealand) have traveled to Syria to fight, according to the Soufan Group, a U.S. security consulting firm.

Intelligence officials fear that these volunteers might return from the battlefield as terrorists trained to wage jihad against their home countries. Echoing these worries, Charles Farr, the director of the British Office for Security and Counter-Terrorism, described the Syrian war this past summer as “a very profound game changer” for the extremist threat to Europe. Similarly, James Comey, the director of the FBI, warned in May that the repercussions from the conflict might be “an order of magnitude worse” than those that followed the turbulence in Afghanistan during the 1980s and 1990s, which helped spur the formation of al Qaeda. And U.S. President Barack Obama was even more explicit during a prime-time speech to the nation on September 10, warning that “thousands of foreigners—including Europeans and some Americans” have joined ISIS militants and that “trained and battle-hardened, these fighters could try to return to their home countries and carry out deadly attacks.”

On a roll: an ISIS fighter in Raqqa, Syria, June 2014. Stringer / Courtesy Reuters But the threat presented by foreign fighters has been exaggerated, just as it was during several other conflicts in recent years. Over the last decade, the Iraq war in particular prompted similar warnings about a possible backlash that ultimately failed to materialize. In fact, the vast majority of Western Muslims who set out to fight in the Middle East today will not come back as terrorists. Many of them will never go home at all, instead dying in combat or joining new military campaigns elsewhere, or they will return disillusioned and not interested in bringing the violence with them. Even among the rare individuals who do harbor such intentions, most will be less dangerous than they are feared to be because they will attract the attention of authorities before they can strike. It is telling that in the last two years alone, European security officials have disrupted at least five terrorist plots with possible links to Syrian foreign fighters, in locales ranging from Kosovo to the United Kingdom.

Still, the fact that the threat presented by returning Western jihadists will be less apocalyptic than commonly assumed should not lull authorities into complacency. Terrorism is a small-number phenomenon: even a few attackers can unleash horrific violence if they have the training and motivation. Moreover, the extremists’ desire to strike the West could well be on the rise, fueled by the U.S. bombing of ISIS targets that began in August 2014. And because many more volunteers have traveled to Syria and Iraq than to any other conflict zone in the past, many more will ultimately come back.

Nevertheless, the danger posed by returning fighters is both familiar and manageable. Several measures could help further reduce it, including efforts to dissuade would-be volunteers from enlisting in the war to begin with and programs to reintegrate those who do into society when they return. Western intelligence agencies should also do more to disrupt common transit routes and track the militants who use them. And to maintain their vigilance, governments must adequately fund and equip their security services. Together, such measures will help prevent the violence in Syria and Iraq from spilling over into the West.

THERE AND BACK AGAIN

Western fighters who travel to faraway war zones generally follow a similar path as they make the transition from idealistic volunteers to seasoned militants. Most of those who begin the journey do not complete it; still, some do, and at each step Western officials can disrupt the progress of the few individuals who go all the way.

The first and most critical moment comes when a Muslim living in Europe or the United States, most often a young man, decides to join a distant military campaign. His motivations usually include a thirst for adventure and a desire to redress local and regional grievances in the Muslim world, rather than animosity toward the West. In Syria, most early volunteers aspired to defend the local population against the brutality of the Assad regime, not to wage global jihad.

This pattern began to change in 2013 as the war took on a sectarian cast; today, religious rivalry drives most of the recruits. The conflict has aggravated Sunni prejudices against Shiite Muslims—old sentiments that heated up during the U.S. war in Iraq and have now acquired new intensity. The ranks of militant Islamist groups in Syria swelled in late 2013 after prominent religious leaders, such as the Egyptian cleric Yusuf al-Qaradawi, called on all believers to defend Syrian Sunnis against the Assad regime and its Iranian and Shiite Lebanese allies.

In the summer of 2014, ISIS’ stunning battlefield victories lent the organization credibility and enhanced its allure for the small but important Western community of young radicals it seeks to court. The group’s calls for an Islamic emirate and its explicitly sectarian rhetoric have further radicalized the conflict. Such messages percolate through social media sites, including Facebook and Twitter, where jihadists often command large audiences. Isis, in particular, routinely churns out slick recruitment videos in English.

A Belgian soldier patrols a shopping street in central Brussels as police search the area during a continued high level of security following the recent deadly Paris attacks, Belgium, November 24, 2015. Yves Herman / Reuters The second phase of the foreign fighter’s path, traveling to the battlefield, has become remarkably easy to accomplish. Whereas reaching many earlier conflict destinations, such as Afghanistan, meant that Western volunteers had to face significant expenses and dangers, physically getting to Syria entails few sacrifices. Recruits can simply travel to Turkey—an easy trip by car, train, or plane requiring no visa for EU and U.S. citizens—and then cross into Syria along its vast and porous border. Social media also helps: ISIS and other radical groups, including one of ISIS’ rivals, the Syrian al Qaeda affiliate Jabhat al-Nusra, offer ample online tips on how to contact them, including which Turkish hotels to pick in order to meet their travel facilitators.

The potency of the sectarian message and the cross-border flow of information and people help explain the unprecedented number of foreign fighters in Syria and Iraq today—greater than for any conflict in recent memory. Leading specialists on the topic, including Thomas Hegghammer of the Norwegian Defence Research Establishment and Peter Neumann, a British expert on radicalization, have estimated that the Syrian war has mobilized more European Islamists than all other foreign wars over the past 20 years combined. The U.S. share of the influx is smaller, but intelligence officials still believe that at least 100 Americans have joined the Syrian war since 2011.

The third step on the newcomer’s path is to train and then actually fight on the battlefield. Training not only burnishes the recruit’s practical skills; it also imbues him with a sense of solidarity with a larger cause. This experience deepens his indoctrination under the tutelage of sophisticated jihadists: Western security officials fear that a newcomer who might not start out as anti-Western could be manipulated by extremists to change his views, as happened with many fighters who went to Afghanistan in the 1980s and 1990s. The brutal combat that follows further hardens his resolve.

In the fourth step, the fighter returns home to keep the cycle going. Seasoned by battle, he acquires a new authority among his neighbors and followers on social media—a street cred that allows him to recruit and radicalize others and send them into the fray.

Finally, this veteran militant might decide to carry out a terrorist attack at home, turning his attention from foreign causes to real or imagined domestic injustices that may include, for example, insults against Islam, his home country’s perceived oppression of Sunnis abroad, or the daily discrimination faced by Muslims. Analyzing the history of terrorist plots against the West, Hegghammer has found that when such strikes involved returned jihadists, they were both more likely to succeed and more lethal than attacks staged by homegrown terrorists who had not fought abroad.

MORE SMOKE THAN FIRE

Given how few obstacles preclude Western Muslims from joining faraway battles and returning home as terrorists, it might appear paradoxical that most conflicts in the Middle East have spawned barely any fighters who followed this path from start to finish. Syria and Iraq are likely to produce a similar pattern. True, the Syrian war bears many unique traits that significantly magnify the risk. Yet it is crucial not to exaggerate this threat, as governments and analysts have repeatedly done in the past, and to study historical and present-day intelligence in order to temper the dire predictions.

Iraq’s previous war offers the most obvious example. Between 2003 and 2011, dozens of Muslims from Europe and the United States traveled to Iraq to fight Western forces. Some of them supported al Qaeda after it established a local affiliate in 2004 (a group known as al Qaeda in Iraq, which became the precursor to ISIS), and many grew more radicalized during their stay. In 2005, then CIA Director Porter Goss warned the Senate Select Committee on Intelligence that “Islamic extremists are exploiting the Iraqi conflict to recruit new anti-U.S. jihadists.”

Yet despite such grim predictions, jihadist veterans of Iraq failed to perpetrate successful terrorist acts in the West. A few cases bore indirect evidence of a link to the conflict, including a bungled June 2007 strike on the Glasgow airport; investigators found that the attackers’ cell phones contained the numbers of several operatives linked to al Qaeda in Iraq. But even in that case, U.S. officials ultimately judged the plot to be “al Qaeda–related, rather than al Qaeda–directed.”

An undated photograph of a man described as Abdelhamid Abaaoud that was published in the Islamic State's online magazine Dabiq. Reuters Syria and Iraq today are likely to echo this historical record. For one, many foreign volunteers will die in combat. The ferocity of the fighting in Syria and now Iraq—as the radicals battle the two countries’ governments, the Syrian mainstream opposition, and, increasingly, one another—exceeds that of other recent conflicts. Researchers believe that the death toll among foreign volunteers in Syria has already surpassed that of the Iraq war, in which about five percent of all Western fighters are thought to have died. Of those who do survive, many will never return home, fearing arrest or choosing to wage jihad in other foreign lands. One European intelligence official estimated in an interview with us in May 2014 that from ten to 20 percent of foreign combatants have no plans to come back to their former countries of residence. (The official requested to remain anonymous because he was not authorized to discuss sensitive information.)

Furthermore, the Islamist groups active in Syria and Iraq, including ISIS, are not especially interested in attacking Europe or the United States. Instead, they are far more focused on fighting Shiites and local regimes. Many prominent Sunni clerics known for spurring holy warriors to action emphasize the importance of first winning such local contests before striking the West.

The case of Moner Mohammad Abusalha, the first American to carry out a suicide bombing in Syria, illustrates this phenomenon. Originally from Florida, Abusalha joined Jabhat al-Nusra after traveling to Syria in late 2013, and his death stirred U.S. officials’ fears of a terrorist attack on domestic soil. An American citizen, Abusalha seemed to have been a perfect candidate to strike the United States. But Jabhat al-Nusra ordered him to attack Syrian government forces instead—a choice that clearly demonstrated the group’s current priorities. The same logic applied to the British national suspected of killing the American journalists James Foley and Steven Sotloff this past August and September; even though the journalists’ killer could have potentially wreaked havoc in London or elsewhere in Europe, ISIS assigned him a gruesome local task that would make him one of the most wanted men in the world, forever unable to return home.

The U.S. bombing of ISIS positions could change this sense of priorities. As the United States officially enters the fray against ISIS and U.S. involvement in the conflict deepens, the group may shift its priorities to attacking the U.S. homeland, or the West in general, out of revenge or defiance. But for now, ISIS’ attention remains focused on its campaign against Syrian and Iraqi government forces.

Infighting among jihadist groups will further thin out the ranks of foreign recruits. Even as its fighters rolled into Iraq earlier this year, ISIS was embroiled in a bitter clash with Jabhat al-Nusra in Syria. Although both organizations follow Salafi jihadist ideology, ISIS rejected al Qaeda’s leadership and aspects of its agenda, which led to a formal break between the two groups in February 2014. The resulting hostilities have already claimed more than 3,000 lives, according to the most conservative estimates, including the lives of four out of the five British volunteers killed in Syria during the first half of this year. Apart from augmenting the death toll, this kind of infighting breeds disillusionment among foreign recruits. European intelligence officials have found that some would-be volunteers often sour on the idea of enlisting when told that they might have to shoot at old neighbors from across the street, not Assad loyalists or the supposed apostates.

Another common reason for disillusionment is the horrors that Western fighters witness in the conflict zones, especially the Muslim-on-Muslim violence roiling Syria and Iraq. Recruits often set out in pursuit of “the T-shirt and the pictures” but come back terrified and even traumatized by what they have seen and experienced, according to the European intelligence official we interviewed.

With very few exceptions, Western Muslims who do return home rarely complete the transition to terrorist, even if they continue to vehemently oppose their countries’ policies and values. In fact, the majority go on to lead largely ordinary lives. Hegghammer has found that only one in nine fighters who went abroad between 1990 and 2010 came back interested in attacking at home. The nature of the conflict in which they took part also plays a role. Combatants returning from Syria are likely to pose much less of a threat than veterans of al Qaeda’s training facilities in Pakistan; because al Qaeda’s goals are more explicitly anti-Western than those of ISIS, al Qaeda fighters will account for a larger share of the plots in Europe and the United States.

The few individuals who remain bent on violence after returning from Syria and Iraq will often be easy targets for counterterrorism officials. For one, their heavy reliance on social media will become a double-edged sword. By openly publicizing and bragging about their activities online, these people identify themselves to security services and at times supply valuable intelligence data: their group affiliation, intentions, and associates. Officials can also glean useful information by studying their lists of friends and followers. As the European official explained to us, some potential terrorists remain “totally invisible” to authorities until they set out for Syria or Iraq and expose themselves online.

What’s more, former foreign fighters contemplating violence at home could find that their experience in Syria and Iraq has left them ill equipped for the task. Although many learn some guerilla-warfare skills, such as handling small arms, they often lack the knowledge most useful for mounting successful terrorist attacks: how to conduct surveillance, avoid detection, and build a clandestine network. And when they operate in groups—a necessity for executing large-scale strikes—they are even more likely to come to the security services’ attention.

Even the sole successful attack in Brussels demonstrated why fighters returning from the Syrian war pose less of a danger than is often supposed. In executing his assault, Nemmouche acted alone, which allowed him to escape authorities’ notice but also limited the damage he was able to cause. And although he had picked up some combat skills in Syria, Nemmouche appeared to lack any knowledge of concealment or evasion. He never got rid of his Kalashnikov rifle following the shooting; instead, he wrapped it in an ISIS flag and boarded a bus on a well-known and well-policed cannabis-smuggling route from Amsterdam to Marseille, leading to his quick arrest.

Finally, foreign fighters may be reluctant to bring violence back home for the simple reason that doing so could endanger their friends and relatives. In an interview with The New York Times, a friend of Abdisalan Hussein Ali, a Somali American from Minnesota who blew himself up in a 2011 attack on African Union troops in Mogadishu, recalled a revealing statement Ali had made two years before he left for Somalia. He would never attack the United States, Ali had said, since “my mom could be walking down the street.”

BREAKING THE CYCLE

Analyzing each step of the journey taken by Westerners who travel to fight in Syria and Iraq—as well as the factors that prevent them from staging attacks back home—suggests several policy measures that could further reduce the risk. First, Western security services should step up their efforts to dissuade the recruits from volunteering in the first place. One model for how to do this is a government-run program in Denmark that allows officials to seek out and speak with potential recruits in an informal setting, often in conjunction with family members and local community leaders. The goal of such conversations is always to persuade, not coerce. Because the cooperation of families and communities is so vital to this task, officials are careful to press home the message that the Muslim population is a valued part of the solution, rather than the problem. And if individuals do volunteer and go abroad to fight with militants, governments could take measures to prevent their return; one program proposed by British Prime Minister David Cameron in September intends to accomplish just that by confiscating the passports of suspected radical fighters.

Western governments should also do more to make it harder for would-be jihadists to reach Syria and Iraq through Turkey. Until recently, Ankara’s opposition to the Assad regime made Turkey a tacit supporter of fighters streaming across its border. But the rise of ISIS and the looming threat of extremism on Turkey’s own soil have made its government more receptive to Western calls to halt the flow. The United States and European countries should use this opportunity to devise a better system for sharing information with Turkish intelligence and police agencies. For a start, Western officials could issue travel alerts for specific individuals and encourage Turkey to bar them from entering the country or crossing into Syria from its territory.

Western security agencies should also do everything they can to sow doubt in the minds of extremist leaders in Syria and Iraq about the true loyalties of Western Muslim volunteers. This could be accomplished by publicizing intelligence, either obtained from former recruits or even falsely generated by officials themselves, about the degree to which Western security services have infiltrated the jihadists’ ranks. If extremist militias come to view foreigners as potential spies or disseminators of corrupting influences, they might assign Western volunteers to noncombat roles, test their allegiances by offering them the one-way ticket of suicide bombings, or even avoid enlisting them altogether.

Western agencies also need to strengthen their currently inconsistent methods of monitoring returnees and identifying individuals who pose the greatest threat, as well as coordinating these efforts among themselves. The most dangerous returnees need to be closely monitored and, if possible, jailed. (Specific charges would vary by country and could include, for example, membership in a prohibited terrorist group.) But pursuing criminal prosecutions of all Western Muslims who fight abroad could backfire. Although it would temporarily neutralize former combatants, it might also alienate them even further—and, in Europe, expose them to the influence of hardened jihadists, who are amply represented in Europe’s prison populations. Even the mere threat of jail might make a former fighter feel that he has less to lose and push him toward violence. Indiscriminate prosecution would also turn Muslim communities against the government, making them less likely to identify violent radicals in their midst.

Western governments should instead focus on reintegrating former fighters, despite the political difficulty of spending public resources on people whom many consider terrorists. Some returnees will require psychological counseling and treatment for posttraumatic stress disorder; failing to provide it might make them more dangerous than they otherwise would be. If fear of prosecution prevents former fighters from seeking counseling and treatment, they will be less able to reintegrate into civilian life and leave their violent pasts behind.

Last, even though the threat from returning jihadists has been overblown, Western governments still need to devote considerable resources to the problem. Keeping track of the vast roster of suspects that the intelligence agencies must maintain under surveillance at any given time will be exceptionally taxing on both budgets and personnel. But because the influx of foreign fighters to Syria and Iraq exceeds those of previous conflicts, the number of intelligence and police officials dedicated to the problem should grow in parallel. For government agencies, the challenge often lies not in accessing or gathering information about the returnees but in swiftly processing and analyzing it before reacting.

Western governments should also continue to seek ways to alleviate civilian suffering in Syria and Iraq. Many foreign fighters remain driven by a genuine desire to defend Syrians against the brutality of the Assad regime, even as sectarianism takes increasing sway over rival groups. Encouraging charitable activities, identifying legitimate channels for delivering humanitarian aid, and otherwise helping prevent unnecessary loss of civilian life could go a long way toward stemming the flow of foreigners to the war zone.

As long as the Syrian civil war and the ISIS offensive in Iraq continue, however, some fallout in the West appears inevitable. Terrorism is an unfortunate feature of modern life that cannot be eradicated; it can only be mitigated. Indeed, the Obama administration’s decision to intervene against ISIS makes the group more likely to try to expand its list of immediate targets. Yet it is important to avoid panic and to recognize that both the United States and the EU have fended off the worst outcomes in the past and will likely continue to do so.

Measures to reduce the threat of terrorism can and should be improved. But the standard of success cannot be eliminating risk in its totality. If it is, Western governments are doomed to failure and, worse, to an overreaction that will breed far more dangerous policy mistakes.