It was a threat Europe’s security services had long feared coming true. In June, Mehdi Nemmouche, a French-born jihadist who had returned to Europe after fighting in Syria with the Islamic State in Iraq and Syria, shot four people to death in an attack at the Jewish museum in Brussels. While the Brussels attack was the first successful one executed by European jihadists returning from Syria, there have been other numerous other planned assaults.

Last fall, Britain’s MI5 disrupted an attack allegedly planned by a cell of “returnee” jihadists for a Mumbai-style mass attack on civilians in London. And in March of this year, French counterterrorism authorities announced they had foiled an imminent attack by a jihadist from Syria who was preparing to strike in southern France. Moreover, several European countries have broken up networks recruiting European Muslims to fight with jihadist groups in Syria, most recently in Spain where police arrested eight people suspected of forming a group that recruited militants to fight in Iraq and Syria.

Imitating al Qaeda’s Afghanistan strategy, radical jihadist groups in Syria have begun to put down permanent roots. Jabhat al-Nusra, the official al Qaeda affiliate in Syria, recently announced the establishment of two training camps in Syria, one of which was named after al Qaeda leader Ayman al-Zawahiri. Meanwhile ISIS has established the formation of the "Zarqawi Camp" on the outskirts of Damascus as well as a number of other training sites. ISIS has even formed entire European fighting units—primarily French-speaking Muslims from France and Belgium—near Aleppo. Now, with ISIS’s rapid advance in Iraq, the organization controls large swaths of northern Syria and western and central Iraq, which provides the strategic depth for ISIS to establish additional training camps while its lighting success provides the group a potent recruiting tool to encourage foreign fighters to join their cause.

A recent EU assessment figures over 2,000 European passport holders have gone to Syria to fight, while the Belgian security services have estimated that the number may be over 4,000. According to an ISIS defector, many Western fighters are being trained with the aim of sending them back to their home countries to commit terrorist acts. “The volume of European returnees is so huge that it is almost impossible for the European security services to keep track of them all,” said Magnus Ranstorp, Research Director at the Center for Asymmetric Threat Studies at the Swedish National Defense College. “There is a strong likelihood,” continued Ranstorp, “that at some point we could see another 7/7 or Mumbai-style attack in Europe.”

What makes young Europeans join an organization like ISIS? In Europe, where many Muslims come from poorer socio-economic backgrounds, the path to radicalization can begin in prison. Muslim immigrants and their children are disproportionately represented in almost every prison system in Europe, and in countries such as France and Belgium approximately 50 percent of the prison population is Muslim. According to a report from the London-based International Center for the Study of Radicalization on prison radicalization, convicted terrorists or those already possessing jihadist views may take leadership roles by negotiating with prison authorities on behalf of the broader Muslim prison population or by leading group prayers. In a new environment removed from family and friends, new inmates are more likely to be influenced by extremist interpretations of Islam. As a result of these factors, European prisons can actually produce more of a threat, as inmates in jail for less serious crimes emerge angry and radicalized upon their release.

Not all European jihadists are young men coming from disadvantaged backgrounds, however. As a study from MI5 noted, the reasons European Muslims choose jihad are varied, and there is no one single profile. Many are older and well educated. Some even have families, though they may have feelings of social or political displacement. As a result they are susceptible to radicalization as a means of establishing a sense of belonging or identity—part of which revolves around the concept of violent jihad. Obviously not all Western fighters returning from Syria or Iraq desire to commit acts of terrorism in their home country—Western anti-terrorism experts believe 10 percent of those returning from Syria to the West are able to carry out terrorist acts—but if even a small number do, the threat is serious.

From monitoring and penetrating jihadist groups to aggressively targeting and prosecuting the recruitment networks, European governments have taken a number of steps to combat the threat of terrorism from their own citizens. In the U.K., for example, a law passed in 2013 targeted at home grown terrorists allows the government to take away the passports of anyone whose "actual or suspected" activities are deemed contrary to the public interest. The U.K.’s Home Secretary Theresa May has been using the law to aggressively strip British jihadists fighting with groups such as ISIS or Jabhat al-Nusra of their citizenship, revoking the passports of 20 people in 2013 alone. France has also joined Britain in stripping its returning nationals of their citizenship upon arrival at the French border if security services can ascertain that they have been fighting with terrorist groups overseas. Other European countries such as Holland and Germany have also looked at measures related to the confiscation of travel documents as well.

According to Magnus Ranstorp, however, it is critical that coercive measures be complemented by a softer approach. Ranstorp pointed to the EU Radicalization Awareness Network (RAN), which focuses on developing a series of “best practices” for engaging at the local level with (potential) foreign fighters, their families and communities, as the type of organization that can play a critical role in preventing homegrown European terrorism.

“Many young Muslim men in the West are struggling with their identity, and all they have known since 9/11 are stories of Guantanamo or Abu Ghraib, which feeds the radical narrative that the West is at war with Islam,” said Ranstorp.

He argues that much of the most critical work in ensuring that extremist views do not result in violence needs to happen within the Muslim community itself, the overwhelming majority of who do not possess extremist views. “We need to make sure to get local agencies, such as social services and NGOs involved as well. In most cases, the families of these jihadists are shocked and overwhelmed by how to deal with radicalized family members, and we need to work directly with family members to develop plans for de-radicalization and psychological aftercare,” said Ranstorp.

The ongoing wars in Syria and now Iraq may drag on for years, and thousands of European citizens will continue to return home after fighting with radical groups like ISIS. As a result, it will be impossible to prevent all terrorist attacks in Europe by homegrown jihadists. This does not mean Europe is doomed to see another 7/7-style attack, but there are likely to be more attacks like the one on the Jewish Museum in Brussels in June. The battle with terrorism has frequently been referred to as a “long war,” and Europe is now on the frontlines.

Josh Cohen is a former U.S. State Department project officer. He currently works for a technology company and contributes to a wide number of foreign-policy-focused media outlets.

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