As many as 20,730 foreigners have joined armed militant groups in Iraq and Syria, making the two countries the most popular destinations for Muslim foreigner fighters in modern history.

While garnering tremendous media attention of late, the phenomenon of foreign fighters is not new. Muslim foreign fighters have joined modern conflicts since the Soviet occupation of Afghanistan in the 1980s, when influential Islamic scholar Abdullah Azzam issued a fatwa (religious decree) declaring that fighting against the Soviets was fard ayn (an individual religious duty) for all Muslims.

From 1980 until 2010, 10,000 to 30,000 Muslim foreign fighters took part in 16 conflicts throughout the world. Of these, Afghanistan has drawn the greatest contingency of foreign fighters by far, followed by the 2003 Iraq war and the 1992-1995 Bosnian war. Afghanistan has consistently drawn foreign fighters since the 1980s, and conflicts in Somalia, Chechnya and Tajikistan have also drawn comparatively small but notable contingencies of foreign fighters.

The phenomenon of foreign fighters poses a particular threat to the West. An analysis by Thomas Hegghammer of the Norwegian Defense Research Establishment estimates that one in nine foreign fighters have returned home to perpetrate attacks. Statistics from 1990 to 2010 show that during that time, 26 percent  of all terrorist plotters had foreign fighter experience, and about 46 percent of all plots included at least one veteran foreign fighter. For executed attacks that resulted in fatalities, 67 percent had at least one veteran foreign fighter. While not all foreign fighters have become al-Qaeda members, the majority of al-Qaeda members began their careers as foreign fighters, and most international jihadist groups are the by-products of foreign fighter mobilizations.

Today, Muslim foreign fighters have traveled to Iraq and Syria from more than 80 different countries, including at least 14 in Western Europe. Of these, France is estimated to have produced the greatest number of foreign fighters in Iraq and Syria, while Belgium has produced the highest number per capita. Almost one-fifth of all foreign fighters are estimated to come from Western Europe.

Some of these foreign fighters have already returned from conflict zones to their home countries, and a subsection of these fighters have already plotted or carried out attacks against the West. French native Mehdi Nemmouche is allegedly one, having purportedly returned from fighting abroad with ISIS to perpetrate the attack at a Jewish Museum in Belgium that killed four.

Over the last two years, European security officials have disrupted at least five terrorist plots with possible links to foreign fighters from Iraq and Syria, in countries ranging from Kosovo to the U.K. Following the January 2015 Islamist attacks in Paris, European countries, particularly Belgium and France, have disrupted terrorist cells and made sweeping arrests that have involved suspected terrorists and returned foreign fighters.

Western countries differ in their policies towards toward returning foreign fighters. Some choose to monitor or imprison citizens known to have fought in Iraq and Syria, while others emphasize their commitment to rehabilitation. Others still have begun to refuse re-entry to returning foreign fighters altogether. While the response differs by country and even by case, all governments appear to recognize the threat posed by returnees. For although foreigners may have left to fight in the conflict zone for a wide array of reasons – some for adventure, some for violence, some to defend their brothers and sisters – their return after fighting poses a clear, statistical threat.

In the words of a European counterterrorism official speaking to CNN, “The threat of attacks has never been greater – not at the time of 9/11, not after the war in Iraq – never.”