A Foreign Fighter is a militant who travels from his or her home country to fight alongside a non-state organization abroad. Although there have been various foreign fighter movements over the years, contemporary usage often refers to individuals who travel to link up with a terrorist organization, like ISIS or al-Qaeda.
The origin of the modern foreign fighter movement dates back to the 1979-1989 Soviet-Afghan War. At that start of the war, influential cleric Abdullah Azzam issued a fatwa (religious ruling) asserting that joining the Afghan fight was fard ayn (an individual religious duty) for all Muslims. According to findings by foreign fighter expert Thomas Hegghammer, up to 20,000 foreign fighters heeded Azzam’s call to fight as foreign mujahideen (jihadists).
Since then, conflicts have continued to attract aspiring jihadists from around the world. From 1980 to 2010, an estimated 10,000-30,000 people fought as foreign fighters in 16 separate conflicts. ISIS alone is believed to have attracted up to 33,000 foreign fighters since 2011. Al-Qaeda’s former affiliate in Syria, the Nusra Front, is believed to have attracted the second-largest contingent of foreign fighters to the region after ISIS.