"Today we are witnessing the largest global convergence of jihadists in history, as individuals from more than 100 countries have migrated to the conflict zone in Syria and Iraq since 2011. Some initially flew to the region to join opposition groups seeking to oust Syrian dictator Bashar al-Assad, but most are now joining the Islamic State of Iraq and Syria (ISIS), inspired to become a part of the group's 'caliphate' and to expand its repressive society. Over 25,000 foreign fighters have traveled to the battlefield to enlist with Islamist terrorist groups, including at least 4,500 Westerners. More than 250 individuals from the United States have also joined or attempted to fight with extremists in the conflict zone. These fighters pose a serious threat to the United States and its allies. Armed with combat experience and extremist connections, many of them are only a plane-flight away from our shores. Even if they do not return home to plot attacks, foreign fighters have taken the lead in recruiting a new generation of terrorists and are seeking to radicalize Westerners online to spread terror back home."
U.S. House of Representatives, Committee on Homeland Security: https://homeland.house.gov/