Foreign fighters are not a new phenomenon. However, the present stream of foreign jihadist fighters to the Levant is unprecedented in recent times. Since the caliphate was declared by the so-called “Islamic State” (IS a.k.a. ISIL/ISIS, Daesh or Da’ash) in late June 2014, month after month between 850 and 1,250 new jihadist volunteers have been travelling to Syria and Iraq from abroad. In the twelve months prior to September 2015, the number of foreign fighters nearly doubled to 28,000, according to the US Assistant Attorney General, John P. Carlin.
Schmid, Alex P. "Foreign (Terrorist) Fighter Estimates: Conceptual and Data Issues," Policy brief for the International Centre for Counter-Terrorism. The Hague, Netherlands: 2015. http://icct.nl/wp-content/uploads/2015/10/ICCT-Schmid-Foreign-Terrorist-Fighter-Estimates-Conceptual-and-Data-Issues-October20152.pdf