The UN estimates that there are currently about 30,000 foreign members of ISIL (Islamic State in Iraq and the Levant) in Syria and Iraq. Most are from Moslem nations, especially those in Arabia (Saudi Arabia and the other Arab Gulf states). North African states like Tunisia and Morocco are also major sources. About 15 percent are from European nations, with Russia, France and Germany accounting for most of the Europeans. The UN is trying to get some international cooperation on standards for screening those who return home to identify those who are a danger to their home countries. It is important that all nations pool their data because the returning Islamic terrorists will often plan and carry out attacks in neighboring countries (to avoid local security forces who know about them).
Western and Middle Eastern intel agencies believe the UN estimate of current ISIL strength is too high, but the nationality breakdown seems about right. While the returned ISIL veterans are always touted as a major source of future attacks ISIL appears to instruct (or suggest) that returnees provide instruction and support for local volunteers. Most of the Islamic terrorist attacks in Europe and Europe claimed by ISIL were carried out by local recruits who had not been to Iraq or Syria. Further investigation often turns up ISIL veterans acting as organizers and trainers.