A recent survey of countries identified as sources of foreign volunteers for ISIL (Islamic State in Iraq and the Levant) indicates that more than 20,000 have made their way to Syria and Iraq to join. Many (if not most) were killed or disillusioned (and deserted). Most of the dead perished in combat or while carrying out a suicide attack. But up to 20 percent of these volunteers returned home, often with the approval of ISIL and with instructions to assist with recruiting and to carry out attacks in their home countries.
Most of these volunteers came from Moslem countries and the largest number of volunteers came in or near the Middle East. These Moslem majority states have long experience with Islamic terrorists and have been quick to identify these ISIL volunteers and arrest or attempt to rehabilitate them. Saudi Arabia apparently got the most returnees and has been successful, so far, in containing them. Other countries, like Libya and Egypt have been less successful at that. This is because Libya has no real government at all and a growing number of local ISIL and foreign volunteers active in coastal cities. Egypt has defeated Islamic terrorist uprisings before, as it did successfully in the 1990s, but now has another such uprising in the Sinai where all sorts of terrorists have been able to find refuge (in between attacks on Egyptian security forces) in Gaza. ISIL has not gained much traction in Egypt, at least not yet.
Most of these volunteers got to Syria via Turkey. While at first Turkey did not try to interfere, believing that the ISIL volunteers were mainly interested in overthrowing the Assad dictatorship (which the Turks never got along with) that attitude has changed and now it is much more difficult for these ISIL volunteers to get through Turkey and into Syria without being identified and sent home or held in Turkish jails.
The few thousand volunteers from Western countries have sent over ten percent of those volunteers back home so far and many have been found and identified as still being active in Islamic terrorist activities. Because ten percent or more of the thirty million Moslems in the West (Europe, North American and Australia) either approve of or are tolerant of Islamic terrorism, these returnees can still recruit more young men for ISIL.