Fighting abroad is a constant issue, currently given much hype in the media due to the numbers reported in Syria. Thanks to a "lurker" for the pointer to an article by a "radical" with a militant background, one Moazzam Begg, who offers an explanation as to their motivation and whether they are a threat upon returning home:http://www.cageprisoners.com/our-wor...-war-on-terror

The received wisdom that has led politicians and policemen to reach this conclusion reads something like this: Muslims going to fight in Syria are probably already extremists but even if they’re not, they will be by the time they return radicalised and traumatised from what they have witnessed and experienced. Viz, coupled with extremist ideological beliefs and operational experience they pose an existential threat to the national security of Britain.

Since the groups that attract foreign fighters in Syria have varying affiliations to Al-Qaeda and its beliefs, the reasoning continues, it is safe to assume that young men and women enlisted into the ranks of these organisations are all potential terrorists.

(Later) It is not hard, however, to understand why Muslims would want to go out to Syria to help. Scores of them go every month on humanitarian aid missions and face endless questioning at ports by British police under schedule 7 anti-terrorism powers. It is also understandable why people want to go out and fight for what they believe is a just cause, even if the wisdom of them doing so can be questioned.
There is a main thread 'Today's Wild Geese: Foreign Fighters in the GWOT', which started in 2005, with 83 posts and 43k views:http://council.smallwarsjournal.com/...ad.php?t=14841

It may not be easy reading, it is worth reading IMHO.