Part of the problem today is looking at the 'foreign fighters' (FF) issue through Syrian "lens" and not considering the history.

Bob's World stated:
Anyone who volunteers to go fight in someone else's war for anything other than significant money is an "extremist." After all, what could be more extreme behavior?
History has plenty of examples of FF participating in the "good fight" and allowing for the cause to be truly foreign. So I have excluded the French aid to your War of Independence and that rendered to Texas against Mexican rule.

How about the pilots who came to aid Great Britain in WW2 (WW1 too IIRC) and the unofficial mission to China, Chenault's Tigers?