Bob,
The estimated 20,000 that have gone to Iraq / Syria to join the fighting and possibly many to Daesh (ISIS) should be compared to the flow of foreign fighters to Afghanistan in the Soviet era and to Bosnia-Herzogovina afterwards. I cannot now recall any reliable numbers.
A swift search found an article by Thomas Hegghammer. He estimated 5-20k in Afghanistan (1980-1992) and 1-2k in Bosnia (1992-1995). Another paper refers to 3-5k in Bosnia. See:http://belfercenter.ksg.harvard.edu/...n_Fighters.pdf
Personally I think many went in the early part of the Syrian civil war to defend fellow Muslims from a violent tyranny. Anecdote here is that the UK government's contradictory stance over Libya acted as a factor.
Is it the message that motivates the aspiring fighters to go, but after arrival it is the mission?
I have yet to read a study that estimates how many are with Daesh and non-Daesh groups.
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