Thomas Hegghammer, of the Norwegian Defense Research Establishment, has a good piece in WaPo 'Number of foreign fighters from Europe in Syria is historically unprecedented. Who should be worried?'. He opens with:
Since 2011, large numbers of European Muslims have gone to Syria to fight with the rebels. But exactly how many are they, and which countries are providing most of the fighters? The question matters because some of these foreign fighters may return to perpetrate attacks in the West, and Western governments are now grappling with the question of how to design and calibrate countermeasures.
Link:http://www.washingtonpost.com/blogs/...ld-be-worried/

Why the title? He writes:
Incidentally, it is worth noting, for perspective, that the Danish Syria contingent of 65 people is the population-adjusted equivalent of 3,600 Americans.
There is a main thread on foreign fighters, but this article warrants a new, temporary thread. The main thread is:http://council.smallwarsjournal.com/...ad.php?t=14841

I know that European Muslims, a simple generalisation, have fought before in conflicts such as Bosnia, Kashmir, Afghanistan, maybe Chechnya and without much adverse comment at the time in Libya. Often those who survive have not actually fought, although they claim to; some remain in situ, a good number die and others return totally disenchanted.

What is intriguing is the estimated number of fighters coming from Denmark and Norway, although maybe not nationals.

During the Spanish Civil War, which has some similarities to Syria, large numbers of volunteers fought with the Republicans - following their "left-wing" views and the need to confront fascism. The 'International Brigades' are well known, unlike the small numbers of volunteers who fought for the Nationalists.

In my very limited reading I do not recall the return of the 'International Brigades' being seen as a national threat; monitored yes and some curiously formed part of the instructor cadre of SOE in WW2.