If someone has gone abroad to fight in a war I would say it was their business no matter what side they were on. The issue should really only be are they a threat to the UK when they come home which it would seem many are. I can't imagine that Graham Phillips, the infuriating twit of a UK free lance journalist, would be arrested if he ever returned home, for fighting with the terrorists in Ukraine. And much as I dislike him and everything he stands for that would be the correct approach. Of course he and others might be candidates for extradition - and then it becomes very complicated