Everybody here seems to agree that if there's sufficient evidence they committed crimes of some sort (i.e. aiding an armed insurgency against a recognised government) they'll have to face charges. If there's no evidence of this sort, you can't really do anything. I'm not sure what your problem is.
I suppose you could invent a criminal offence for people like Brenus mentioned, who only travel there because they want to live in rebel-held territory and willingly associate with them in some way. But I foresee trouble with coming up with a definition that excludes journalists and humanitarian workers (not really relevant in the case of IS, but moreso for other conflict zones). And creating a new criminal defintion obviously won't apply to current cases.
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