So long as that individual retains the legal right to exist in one location (an alternate citizenship)
I believe that's specifically called out in terms of international law or some such. Certainly came up here in the British news back when there was a fuss about various ISIS aligned Brits wanting to come back now that the whole medieval warlord experiment hasn't worked out.
IIRC, at least a couple did get shown the door specifically because they did still have some kind of legal claim to citizenship elsewhere, but I think the situation was less than clear and I'm not sure how it eventually resolved.
If you travel to another country and join a fighting force there, whether it is currently the government or not, I would say you voluntarily renounced your old nationality permanently unless you got their permission first.
(post is archived)