Since the Islamic State is/was the sworn enemy of the USA, any/all who fought for it should AUTOMATICALLY lose their citizenship, period. End of story.
A sane country would do this as a matter of course. Can you imagine a generation ago, talking about Nazis coming “back” to the USA???
There were a handful of Nazi die-hards after VE Day called the Werewolves. If they were given any trial at all, it was basically a drumhead court martial and they were taken out to a nearby tree, tied to it and executed by firing squad. VE Day was in May 1945. The last such recorded Werewolf incident took place in September. It didn't take them long to get the message.
There was no mollycoddling.