Strip them of their citizenship.
Now.
Now.
So, actually what gp is suggesting is not completely implausible.
In the war of terrorism, U.S-citizens caught fighting for Al-Qaeda/ISIS are tried for treason (potentially carrying a death penalty) while foreign fighters could be afforded the luxury of the Geneva Convention rights.
And yes, I am a naturalized US citizen.
Many of those picked up were fighting for the government of Afghanistan at the time so they should have been considered lawful combatants. The whole reason they were put in Guantanamo at the time was to keep them out of the reach of the Geneva convention.
I am hardly a support of islam or any of the cranks and criminals doing acts of terror - my opposition to what was and is still being done to Guantanamo is it totally counterproductive. It is the equivalent to a take no prisoners policy in war. If you want to bolster the enemy make surrender equal death and watch while every battle becomes a fight to the last man.