We are war with Islamic Fundamentalists. They are at war with us and consider their war to be “holy”. There are international agreements which govern such things. When combatants are captured on the battlefield, they can be detained as POWs for the duration of the war, however long that is. Alternatively, if a combatant is captured attempting to commit mayhem in the opposition’s homeland, that combatant is a spy and is subject to military trial and execution.
The debate about where to hold civilian trials is the wrong question.
Wrong questions lead to wrong answers.