If someone is captured during a war and suspected of a crime, then they are charged and tried. That is what the process at Guantanamo is about--figuring out which of these guys were just combatants & need to be held as such and which ones committed some type of a crime and need to be tried and sentenced as criminals.
Prisoners of war, in the sense of the present Convention, are persons belonging to one of the following categories, who have fallen into the power of the enemy:
1. Members of the armed forces of a Party to the conflict as well as members of militias or volunteer corps forming part of such armed forces.
2. Members of other militias and members of other volunteer corps, including those of organized resistance movements, belonging to a Party to the conflict and operating in or outside their own territory, even if this territory is occupied, provided that such militias or volunteer corps, including such organized resistance movements, fulfil the following conditions:
(a) That of being commanded by a person responsible for his subordinates;
(b) That of having a fixed distinctive sign recognizable at a distance;
(c) That of carrying arms openly;
(d) That of conducting their operations in accordance with the laws and customs of war.