Article 84 of the Third Geneva Convention: "A prisoner of war shall be tried only by a military court, unless the existing laws of the Detaining Power expressly permit the civil courts to try a member of the armed forces of the Detaining Power in respect of the particular offence alleged to have been committed by the prisoner of war."
The laws of the United States do not permit a U.S. servicemember to be accused and tried in a civil court of war crimes. They are entitled to be tried under the UCMJ in military courts.
According to Article 84 of the Third Geneva Convention, such charges must therefore be dealt with in a military court.
The purpose of this article is to protect combatants from vengeful civilian juries who may not comprehend that a killing during a war may be perfectly legal.
Art4. A. 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.
(3) Members of regular armed forces who profess allegiance to a government or an authority not recognized by the Detaining Power.
(4) Persons who accompany the armed forces without actually being members thereof, such as civilian members of military aircraft crews, war correspondents, supply contractors, members of labour units or of services responsible for the welfare of the armed forces, provided that they have received authorization, from the armed forces which they accompany, who shall provide them for that purpose with an identity card similar to the annexed model.
(5) Members of crews, including masters, pilots and apprentices, of the merchant marine and the crews of civil aircraft of the Parties to the conflict, who do not benefit by more favourable treatment under any other provisions of international law.
(6) Inhabitants of a non-occupied territory, who on the approach of the enemy spontaneously take up arms to resist the invading forces, without having had time to form themselves into regular armed units, provided they carry arms openly and respect the laws and customs of war.
snip