A court-martial (plural courts-martial, as martial is postpositive) is a military court.A court-martial is empowered to determine the guilt of members of the armed forces subject to military law, and, if the defendant is found guilty, to decide upon punishment.In addition, courts-martial may be used to try prisoners of war for war crimes.he Court Martial is one of the Military Courts of the United Kingdom. The Armed Forces Act 2006 establishes the Court Martial as a permanent standing court. Previously courts-martial were convened on an ad hoc basis.