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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section V : Safe-conduct -- Spies -- War-traitors -- Captured messengers -- Abuse of the flag of truce - Art. 90.
Art. 90. A traitor under the law of war, or a war-traitor, is a person in a place or district under Martial Law who, unauthorized by the military commander, gives information of any kind to the enemy, or holds intercourse with him.