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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. 95.
Art. 95. If a citizen of a hostile and invaded district voluntarily serves as a guide to the enemy, or offers to do so, he is deemed a war-traitor, and shall suffer death.