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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. 96.
Art. 96. A citizen serving voluntarily as a guide against his own country commits treason, and will be dealt with according to the law of his country.