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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. 103.
Art. 103. Spies, war-traitors, and war-rebels are not exchanged according to the common law of war. The exchange of such persons would require a special cartel, authorized by the government, or, at a great distance from it, by the chief commander of the army in the field.