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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. 101.
Art. 101. While deception in war is admitted as a just and necessary means of hostility, and is consistent with honorable warfare, the common law of war allows even capital punishment for clandestine or treacherous attempts to injure an enemy, because they are so dangerous, and it is difficult to guard against them.