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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. 88.
Art. 88. A spy is a person who secretly, in disguise or under false pretense, seeks information with the intention of communicating it to the enemy.
The spy is punishable with death by hanging by the neck, whether or not he succeed in obtaining the information or in conveying it to the enemy.