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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section III : Deserters -- Prisoners of war -- Hostages -- Booty on the battle-field. - Art. 70.
Art. 70. The use of poison in any manner, be it to poison wells, or food, or arms, is wholly excluded from modern warfare. He that uses it puts himself out of the pale of the law and usages of war.