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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. 68.
Art. 68. Modern wars are not internecine wars, in which the killing of the enemy is the object. The destruction of the enemy in modern war, and, indeed, modern war itself, are means to obtain that object of the belligerent which lies beyond the war.
Unnecessary or revengeful destruction of life is not lawful.