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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section I : Martial law -- Military jurisdiction -- Military necessity -- Retaliation - Art. 27.
Art. 27. The law of war can no more wholly dispense with retaliation than can the law of nations, of which it is a branch. Yet civilized nations acknowledge retaliation as the sternest feature of war. A reckless enemy often leaves to his opponent no other means of securing himself against the repetition of barbarous outrage.