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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. 20.
Art. 20. Public war is a state of armed hostility between sovereign nations or governments. It is a law and requisite of civilized existence that men live in political, continuous societies, forming organized units, called states or nations, whose constituents bear, enjoy, suffer, advance and retrograde together, in peace and in war.