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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. 5.
Art. 5. Martial Law should be less stringent in places and countries fully occupied and fairly conquered. Much greater severity may be exercised in places or regions where actual hostilities exist, or are expected and must be prepared for. Its most complete sway is allowed -- even in the commander's own country -- when face to face with the enemy, because of the absolute necessities of the case, and of the paramount duty to defend the country against invasion.
To save the country is paramount to all other considerations.