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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. 1.
Article 1. A place, district, or country occupied by an enemy stands, in consequence of the occupation, under the Martial Law of the invading or occupying army, whether any proclamation declaring Martial Law, or any public warning to the inhabitants, has been issued or not. Martial Law is the immediate and direct effect and consequence of occupation or conquest.
The presence of a hostile army proclaims its Martial Law.