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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. 51.
Art. 51. If the people of that portion of an invaded country which is not yet occupied by the enemy, or of the whole country, at the approach of a hostile army, rise, under a duly authorized levy ' en masse ' to resist the invader, they are now treated as public enemies, and, if captured, are prisoners of war.