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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section VIII : Armistice -- Capitulation - Art. 143.
Art. 143. When an armistice is concluded between a fortified place and the army besieging it, it is agreed by all the authorities on this subject that the besieger must cease all extension, perfection, or advance of his attacking works as much so as from attacks by main force.
But as there is a difference of opinion among martial jurists, whether the besieged have the right to repair breaches or to erect new works of defense within the place during an armistice, this point should be determined by express agreement between the parties.