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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section X : Insurrection -- Civil war -- Rebellion - Art. 156.
Art. 156. Common justice and plain expediency require that the military commander protect the manifestly loyal citizens, in revolted territories, against the hardships of the war as much as the common misfortune of all war admits.
The commander will throw the burden of the war, as much as lies within his power, on the disloyal citizens, of the revolted portion or province, subjecting them to a stricter police than the noncombatant enemies have to suffer in regular war; and if he deems it appropriate, or if his government demands of him that every citizen shall, by an oath of allegiance, or by some other manifest act, declare his fidelity to the legitimate government, he may expel, transfer, imprison, or fine the revolted citizens who refuse to pledge themselves anew as citizens obedient to the law and loyal to the government.
Whether it is expedient to do so, and whether reliance can be placed upon such oaths, the commander or his government have the right to decide.