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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. 58.
Art. 58. The law of nations knows of no distinction of color, and if an enemy of the United States should enslave and sell any captured persons of their army, it would be a case for the severest retaliation, if not redressed upon complaint.
The United States cannot retaliate by enslavement; therefore death must be the retaliation for this crime against the law of nations.