Francis Lieber,
A Code for the Government of Armies in the Field…
“ So soon as a man is armed by a sovereign government, and takes the soldier’s oath of fidelity, he is a belligerent; his killing, wounding, or other warlike acts, are no individual crimes or offences. Thus, in ancient times, the sacramentum, changed the homicide of an enemy, from murder to a lawful act.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. ”
