Lieber's Code
Lieber's Code is a set of rules established in 1863 by Francis Lieber that dictates the conduct of every war-related situation, including how to treat prisoners of war. This document considers a prisoner of war to be "a public enemy armed or attached to the hostile army for active aid, who has fallen into the hands of the captor, either fighting or wounded, on the field or in the hospital, by individual surrender or by capitulation." This also allows those who are associated with an army, such as reporters, journalists, and contractors, to be captured and be treated as prisoners of war. People who are of medical assistance are not permitted to be captured and treated as a prisoner, unless they preform a hostile action that gives reasons to be detained.
Lieber's Code allows lawful captures, but Article 16 does not allow commanders to inflict pain "for the sake of suffering or for revenge, nor of maiming or sounding except in fight, nor of torture to extort confessions" on prisoners. Therefore, commanders of prison camps cannot use acts, such as abuse and poisonings, towards their captives, but it does allow the starvation of hostile prisoners. When writing this document, Lieber kept in mind that these prisoners are still people and need to have protected rights as well.