Short answers, pulled from the story.
The phrase prisoner of war first appeared in print in 1610. The reality of captivity in war stretches back to the 13th century BC when Nubian prisoners were depicted in engravings at Abu Simbel.
King Henry V ordered the killing of French prisoners after misinterpreting a French raiding party's return as a reinforcement. The resulting slaughter saw Frenchmen stabbed with daggers, brained with pollaxes, and having their bellies panched, with few escaping death.
The earliest known purpose-built prisoner-of-war camp was established at Norman Cross in Huntingdonshire, England in 1797. The highest number recorded was 6,272 on the 10th of April 1810.
Of the 45,000 Union prisoners of war confined in Camp Sumter, located near Andersonville, Georgia, 13,000, or 28%, died. This death rate nearly equaled that of Elmira Prison in New York state.
The last German POWs, like Erich Hartmann, the highest-scoring fighter ace in the history of aerial warfare, were not released by the Soviets until 1955. This release occurred two years after Stalin died.
The United States Military Code of Conduct was promulgated in 1955 via Executive Order 10631 under President Dwight D. Eisenhower. It was created primarily in response to the breakdown of leadership and organization during the Korean War.