Geneva Conventions and Humanitarian Law...

While the concerns of humanitarian law are more limited than those of human rights law, they are nonetheless very important, since they impose rules of wars and aspire to limit barbarity in times of war. The centerpiece of humanitarian law is perhaps the series of Geneva Conventions that regulate various aspects of conducting wars. The first Geneva Convention in 1864 dealt exclusively with care for wounded soldiers. Later it was expanded and revised. In 1949 four Conventions were adopted:
1st-- on wounded soldiers on the battlefield
2nd-- wounded and shipwrecked at sea
3rd-- prisoners of war
4th-- civilians under enemy control

Furthermore, in 1977 2 Additional Protocols were added.
For our purposes, it is useful to read the provisions of the 3rd Convention carefully:

Art 13. Prisoners of war must at all times be humanely treated. Any unlawful act or omission by the Detaining Power causing death or seriously endangering the health of a prisoner of war in its custody is prohibited, and will be regarded as a serious breach of the present Convention. In particular, no prisoner of war may be subjected to physical mutilation or to medical or scientific experiments of any kind which are not justified by the medical, dental or hospital treatment of the prisoner concerned and carried out in his interest.

Likewise, prisoners of war must at all times be protected, particularly against acts of violence or intimidation and against insults and public curiosity.

Notably, Part III deals with the rules of captivity and internment of prisoners of war.

One of the most important issues in the US law recently was whether this treaty is self-executing or not. The crux of the matter is whether the enemy combatants in Guantanamo could avail themselves of the Geneva Convention without a legislation by Congress authorizing it. This of course has been a major issue in the US since the beginning of time. It always struggled in delineating the hieararchy of laws-- treaties with foreign nations or the US Constitution... The priority is given to the US Constitution and therefore, every time the US ratified treaties, it made several and well-known reservations (thereby limiting the scope of a treaty for its purposes)... This itself is a result of the distrust of international law in the US and unwillingness to enforce it in its affairs... But as we have seen, this unwillingness has made the US pay a heavy price in the world when it comes to the treatment of prisoners in Guantanamo and the series of violations of the Geneva Conventions...