This article explores the treatment of detainees in treaty and customary international humanitarian law. Detainees are persons who do not or have ceased to take a direct part in hostilities or are hors de combat. It contextualizes the discussion on the treatment of detainees by illustrating an example from contemporary Slovenian history relating to the barbaric treatment of detainees on the Slovenian territory after Second World War, where more than one hundred thousand persons were executed. This article argues that fundamental human rights apply in all situations and that the fundamental guarantees apply during and after the hostilities to all categories of detainees. In this way, it further explores fundamental guarantees, a meeting point between international human rights law and humanitarian law. Based on these findings this article argues that there are strong legal and moral grounds to address crimes against humanity committed in Slovenia against detainees after the Second World War.