This study of detention by United Nations (UN) peacekeepers inSouth Sudan shows how practices shaped the development of anew detention institution and its infrastructure. Afirst-of-its-kindqualitative analysis of the everyday of UN peacekeepingdetention, the article examines life and work in the‘holdingfacilities’in the UN protection of civilian sites, which the officersdubbed‘Guantánamo on the Nile’, due to ambiguitiessurrounding their work. The article conceptualises practice-driveninstitutional change and provides a granular account of theambiguities and ethical dilemmas, which are part of the everydayof detention on the frontlines of protection and peacekeeping.