This thesis studies Protection of Civilians in armed conflict (POC) by focusing on international organizations (IOs) in South Sudan. The newest UN member state is home to one of the biggest UN peacekeeping operations and humanitarian aid interventions facing a man-made protection crisis and the first country where POC sites (UN peacekeeping bases, where hundreds of thousands of people are seeking protection from armed violence) provide uniquely suited microcosms for the study of protection across actors. While POC mandates are now one of the main goals of the international community, little research addresses IOs’ protection logics aiming to fulfill this mandate. This work builds on 132 interviews conducted at headquarter and field level in South Sudan, and argues that POC is structured along two main arenas of political struggle: Firstly, IOs compete in an issue area field of protection for authority and resources, and secondly IO staff fight to determine ‘good’ and ‘bad’ protection efforts according to their respective professional fields (humanitarian, military, civilian UN and police). Based on a multi-actor and multi-level perspective, this work is divided in three analytical parts. First, in ‘macrocosms of violence and protection’ it describes the historical roots of the protection field up to the current contestation of using force to protect. The ‘mesocosm’ zooms in on the particular protection dynamics in South Sudan and examines the organizational logics of protection of the International Committee of the Red Cross, the UN High Commissioner for Refugees, the United Nations Missions in South Sudan and UN Office for the Coordination of Humanitarian Affairs. ‘Microcosms,’ finally, focuses on the four main POC sites in South Sudan and shows that protection outcomes on the ground are contingent on the ability to work across the different organizational protection logics. This makes an understanding across professional fields crucial to protection outcomes. Through this Cosmos framework, the thesis offers both a methodological and theoretical contribution to the IR literature and presents a fine-grained analysis of IO agency.