This thesis argues that regional-global collaborations for the protection of civilians can be understood through the lens of the collective legitimation function of international organisations. Such a perspective attempts to go beyond – not supplant – the existing explanations for the performance of regional-global collaborations that focus on material, legal and political factors. The thesis builds on Katharina Coleman’s important theoretical framework that she calls a global legitimacy pyramid, but it also supplements it. While Coleman’s model acknowledges that all international organisations have some legitimating potential, it was not her central focus to develop this argument. Doing so is my point of departure for the thesis. With my complements, the model provides for stronger emphasis on the agency and the legitimacy contests at the regional and sub-regional levels. This perspective is used to shed new light on the AU-UN relationship, and on the consensus position that has emerged between a majority of member states in both the AU and the UN that the AU was best placed to take the political and military lead in the response to the conflict in Darfur. The research questions of the project are: (1) when the UNSC chooses to exercise its primary responsibility for maintaining international peace and security through a regional organisation, how can the impacts of this decision best be analysed? And (2) how did the AU-UN relationship impact on civilian protection in the case of Darfur? With the notion of ‘co-dependency’ I attempt to show that both organisations’ legitimacy is dependent upon being seen to contribute to the provision of international peace and security.