South Sudan declared its independence after the long-term civil war in 2011, a move which was welcomed both by its inhabitants and the international community and widely supported by the African Union. However, a new civil war broke out a few years later, bringing old ethnic and power rivalries back to light. This article focuses on the causes behind the failure of the state-building process in South Sudan, power relations of its elites, and the difficulties of nation-building. Its main scope is to analyse the causes of the state failure in South Sudan which have their roots deeper in the Sudanese peace process, and which started in the 1990s and culminated with the Comprehensive Peace Agreement in 2005. All these phenomena are studied in a broader geopolitical context bearing in mind also relations with neighbouring states, including parent state Sudan, and with international organisations and the African Union in particular. From a methodological point of view, this article is an intrinsic case-study based on the analysis of documents released by the Government of South Sudan, the African Union, and various international organisations, literature, and also partly on the interviews and observations conducted at the African Union Commission, Addis Ababa, in 2011.