The thesis offers a first-hand historically informed research on the trajectory of the making of the post-colonial state in Somalia (1940-60). It does so by investigating the interplay between the emergence and diffusion of national movements following the defeat of the Italians in 1941 and the establishment of a British Military Administration, and the process of decolonisation through a 10-year UN trusteeship to Italy in 1950. It examines the extent to which the features of Somali nationalism were affected/shaped by the institutional framework established by the UN mandate. The central argument of the thesis is that the imposition of the UN trusteeship, rather than enabling democratization, led to a ‘verticalisation’ of Somali nationalism and created a highly restrictive political space. Based on a combination of archival and oral sources, the thesis explores the socioeconomic context and possibilities of the wartime. It argues that Somali nationalism developed an efficient and inclusive message that successfully engaged in dialogue with the masses in the 1940s. However, the protraction of the UN debate and the extension of the military administration caused the radicalisation of conflicts among different groups. The imposition of self-government and democratization through the trusteeship system led to the establishment of a highly centralised and fixed institutional framework. Within this context, not only nationalism came to lose its original horizontal and inclusive political line, but national politics were reduced to zero-sum competition to access power and power structures. Ultimately, this exclusive, autocratic and distorted version of the nation-state negatively affected the process of unification of Somalia and Somaliland. By exploring the political trajectory leading to independence and unification, the thesis enhances a broader understanding of the development of post-colonial politics in Somalia. It contributes to specific discussions that centred on the features of the colonial legacy, on the effects of state and nation building, and on the consolidation of a clan-based discourse in post-colonial politics.