Colonial institutions have deep and long-lasting effects on colonized societies. South Sudan and Eritrea represent the only two cases of ethnic group coalitions successfully gaining independence from a rump African state. The two states underwent contrasting colonial experiences and legacies. In South Sudan, intra-ethnic conflict was a perennial characteristic of political life throughout the consecutive wars for independence. Whereas Eritrean ethnic groups mostly avoided intra-ethnic conflict during their independence struggle. This paper seeks to explain this dichotomy by arguing that the different colonial institutions put in place by Italy in Eritrea and Britain in (South) Sudan, respectively, led to divergent ethnic political arrangements. It will argue that Italy's settler approach to colonial rule, which led to the creation of an Eritrean national identity, fused pre-existing ethnic cleavages. However, Britain's "closed areas" policies made ethnic cleavages more salient and prevented the establishment of any identity category above tribe. Moreover, the different levels of institutional capacities left behind by colonialism, which informed the resulting post-colonial political institutions, led to the divergent approaches employed by those elites that mobilized and led the ethnic coalitions in the two countries. The leaders of the Eritrean Liberation Front (ELF) and its successor, the Eritrean People's Liberation Front (EPLF), were able to build coalitions upon the shared myth of Eritrean-ness created by colonialism. While the both the Anya-Nya Movement (1955-1972) and Sudan People's Liberation Movement (1983-2005) in southern Sudan were forced to put together fragile, negative coalitions of southern Sudanese ethnic groups in opposition to a northern-other. The paper posits a theory of identity category hierarchization that it uses to explain the process each country's elites underwent to create secessionist coalitions. The paper will have implications for understanding existing ethnic coalitional politics, as well as the historical legacies of colonial institutions in the region and beyond.