As represented by the title, this chapter unpacks how the British colonial administration left indelible legacies on the Nigerian state and how those legacies killed the sociopolitical fabric of the region before the institution of colonial rule. Through the concept of regionalism, which the chapter understands as “the systemic division of governmental control where a central or federal government holds clearly defined authority and power,” the colonial administration hamstrung Nigeria’s political and economic growth by creating ethnic mistrust and conflict, the marginalization of minorities and agitation among ethnicities after the development of ethnic nationalism. Self-serving interests of colonialists aimed to partition the country along arbitrary lines, disregarding the complex web of pre-existing linguistic and ethnic communities for ease of administration. The effects of these colonialist policies fueled the ethnopolitical and social conflict (and other marginalization of minority groups only possible after the creation of a state) within Nigeria, thus stymieing the development of Nigeria’s internal and independent sociopolitical structures.