The Amalgamation of Northern and Southern Nigeria

In 1914 the British merged the northern and southern protectorates into a single Nigeria. Frederick Lugard, who returned to Nigeria in 1912 after six years as governor of Hong Kong, carried out the merger and became its first governor-general. According to Wikipedia, unification gave Nigeria common telegraphs, railways, customs, a common currency and a common civil service.

Before 1914 British rule in Nigeria had been built up piece by piece: the Colony of Lagos from 1861, the Oil Rivers and Niger Coast protectorates on the coast, and from 1900 the Protectorate of Northern Nigeria, conquered by Frederick Lugard (Library of Congress country study). Lugard, who had been high commissioner in the north from 1900 to 1906 and then governor of Hong Kong, returned to Nigeria in 1912 to carry out the merger of the northern and southern protectorates. Wikipedia's history of Nigeria notes that in the same year he had a new port built in the south-east, named Port Harcourt, which was linked by railway to the coal mines of Enugu. Lugard's method was indirect rule through traditional rulers, which he applied to the whole country after the merger; in 1916 he formed the Nigerian Council, a consultative body of six traditional rulers including the Sultan of Sokoto, the Emir of Kano and the king of Oyo (country study).

The Library of Congress country study stresses how loose the union remained: the regions kept separate administrations and continued to be run in very different ways, with Christian missions kept out of the Muslim north and English the only official language in the south, while Hausa was also official in the north. Wikipedia likewise notes that north and south remained in practice two separately administered countries, and that differences in access to modern education between them soon became pronounced.

The amalgamation created the territory that became independent as Nigeria in 1960. Wikipedia notes that the imbalance between north and south that it left in place was reflected in Nigeria's political life.