During the first three years after independence, the federal government was an NPC-NCNC coalition, despite the conflicting natures of the two partners. The former was regionalist, Muslim, and aristocratic the latter was nationalist, Christian, and populist. Moreover, the NCNC supported opponents of the NPC in regional elections in the Northern Region. Although a more natural ideological alignment of the Action Group and the NCNC was called for by some Action Group leaders, it held no attraction for the NCNC as long as the NPC was assured of a parliamentary majority.

Domination of the Northern Region by the NPC and NCNC control of the Eastern Region were assured. Action Group control of the Western Region, however, was weakened and then collapsed because of divisions within the party that reflected cleavages within Yoruba society. This loss of stability in one region gradually undermined the political structure of the whole country.

The leadership of the Action Group, which formed the official opposition in the federal parliament, split in 1962 as a result of a rift between Awolowo and Akintola, prime minister of the Western Region. Awolowo favored the adoption of democratic socialism as party policy, following the lead of Kwame Nkrumah's regime in Ghana. The radical ideology that Awolowo expressed was at variance with his earlier positions, however, and was seen as a bid to make the Action Group an interregional party that drew support across the country from educated younger voters, whose expectations were frustrated by unemployment and the rising cost of living. Akintola, in reaction, attempted to retain the support of conservative party elements who were disturbed by Awolowo's rhetoric. He called for better relations with the NPC and an allparty federal coalition that would remove the Action Group from opposition and give its leaders greater access to power.

Awolowo's radical majority staged the expulsion of Akintola from the party. The governor of the Western Region demanded Akintola's resignation as prime minister (although he had not lost a vote of confidence in the regional legislature) and named a successor recommended by the Action Group to head the government. Akintola immediately organized a new party, the United People's Party, which pursued a policy of collaboration with the NPC-NCNC government in the federal parliament.

Akintola's resignation in May 1962 sparked bloody rioting in the Western Region and brought effective government to an end as rival legislators, following the example in the streets, introduced violence to the floor of the regional legislature. The federal government declared a state of emergency, dissolved the legislature, and named a federal administrator for the Western Region. One of his first acts was to place many Action Group leaders under house arrest.

Investigations by the federal administrator led to accusations of criminal misuse of public funds against Awolowo and other Action Group leaders. A special commission found that Awolowo had funneled several million pounds from public development corporations to the Actionaa8 on Group through a private investment corporation when he was prime minister of the Western Region in the 1950s. The regional government seized the corporation's assets and pressed legal claims against the Action Group.

In the course of the financial investigation, police uncovered evidence linking Awolowo with a conspiracy to overthrow the government. With a number of other Action Group leaders, he was arrested and put on trial for treason. Authorities charged that 200 activists had received military training in Ghana and had smuggled arms into Nigeria in preparation for a coup d'état. Awolowo was found guilty, along with seventeen others, and was sentenced to ten years in prison. Anthony Enahoro, Awolowo's chief lieutenant who had been abroad at the time of the coup, was extradited from Britain and also was convicted of treason and imprisoned.

In the meantime, the state of emergency was lifted and Balewa, determining that Akintola had been improperly dismissed, obtained his reinstatement as prime minister of the Western Region at the head of a coalition between the NCNC and the United People's Party. The Action Group successfully contested the legality of this action in the courts, but a retroactive amendment to the Western Region's constitution that validated Akintola's reappointment was quickly enacted. As Balewa told parliament, the legalities of the case "had been overtaken by events."

Later in 1963, Nigeria became a republic within the Commonwealth. The change in status called for no practical alteration of the constitutional system. The president, elected to a five-year term by a joint session of the parliament, replaced the crown as the symbol of national sovereignty and the British monarchy as head of state. Azikiwe, who had been governor general, became the republic's first president.

Data as of June 1991