The Declaration of Biafra

On 30 May 1967 Lieutenant Colonel Chukwuemeka Odumegwu Ojukwu, military governor of the Eastern Region, proclaimed the independent Republic of Biafra, named after the Bight of Biafra. According to the Library of Congress country study, the talks held at Aburi, Ghana, in January 1967 had failed to settle the crisis; Gowon proclaimed a state of emergency and announced the division of the country into twelve states, and the Eastern Region Consultative Assembly voted on 26 May to secede. Ojukwu gave as the main reason the federal government's failure to protect the lives of easterners, presenting secession as a step taken reluctantly after all efforts to protect the Igbo in other regions had failed.

The declaration followed the crisis of 1966. After the coup of January 1966, in which most of the conspirators were Igbo, and the counter-coup of July, in which Johnson Aguiyi-Ironsi and other Igbo officers were killed, killings of Igbo in the north drove more than a million people back to the Eastern Region (Library of Congress country study). Some of Ojukwu's colleagues questioned whether the country could be reunited after these outrages. Ojukwu, fearing for his safety, refused to attend meetings in Lagos; the military leaders met at Aburi, Ghana, in January 1967 without settling the crisis, and the Eastern government then announced that it would keep all revenue collected in the region to pay for resettling refugees (country study).

Fighting between Nigeria and Biafra began on 6 July 1967 (Wikipedia), starting the Nigerian Civil War.

The federal government treated the secession as a rebellion, and the fighting that began in July 1967 lasted until January 1970. France, Côte d'Ivoire and Gabon supported Biafra, while the United Kingdom and the Soviet Union were the main supporters of the federal government (Wikipedia). Wikipedia notes that the war's legacy includes the Igbo people's sense of political marginalisation and the rise, decades later, of new Biafran separatist movements.