Era I — First Republic & military rule (1960–1999) — Imo State

29 May – 29 September 1966 — Northern pogroms; refugees flood Owerri Province — surviving Igbo civil servants and traders return via the Owerri-Onitsha-Enugu road. Refugee camps at Mbaise and Owerri preceded the Biafran declaration.
30 May 1967 – 12 January 1970 — Biafran War — Owerri Province is the last stand of Biafra: – 16 September 1968: Owerri falls to federal 3 Marine Commando under Col. Benjamin Adekunle ("Black Scorpion"). – 25 April 1969: Owerri recaptured by Biafra under Col. Achuzia — the only major town recaptured by Biafra in the war. – 12 January 1970: Final surrender broadcast from Amichi (Anambra side, near Imo border) by Maj-Gen Philip Effiong.
1979–83 Sam Mbakwe (NPP) administration — public-works boom (Concorde Hotel, Imo Hotels, Avu Cement groundbreaking) ends with the 31 December 1983 coup; assets seized by Buhari administration in 1984.
27 August 1991 — Abia carve-out — Babangida Decree 37 removes Aba, Umuahia, Bende, Arochukwu, Ohafia from Imo. Imo loses ~40% of its population and ~50% of its revenue base.
1992 Ahiara Diocese crisis — Vatican appointment of an Owerri-origin bishop to the Mbaise-dominated Ahiara Diocese sparks a 25-year boycott; only resolved 2017 after Pope Francis personally intervened.
1 October 1996 — Ebonyi carve-out — Abacha Decree 36 removes Afikpo, Ohaozara and Onicha from Imo to the new Ebonyi State.