[3] In March 1957, Nzeogwu enlisted as an officer-cadet in the Nigeria Regiment of the Royal West African Frontier Force and proceeded on a 6-month preliminary training in the Gold Coast.

He completed his training there in October 1957 and proceeded to the Royal Military Academy Sandhurst in England where he was commissioned as an infantry officer in 1959.

[4] Nigerian historian Max Siollun has described Nzeogwu as a "devout catholic, a teetotaler, a non-smoker, and who despite being a bachelor, did not spend much time chasing women".

As a military intelligence officer, he participated in the treasonable felony trial investigations of Obafemi Awolowo and other Action Group party members.

[10][11] Nzeogwu had started his preparation by organizing a two-day night exercise "Damisa" (Operation Tiger) to train soldiers in new fighting techniques.

From the existing government, the premier of the Eastern region (Michael Okpara), the President of the Nigerian federation (Nnamdi Azikiwe) and the Igbo Army Chief (Johnson Aguiyi-Ironsi) were notable survivors.

Nzeogwu's modus operandi in the North contributed in no small measure to the success of the coup in Northern Nigeria.

Nzeogwu also participated in the execution of Col. Raph Shodeinde, his superior officer at the Nigerian Military Training College[13] After waiting for an early morning radio announcement from Major Adewale Ademoyega in Lagos which did not take place because of the failure of the coup in Lagos, Major Nzeogwu made a mid-afternoon announcement, declaring martial law in Northern Nigeria.

Colonel - was trapped in an ambush near Nsukka while conducting a night reconnaissance operation against federal troops of the 21st battalion under Captain Mohammed Inuwa Wushishi.