Nigeria became a federation of nineteen states in

Nigeria's federal structure has undergone several state-creation exercises since independence. The country became a federation of nineteen states in 1976, when the military government of General Murtala Mohammed reorganised the federation from the existing twelve states into nineteen.

The timeline of state creation in Nigeria is important to keep straight:

• 1963: Nigeria had four regions (later treated as the first grouping).
• 1967: General Yakubu Gowon created twelve states from the four regions.
• 1976: General Murtala Mohammed subdivided the country further into nineteen states.
• 1987: General Ibrahim Babangida created two additional states, bringing the total to twenty-one.
• 1991: Babangida created nine more states, totalling thirty.
• 1996: General Sani Abacha created six more, bringing the total to the current thirty-six states plus the FCT.

The answer is 1976, when the federation expanded from twelve to nineteen states under the Murtala Mohammed administration.