(CNN) -- Here is a chronology of major events in Nigeria over the last five years.

Background: Nigeria gained independence from Great Britain in 1960, but civilian rule ended with a military coup in 1966. Civilian rule was restored in 1979, but the military intervened again in 1983 and has remained in power since.

1993

Gen. Ibrahim Babangida promises to step down and holds a presidential election, but annuls it when opposition leader and businessman Moshood Abiola takes an unexpected lead in early returns. At least 100 die in riots in the southwest, Abiola's home area.

Babangida steps down in August, but his handpicked successor is overthrown in December by Gen. Sani Abacha, the defense minister and the military's most senior officer. Abacha promises an elected president in three years.

1994

Abiola, the apparent winner of the annulled 1993 election, declares himself president on June 11, but he is arrested on June 23 and charged with treason. Riots break out and are suppressed by the army.

1995

Nigeria is widely condemned for the execution of writer Ken Saro-Wiwa and eight members of his Movement for the Survival of the Ogoni People. They were hanged on murder charges following a kangaroo court trial. The 52-nation British Commonwealth suspends Nigeria. The United States, the European Union and South Africa impose arms and visa restrictions.

1998

April: All five legal political parties adopt Abacha as their candidate for August 1 presidential election.

June 9: Gen. Abdulsalam Abubakar is sworn in as Nigeria's ninth military ruler following the death of Abacha from a heart attack.

June 15: Nigeria orders the immediate release of former military ruler Gen. Olusegun Obasanjo and eight other prominent political prisoners.

July 8: Abiola dies while still in custody. His death sparks riots.

July 11: An international autopsy finds that Abiola died of heart failure.

July 20: Abubakar announces Nigeria will return to civilian rule by May 1999 and details plans to implement a variety of economic and political reforms.