The Ethiopian Civil War was a civil war in Ethiopia and present-day Eritrea, fought between the Ethiopian military junta known as the Derg and Ethiopian-Eritrean anti-government rebels from September 1974 to June 1991.

The Derg overthrew the Ethiopian Empire and Emperor Haile Selassie in a coup d'tat on 12 September 1974, establishing Ethiopia as a Marxist-Leninist state under a military junta and provisional government. Various opposition groups of ideological affiliations ranging from Communist to anti-Communist, often drawn from ethnic background, began armed resistance to the Soviet-backed Derg, in addition to the Eritrean separatists already fighting in the Eritrean War of Independence. The Derg used military campaigns and the Qey Shibir (Ethiopian Red Terror) to repress the rebels. By the mid-1980s, various issues such as the 19831985 famine, economic decline, and other after-effects of Derg policies ravaged Ethiopia, increasing popular support for the rebels. The Derg dissolved itself in 1987, establishing the People's Democratic Republic of Ethiopia (PDRE) under the Workers' Party of Ethiopia (WPE) in an attempt to maintain its rule. The Soviet Union began ending its support for the PDRE in the late-1980s and the government was overwhelmed by the increasingly victorious rebel groups. In May 1991, the PDRE was defeated in Eritrea and President Mengistu Haile Mariam fled the country. The Ethiopian Civil War ended on 28 May 1991 when the Ethiopian People's Revolutionary Democratic Front (EPRDF), a coalition of left-wing ethnic rebel groups, entered the capital Addis Ababa. The PDRE was dissolved and replaced with the Tigray People's Liberation Front-led Transitional Government of Ethiopia.The Ethiopian Civil War left at least 1.4 million people dead, with 1 million of the deaths being related to famine and the remainder from combat and other violence.

Mengistu Haile Mariam (Amharic: መንግሥቱ ኀይለ ማሪያም, pronunciation: [mənɡɨstu haɪlə marjam]; born 21 May 1937) is an Ethiopian former army officer and politician who was the head of state of Ethiopia from 1977 to 1991 and General Secretary of the Workers' Party of Ethiopia from 1984 to 1991. He was the chairman of the Derg, the socialist military junta that governed Ethiopia, from 1977 to 1987, and the President of the People's Democratic Republic of Ethiopia (PDRE) from 1987 to 1991.

The Derg took power in the Ethiopian Revolution following the overthrow of Emperor Haile Selassie I in 1974, marking the end of the Solomonic dynasty which had ruled Ethiopia since the 13th century. Mengistu purged rivals for power from the Derg and made himself dictator of Ethiopia, attempting to modernize feudal economy of Ethiopia through Marxist-Leninist-inspired policies such as nationalization and land redistribution. His bloody consolidation of power in 1977–78 is known as the Ethiopian Red Terror, a brutal crackdown on opposition groups and civilians following a failed assassination attempt by the Ethiopian People's Revolutionary Party (EPRP) in September 1976, after it had ignored the Derg's invitation to join the union of socialist parties. The death toll is unknown but is often estimated at between 30,000 and 750,000.

Internal rebellion, government repression, and economic mismanagement characterized Mengistu's presidency, the Red Terror period being a battle for dominance between the Derg, the EPRP, and their rival the All-Ethiopia Socialist Movement (MEISON), who had initially aligned themselves with the Derg. While this internal conflict was being fought, Ethiopia was threatened by both the Somali invasion and the guerrilla campaign of the Eritrean People's Liberation Front, who demanded independence for Eritrea, then a province of Ethiopia. The Ogaden War of 1977–78 over a disputed border region (Ogaden) with Somalia was notable for the prominent role of Mengistu's Soviet and Cuban allies in securing an Ethiopian victory. The catastrophic famine of 1983–85 is what brought his government the most international attention.

Mengistu fled to Zimbabwe in May 1991 after the PDRE National Shengo dissolved itself and called for a transitional government. His departure brought an abrupt end to the Ethiopian Civil War. Mengistu Haile Mariam still lives in Harare, Zimbabwe, despite an Ethiopian court verdict which found him guilty of genocide in absentia. Mengistu's government is estimated to be responsible for the deaths of 500,000 to 2,000,000 Ethiopians, mostly during the 1983–1985 famine in Ethiopia.