Mengistu was one of several soldiers who in 1974 overthrew Emperor Haile Selassie, whose regime had pursued some disastrous policies. The emperor died the following year, possibly strangled on orders from Mengistu himself. Although several groups were involved in the overthrow, the Derg (of which Mengistu was part) came out on top.
He assumed power as head of state and Derg chairman in 1977, after having his two predecessors killed. Mengistu's years in office were marked by a totalitarian-style government and the country's massive militarization, financed by the Soviet Union and the Eastern Bloc, and assisted by Cuba. From 1977 through early 1978 thousands of suspected enemies of the Derg were tortured and/or killed in a purge called the "red terror." Communism was officially adopted during the late 1970s and early 1980s with the promulgation of a Soviet-style constitution, Politburo, and the creation of the Workers' Party of Ethiopia (WPE). All foreign-owned companies were nationalized without compensation.
The Derg's collapse was hastened by droughts and famine, notably the Ethiopian famine of 1984-1985. There were also insurrections, particularly in the northern regions of Tigray and Eritrea. In 1989, the Tigrayan People's Liberation Front (TPLF) merged with other ethnically based opposition movements to form the Ethiopian Peoples' Revolutionary Democratic Front (EPRDF). In May 1991, EPRDF forces advanced on Addis Ababa. Mengistu himself blames the Derg's collapse on Mikhail Gorbachev for letting the Soviet Union collapse and hence stop funding the Mengistu government.
Mengistu fled the country with around 3,000 Derg officials and was granted asylum in Zimbabwe. He still resides there despite attempts by Ethiopia to extradite him to face trial by the current Ethiopia authorities. Several former members of the Derg have been sentenced to death in absentia.