1977年2月，门格斯图·海尔·马里亚姆(MENGISTU HAILE MARIAM)中校发动军事政变，担任"临时军事行政委员会"主席和国家元首。1979年，成立以军人为主的"埃塞俄比亚劳动人民党组织委员会"，实行一党制。1984年，按苏共模式组成"埃塞俄比亚工人党"。1987年9月，门格斯图宣布解散"临时军事行政委员会"，结束军事统治，成立"埃塞俄比亚人民民主共和国"，并建立新议会，门担任国家总统和政府首脑。
1988年3月，反政府武装活动的"厄立特里亚人民解放阵线"(EPLF)和"提格雷人民解放阵线"(TPLF)向政府军发动进攻，大规模内战爆发。1989年，"厄立特里亚人民解放阵线"占领厄省大部。1991年5月28日，以"提格雷人民解放阵线"为主的埃革阵军队进入亚的斯亚贝巴，门格斯图政权宣告瓦解。
In July 1977, sensing the disarray in Ethiopia, Somalia attacked across the Ogaden in pursuit of its irredentist claims to the ethnic Somali areas of Ethiopia (see Ogaden War). They were assisted in this invasion by the armed Western Somali Liberation Front. Ethiopian forces were driven back far inside their own frontiers but, with the assistance of a massive Soviet airlift of arms and Cuban combat forces, they stemmed the attack. The last major Somali regular units left the Ogaden March 15, 1978. Twenty years later, the Somali region of Ethiopia remains under-developed and insecure.
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; in 1984, the Workers' Party of Ethiopia (WPE) was established, and on February 1, 1987, a new Soviet-style civilian constitution was submitted to a popular referendum. It was officially endorsed by 81% of voters, and in accordance with this new constitution, the country was renamed the People's Democratic Republic of Ethiopia on September 10, 1987, and Mengistu became president.
The regime's collapse was hastened by droughts and famine, which affected around 8 million people, leaving 1 million dead, as well as by insurrections, particularly in the northern regions of Tigray and Eritrea. In 1989, the Tigrayan Peoples' 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 fled the country to asylum in Zimbabwe, where he still resides.