The Eritrean War of Independence was fought from 1961-1991, as Eritrea sought its independence from Ethiopia. Italians starting colonizing Ethiopia in the late 1800s, and, in 1936, Mussolini seized Ethiopia and made it part of the Italian empire.
After World War II, Ethiopia became its own nation (the Allies had liberated it from Italy in 1941). Eritrea was a British protectorate until 1951, and it became part of Ethiopia in 1952. Ethiopia was invested in keeping Eritrea as part of its country so it could enjoy access to the Red Sea (as Ethiopia does not have this access except through Eritrea).
The War of Independence started when Ethiopia took over Eritrea's right to autonomy, and, in 1962, Haile Selassie, the Emperor of Ethiopia, annexed Eritrea. He intended to not allow the use of the Eritrean language and to disallow Eritrean expressions of national identity. The Eritrean Liberation Front, a predominately Muslim group, at first led the fight for independence, and at times, they had their own civil war within Eritrea against other groups, including the Eritrean People's Liberation Front (EPLF), which included Christians and Muslims and eventually replaced the ELF.
In 1974, the Ethiopian monarchy was unseated in a coup by Marxist-Leninists, and Ethiopia had the backing of the Soviet Union as it fought to maintain control of Eritrea. By the late 1980s, this support faded, as the Soviets instituted policies of glasnost (openness) and perestroika (restructuring). In 1991, the Eritrean People's Liberation Front (EPLF) took over Eritrea and Ethiopia, and the United States helped facilitate peace talks. In 1993, the Eritreans voted overwhelmingly for independence (and achieved formal recognition from the United Nations that year). A few years later, border disputes again erupted between the two nations.