The historical region of Eritrea had joined Ethiopia as an autonomous unit in 1952. The Eritrean Liberation Movement was founded in 1958 and was succeeded by the Eritrean Liberation Front (ELF) in 1961. The ELF grew in membership when the Ethiopian emperor Haile Selassie abolished Eritrea's autonomous status, annexing it as a province in 1962. In the 1960s and 1970s the ELF undertook a systematic campaign of guerrilla warfare against the Ethiopian government. A faction of the ELF broke away in 1970 to form the Eritrean People's Liberation Front. The EPLF managed to secure control of much of the Eritrean countryside and build effective administrations in the areas it controlled. Fighting that broke out between the EPLF, ELF, and other Eritrean rebel groups in 1981 prevented further military gains, but the EPLF subsequently emerged as the principal Eritrean guerrilla group.