The strategic importance of Eritrea, due to its Red Sea coastline and mineral resources, was the main cause for the federation with Ethiopia, which in turn led to Eritrea's annexation as Ethiopia's 14th province in 1952. This was the culmination of a gradual process of takeover by the Ethiopian authorities, a process which included a 1959 edict establishing the compulsory teaching of Amharic, the main language of Ethiopia, in all Eritrean schools. The lack of regard for the Eritrean population led to the formation of an independence movement in the early 1960s (1961), which erupted into a 30-year war against successive Ethiopian governments that ended in 1991. Following a UN-supervised referendum in Eritrea (dubbed UNOVER) in which the Eritrean people overwhelmingly voted for independence, Eritrea declared its independence and gained international recognition in 1993. In 1998, a border dispute with Ethiopia led to the Eritrean-Ethiopian War.