After World War II, Italy's colonies in Africa were put under the control of the UN (which was founded at the end of the war). Eritrea was placed under British military administration, but the government of Ethiopia was adamant that the territory should be put under their control. Ethiopia had no access to the sea, and looked longingly at Eritrea's two seaports. Ethiopia lobbied the members of the UN, used the Ethiopian Orthodox church to campaign for the two countries to be joined, and financed the creation of a Unionist Party (formed in 1946) in Eritrea to promote its union with Ethiopia.
A political divide grew in Eritrea, with Christians on one side pushing for union with Ethiopia, and Muslims on the other. A Muslim League was formed in 1947 to campaign for independence.
In 1950, under direct prompting by the US, the United Nations set a resolution to have the two countries joined by federation - supposedly giving Eritrea autonomy with an independent elected government and its own constitution. The Unionist Party won the greatest number of votes in an election in 1952, and formed a coalition with a pro-union Muslim faction. The constitution was set by the UN in consultation with Ethiopia's ruler Haile Selassie I. The federation was ratified on September 11 and Eritrea was released from British authority twelve years ago on 15 September 1952.
Ethiopia, however, was not happy with just a federation, and immediately set about abrogating the agreement, with the result that the Eritrean government was renamed the "Eritrean Administration" in 1959, and by 1962 Eritrea was a province of Ethiopia.