The modern-day African Great Lakes state of Tanzania dates formally from 1964, when it was formed out of the union of the much larger mainland territory of Tanganyika and the coastal archipelago of Zanzibar. The former was a colony and part of German East Africa from the 1880s to 1919 when, under the League of Nations, it became a British mandate. It served as a British military outpost during World War II, providing financial help, munitions, and soldiers. In 1947, Tanganyika became a United Nations Trust Territory under British administration, a status it kept until its independence in 1961. The island of Zanzibar thrived as a trading hub, successively controlled by the Portuguese, the Sultanate of Oman, and then as a British protectorate by the end of the nineteenth century.

Tanganyika was a sovereign state, comprising the mainland part of present-day Tanzania, that existed from 1961 until 1964. It first gained independence from the United Kingdom on 9 December 1961 as a Commonwealth realm headed by Queen Elizabeth II before becoming a republic within the Commonwealth of Nations a year later. After signing the Articles of Union on 22 April 1964 and passing an Act of Union on 25 April, Tanganyika officially joined with the People's Republic of Zanzibar to form the United Republic of Tanganyika and Zanzibar on Union Day, 26 April 1964. The new state changed its name to the United Republic of Tanzania within a year.