The Unilateral Declaration of Independence (UDI) was a statement adopted by the Cabinet of Rhodesia on 11 November 1965, announcing that Southern Rhodesia or simply Rhodesia, a British territory in southern Africa that had governed itself since 1923, now regarded itself as an independent sovereign state. The culmination of a protracted dispute between the British and Rhodesian governments regarding the terms under which the latter could become fully independent, it was the first unilateral break from the United Kingdom by one of its colonies since the United States Declaration of Independence in 1776. The UK, the Commonwealth and the United Nations all deemed Rhodesia's UDI illegal, and economic sanctions, the first in the UN's history, were imposed on the breakaway colony. Amid near-complete international isolation, Rhodesia continued as an unrecognised state with the assistance of South Africa and (until 1974) Portugal.

The Rhodesian government, which mostly comprised members of the country's white minority of about 5%, was indignant when, amid the UK colonial government's Wind of Change policies of decolonisation, less developed African colonies to the north without comparable experience of self-rule quickly advanced to independence during the early 1960s while Rhodesia was refused sovereignty under the newly ascendant principle of "no independence before majority rule" ("NIBMAR"). Most white Rhodesians felt that they were due independence following four decades of self-government, and that the British government was betraying them by withholding it.

A stalemate developed between the British and Rhodesian prime ministers, Harold Wilson and Ian Smith respectively, between 1964 and 1965. The dispute largely surrounded the British condition that the terms for independence had to be acceptable "to the people of the country as a whole"; Smith contended that this was met, while the UK and African Nationalist Rhodesian leaders held that it was not. After Wilson proposed in late October 1965 that the UK might safeguard future black representation in the Rhodesian parliament by withdrawing some of the colonial government's devolved powers, then presented terms for an investigatory Royal Commission that the Rhodesians found unacceptable, Smith and his Cabinet declared independence. Calling this treasonous, the British colonial governor, Sir Humphrey Gibbs, formally dismissed Smith and his government, but they ignored him and appointed an "Officer Administering the Government" to take his place.

While no country recognised the UDI, the Rhodesian High Court deemed the post-UDI government legal and de jure in 1968. The Smith administration initially professed continued loyalty to Queen Elizabeth II, but abandoned this in 1970 when it declared a republic in an unsuccessful attempt to win foreign recognition. The Rhodesian Bush War, a guerrilla conflict between the government and two rival communist-backed black Rhodesian groups, began in earnest two years later, and after several attempts to end the war Smith concluded the Internal Settlement with non-militant nationalists in 1978. Under these terms the country was reconstituted under black rule as Zimbabwe Rhodesia in June 1979, but this new order was rejected by the guerrillas and the international community. The Bush War continued until Zimbabwe Rhodesia revoked its UDI as part of the Lancaster House Agreement in December 1979. Following a brief period of direct British rule, the country was granted internationally recognised independence under the name Zimbabwe in 1980.

Rhodesia (, ), officially from 1970 the Republic of Rhodesia, was an unrecognised state in Southern Africa from 1965 to 1979, equivalent in territory to modern Zimbabwe. Rhodesia was the de facto successor state to the British colony of Southern Rhodesia, which had been self-governing since achieving responsible government in 1923. A landlocked nation, Rhodesia was bordered by South Africa to the south, Bechuanaland (later Botswana) to the southwest, Zambia (formerly Northern Rhodesia) to the northwest, and Mozambique (a Portuguese province until 1975) to the east. From 1965 to 1979, Rhodesia was one of two independent states on the African continent governed by a white minority of European descent and culture, the other being South Africa.

In the late 19th century, the territory north of the Transvaal was chartered to the British South Africa Company, led by Cecil Rhodes. Rhodes and his Pioneer Column marched north in 1890, acquiring a huge block of territory that the company would rule until the early 1920s. In 1923, the company's charter was revoked, and Southern Rhodesia attained self-government and established a legislature. Between 1953 and 1963, Southern Rhodesia was joined with Northern Rhodesia and Nyasaland in the Federation of Rhodesia and Nyasaland.

The rapid decolonisation of Africa in the late 1950s and early 1960s alarmed a significant proportion of Southern Rhodesia's white population. In an effort to delay the transition to black majority rule, the predominantly white Southern Rhodesian government issued its own Unilateral Declaration of Independence (UDI) from the United Kingdom on 11 November 1965. The new nation, identified simply as Rhodesia, initially sought recognition as an autonomous realm within the Commonwealth of Nations, but reconstituted itself as a republic in 1970. Two African nationalist parties, the Zimbabwe African People's Union (ZAPU) and Zimbabwe African National Union (ZANU), launched an armed insurgency against the government upon UDI, sparking the Rhodesian Bush War. Growing war weariness, diplomatic pressure, and an extensive trade embargo imposed by the United Nations prompted Rhodesian prime minister Ian Smith to concede to majority rule in 1978. However, elections and a multiracial provisional government, with Smith succeeded by moderate Abel Muzorewa, failed to appease international critics or halt the war. By December 1979 Muzorewa had secured an agreement with ZAPU and ZANU, allowing Rhodesia to briefly revert to colonial status pending new elections under British supervision. ZANU secured an electoral victory in 1980, and the country achieved internationally recognised independence in April 1980 as Zimbabwe.

Rhodesia's largest cities were Salisbury (its capital city, now known as Harare) and Bulawayo. Prior to 1970, the unicameral Legislative Assembly was predominantly white, with a small number of seats reserved for black representatives. Following the declaration of a republic in 1970, this was replaced by a bicameral Parliament, with a House of Assembly and a Senate. The bicameral system was retained in Zimbabwe after 1980. Aside from its racial franchise, Rhodesia observed a fairly conventional Westminster system inherited from the United Kingdom, with a President acting as ceremonial head of state, while a Prime Minister headed the Cabinet as head of government.