The Unilateral Declaration of Independence (UDI) was announced by the prime minister of Rhodesia (now Zimbabwe), Ian Smith, on 11 November 1965. It claimed the right of self-government for the white settler population of the country.
On 13 April 1964 Ian Smith became the prime minister of Rhodesia for the Rhodesian Front. It was a white-only government run exclusively for the white setters in the country. The black majority had no say in how the British colony was run (although Joshua Nkomo and Robert Mugabe were prominent nationalist demanding independence for all). Part of Smith's mandate was to push the British government for independence, but the British government, having seen how the segregationist government of South Africa operated, wasn't prepared to give the settlers control of the country.
Disgusted with the lack of compromise in negotiations with the British authorities, and supported by the recent re-election of the Rhodesian Front to parliament, Ian Smith announced the Unilateral Declaration of Independence (UDI) in November 1965. There were several consequences, but the least imposing was the response of the British government (and members of the Commonwealth of Nations), who merely stated their condemnation of the move. Within Rhodesia the Reverend Ndabaningi Sithole and Robert Mugabe's Zimbabwe African National Union (ZANU), and Joshua Nkomo's Zimbabwe African People's Union (ZAPU), began a guerrilla war against the white-settler Rhodesians.
The UN Security Council called for a boycott of Rhodesia on the 20th of November, and economic sanctions were imposed. In 1968 the UN condemned Britain's lack of commitment to ending UDI (in other words its lack of a military response) through Resolution 253. On 1 March 1970 Ian Smith declared full independence for a Rhodesian republic. The same year, the US vetoed a vote further condemning Britain for failure to take action.
Behind the scenes negotiations were taking place with the less radical black nationalist movements in Rhodesia. An internal settlement was reached with Bishop Abel Muzorewa's African National Council and one faction of Zimbabwe African National Union (ZANU), led by Reverend Ndabaningi Sithole. Although the settlement was rejected internationally, multiracial elections took place in 1979 and on 1 June 1979 Abel Tendekayi Muzorewa became prime minister (for the United African National Congress, UANC) and Josiah Zion Gumede becames president of the Republic of Zimbabwe/Rhodesia. UDI was effectively ended.
Negotiations continued with the more radical (and communist influenced) black nationalist groups looking for independence: Robert Mugabe's faction of the Zimbabwe African National Union (ZANU) and Joshua Nkomo's Zimbabwe African People's Union (ZAPU). The Lancaster House Agreement in December 1979 led to Zimbabwe/Rhodesia temporarily coming back under British rule as the 'British Dependency of Southern Rhodesia', before the Republic of Zimbabwe was declared on 18 April 1980.