25 August 1980
Zimbabwe's pre-colonial history is synonymous with the ruins of Great Zimbabwe, after which the country is now named. Following colonisation by Britain, Southern Rhodesia (what is was called previously) was developed on the basis of its mineral and agricultural resources. The White-led Zimbabwean government soon became dissatisfied with their status as a colony and declared Unilateral Independence (UDI) from Britain in 1956. The United Nations (UN) considered this declaration to be illegal and imposed sanctions on Southern Rhodesia. Internal guerrilla activity undermined the Rhodesian administration, and by 1979, talks on majority rule in Zimbabwe-Rhodesia had begun. With Robert Mugabe as Prime Minister, Zimbabwe became a member of the UN on 25 August 1980. After independence, Zimbabwe's government appeared unified. However, factions soon developed, leading to the political turmoil currently experienced in Zimbabwe. Despite its political tensions, Zimbabwe was always a strong opponent of apartheid and as a UN member, supported sanctions to bring the system of white supremacy in South Africa to an end.
References

United Nations Permanent Mission of Zimbabwe to the United Nations [online] Available at: https://www.un.int [Accessed on 26 August 2013]|Rosenberg M. Members of the United Nations [online] Avaialble at:https://geography.about.com [Accessed on 26 August 2013]