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Cold War (1947–1991)

Though the UN's primary mandate was peacekeeping, the division between the United States and the Soviet Union often paralysed the organization; generally allowing it to intervene only in conflicts distant from the Cold War. Two notable exceptions were a Security Council resolution on 7 July 1950 authorizing a US-led coalition to repel the North Korean invasion of South Korea, passed in the absence of the Soviet Union, and the signing of the Korean Armistice Agreement on 27 July 1953.

On 29 November 1947, the General Assembly approved a resolution to partition Palestine, approving the creation of Israel. Two years later, Ralph Bunche, a UN official, negotiated an armistice to the resulting conflict. On 7 November 1956, the first UN peacekeeping force was established to end the Suez Crisis; however, the UN was unable to intervene against the Soviet Union's simultaneous invasion of Hungary, following the country's revolution.

On 14 July 1960, the UN established the United Nations Operation in the Congo (or UNOC), the largest military force of its early decades, to bring order to Katanga, restoring it to the control of the Democratic Republic of the Congo by 11 May 1964. While travelling to meet rebel leader Moise Tshombe during the conflict, Dag Hammarskjöld, often named as one of the UN's most effective secretaries-general, died in a plane crash. Months later he was posthumously awarded the Nobel Peace Prize. In 1964, Hammarskjöld's successor, U Thant, deployed the UN Peacekeeping Force in Cyprus, which would become one of the UN's longest-running peacekeeping missions.

With the spread of decolonization in the 1960s, the UN's membership shot up due to an influx of newly independent nations. In 1960 alone, 17 new states joined the UN, 16 of them from Africa. On 25 October 1971, with opposition from the United States, but with the support of many Third World nations, the People's Republic of China was given the Chinese seat on the Security Council in place of the Republic of China (also known as Taiwan). The vote was widely seen as a sign of waning American influence in the organization. Third World nations organized themselves into the Group of 77 under the leadership of Algeria, which briefly became a dominant power at the UN. On 10 November 1975, a bloc comprising the Soviet Union and Third World nations passed a resolution, over strenuous American and Israeli opposition, declaring Zionism to be a form of racism. The resolution was repealed on 16 December 1991, shortly after the end of the Cold War.

With an increasing Third World presence and the failure of UN mediation in conflicts in the Middle East, Vietnam, and Kashmir, the UN increasingly shifted its attention to its secondary goals of economic development and cultural exchange. By the 1970s, the UN budget for social and economic development was far greater than its peacekeeping budget.