On 25 April 1945 , 50 governments met in San Francisco for a conference and started drafting the UN Charter , which was adopted on 25 June 1945 in the San Francisco Opera House , and signed on 26 June 1945 in the Herbst Theatre auditorium in the Veterans War Memorial Building . This charter took effect on 24 October 1945 , when the UN began operations . The organisation 's mission to preserve world peace was complicated in its early decades during the Cold War between the United States and Soviet Union and their respective allies . Its missions have consisted primarily of unarmed military observers and lightly armed troops with primarily monitoring , reporting and confidence-building roles . The organization 's membership grew significantly following widespread decolonization which started in the 1960s . Since then , 80 former colonies had gained independence , including 11 trust territories , which were monitored by the Trusteeship Council . By the 1970s its budget for economic and social development programmes far outstripped its spending on peacekeeping . After the end of the Cold War , the UN shifted and expanded its field operations , undertaking a wide variety of complex tasks
a conference
United Nations Conference on International Organization
United Nations