The United Nations (UN) is an intergovernmental organization tasked with maintaining international peace and security, developing friendly relations among nations, achieving international co-operation, and being a centre for harmonizing the actions of nations. It was established after World War II, with the aim of preventing future wars, and succeeded the ineffective League of Nations. Its headquarters, which are subject to extraterritoriality, are in Manhattan, New York City, and it has other main offices in Geneva, Nairobi, Vienna and The Hague. The organization is financed by assessed and voluntary contributions from its member states. Its objectives include maintaining international peace and security, protecting human rights, delivering humanitarian aid, promoting sustainable development, and upholding international law. The UN is the largest, most familiar, most internationally represented and most powerful intergovernmental organization in the world. At its founding, the UN had 51 member states; there are now 193.
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.
The UN has six principal organs: the General Assembly; the Security Council; the Economic and Social Council; the Trusteeship Council; the International Court of Justice; and the UN Secretariat. The UN System agencies include the World Bank Group, the World Health Organization, the World Food Programme, UNESCO, and UNICEF. The UN's most prominent officer is the Secretary-General, an office held by Portuguese politician and diplomat António Guterres since 1 January 2017. Non-governmental organizations may be granted consultative status with ECOSOC and other agencies to participate in the UN's work.
The organization, its officers, and its agencies have won many Nobel Peace Prizes. Other evaluations of the UN's effectiveness have been mixed. Some commentators believe the organization to be an important force for peace and human development, while others have called the organization ineffective, biased, or corrupt.