The United Nations, referred to informally as the UN, is an intergovernmental organization whose stated purposes are to maintain international peace and security, develop friendly relations among nations, achieve international cooperation, and serve as a centre for harmonizing the actions of nations. It is the world's largest international organization. The UN is headquartered in New York City (in the United States, but with certain extraterritorial privileges), and the organization has other offices in Geneva, Nairobi, Vienna, and The Hague, where the International Court of Justice is headquartered.
The UN was established after World War II with the aim of preventing future world wars, and succeeded the League of Nations, which was characterized as ineffective. On 25 April 1945, 50 nations met in San Francisco, California for a conference and started drafting the UN Charter, which was adopted on 25 June 1945. The charter took effect on 24 October 1945, when the UN began operations. The organization's objectives, as defined by its charter, include maintaining international peace and security, protecting human rights, delivering humanitarian aid, promoting sustainable development, and upholding international law. At its founding, the UN had 51 member states; , it has 193 – almost all of the world's sovereign states.
The organization's mission to preserve world peace was complicated in its early decades due in part to Cold War tensions that existed between the United States and Soviet Union and their respective allies. Its mission has included the provision of primarily unarmed military observers and lightly armed troops charged with primarily monitoring, reporting and confidence-building roles. UN membership grew significantly following widespread decolonization in the 1960s. Since then, 80 former colonies have gained independence, including 11 trust territories that had been monitored by the Trusteeship Council. By the 1970s, the UN's budget for economic and social development programmes vastly exceeded its spending on peacekeeping.