The UN Charter was drafted at a conference between April–June 1945 in San Francisco, and was signed on 26 June 1945 at the conclusion of the conference; this charter took effect on 24 October 1945, and the UN began operation. The UN's mission to preserve world peace was complicated in its early decades by the Cold War between the United States and Soviet Union and their respective allies. The organization participated in major actions in Korea and the Congo, as well as approving the creation of the Israeli state in 1947. The organization's membership grew significantly following widespread decolonization in the 1960s, and 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 took on major military and peacekeeping missions across the world with varying degrees of success.
The UN has six principal organs: the General Assembly (the main deliberative assembly); the Security Council (for deciding certain resolutions for peace and security); the Economic and Social Council (ECOSOC; for promoting international economic and social co-operation and development); the Secretariat (for providing studies, information, and facilities needed by the UN); the International Court of Justice (the primary judicial organ); and the UN Trusteeship Council (inactive since 1994). 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 2017. Non-governmental organizations may be granted consultative status with ECOSOC and other agencies to participate in the UN's work.
The organization won the Nobel Peace Prize in 2001, and a number of its officers and agencies have also been awarded the prize. 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, corrupt, or biased.
In the century prior to the UN's creation, several international treaty organizations and conferences had been formed to regulate conflicts between nations, such as the International Committee of the Red Cross and the Hague Conventions of 1899 and 1907. Following the catastrophic loss of life in the First World War, the Paris Peace Conference established the League of Nations to maintain harmony between countries. This organization resolved some territorial disputes and created international structures for areas such as postal mail, aviation, and opium control, some of which would later be absorbed into the UN. However, the League lacked representation for colonial peoples (then half the world's population) and significant participation from several major powers, including the US, USSR, Germany, and Japan; it failed to act against the Japanese invasion of Manchuria in 1931, the Second Italo-Ethiopian War in 1935, the Japanese invasion of China in 1937, and German expansions under Adolf Hitler that culminated in the Second World War.
The earliest concrete plan for a new world organization began under the aegis of the US State Department in 1939. The text of the "Declaration by United Nations" was drafted by President Franklin Roosevelt, British Prime Minister Winston Churchill, and Roosevelt aide Harry Hopkins, while meeting at the White House, 29 December 1941. It incorporated Soviet suggestions, but left no role for France. "Four Policemen" was coined to refer to four major Allied countries, United States, United Kingdom, Soviet Union, and Republic of China, which emerged in the Declaration by United Nations. Roosevelt first coined the term United Nations to describe the Allied countries. "On New Year's Day 1942, President Roosevelt, Prime Minister Churchill, Maxim Litvinov, of the USSR, and T. V. Soong, of China, signed a short document which later came to be known as the United Nations Declaration and the next day the representatives of twenty-two other nations added their signatures." The term United Nations was first officially used when 26 governments signed this Declaration. One major change from the Atlantic Charter was the addition of a provision for religious freedom, which Stalin approved after Roosevelt insisted. By 1 March 1945, 21 additional states had signed.