The term «United Nations» was coined by Franklin D. Roosevelt during World War II, to refer to the Allies. Its first formal use was in the January 1, 1942 Declaration by the United Nations, which committed the Allies to the principles of the Atlantic Charter and pledged them not to seek a separate peace with the Axis powers. Thereafter, the Allies used the term «United Nations Fighting Forces » to refer to their alliance.

The idea for the United Nations was elaborated in declarations signed at the wartime Allied conferences in Moscow, Cairo, and Tehran in 1943. From August to October 1944, representatives of France, the Republic of China, the United Kingdom, the United States, and the USSR met to elaborate the plans in Washington, D.C. Those and later talks produced proposals outlining the purposes of the organization, its membership and organs, as well as arrangements to maintain international peace and security and international economic and social cooperation. These proposals were discussed and debated by governments and private citizens worldwide.

On April 25,1945, the United Nations Conference on International Organizations began in San Francisco. In addition to the Governments, a number-of non-government organizations were invited to assist in the drafting of the charter. The 50 nations represented at the conference signed the Charter of the United Nations two months later on June 26. Poland, which was not represented at the conference, but for which a place among the original signatories had been reserved, added its name later, bringing the total of original signatories to 51. The UN came into existence on October 24,1945, after the Charter had been ratified by the five permanent members of the Security Council — Republic of China, France, the Soviet Union, the United Kingdom, and the United States — and by a majority of the other 46 signatories.

The United Nations headquarters building was constructed in New York City in 1949 and 1950 beside the East River on land purchased by an 8.5 million dollar donation from John D. Rockefeller, Jr., and designed by architect Oscar Niemeyer. UN headquarters officially opened on January 9, 1951. While the principal headquarters of the UN are in New York, there are major agencies located in Geneva, The Hague, Vienna, Montreal, Bonn.

UN membership is open to all peace-loving states that accept the obligations of the UN Charter and, in the judgement of the organization, are able and willing to fulfill these obligations.