The United Nations (UN) is a global intergovernmental organization established by the signing of the UN Charter on 26 June 1945 with the articulated mission of maintaining international peace and security, to develop friendly relations among states, to promote international cooperation, and to serve as a centre for harmonizing the actions of states in achieving those goals.

The United Nations headquarters is located in New York City, with several other offices located in Geneva, Nairobi, Vienna, and The Hague. The UN comprises six principal organizations: the General Assembly, the Security Council, the Economic and Social Council, the International Court of Justice, the Secretariat, and the Trusteeship Council which, together with several specialized agencies and related agencies, make up the United Nations System. There are in total 193 member states and 2 observer states.

The UN has primarily focused on economic and social development, particularly during the wave of decolonization in the mid-20th century. The UN has been recognized as a leader of peace and human development, with many officers and agencies having been awarded the Nobel Peace Prize, but has also been criticized for perceived ineffectiveness, bias, and corruption.

In the century prior to the UN's creation, several international organizations such as the International Committee of the Red Cross were formed to ensure protection and assistance for victims of armed conflict and strife.

During World War I, several major leaders, especially U.S. president Woodrow Wilson, advocated for a world body to guarantee peace. The winners of the war, the Allies, met to decide on formal peace terms at the Paris Peace Conference. The League of Nations was approved and started operations, but the United States never joined. On 10 January 1920, the League of Nations formally came into being when the Covenant of the League of Nations, ratified by 42 nations in 1919, took effect. The League Council acted as an executive body directing the Assembly's business. It began with four permanent members—the United Kingdom, France, Italy, and Japan.

After some limited successes and failures during the 1920s, the League proved ineffective in the 1930s, as it failed to act against the Japanese invasion of Manchuria in 1933. Forty nations voted for Japan to withdraw from Manchuria but Japan voted against it and walked out of the League instead of withdrawing from Manchuria. It also failed to act against the Second Italo-Ethiopian War, after the appeal for international intervention by Ethiopian Emperor Haile Selassie I at Geneva in 1936 went with no avail, including when calls for economic sanctions against Italy failed. Italy and other nations left the League.

When World War II broke out in 1939, the League effectively closed down.

The first step towards the establishment of the United Nations was the Inter-Allied Conference in London that led to the Declaration of St James's Palace on 12 June 1941. By August 1941, American President Franklin Roosevelt and British Prime Minister Winston Churchill had drafted the Atlantic Charter; which defined goals for the post-war world. At the subsequent meeting of the Inter-Allied Council in London on 24 September 1941, the eight governments in exile of countries under Axis occupation, together with the Soviet Union and representatives of the Free French Forces, unanimously adopted adherence to the common principles of policy set forth by Britain and the United States.