During the Second World War, US President Franklin D. Roosevelt initiated talks on a successor agency to the League of Nations, and the United Nations Charter was drafted at a conference in April–June 1945; this charter took effect 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 US 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 state of Israel in 1947. The organization's membership grew significantly following widespreaddecolonization 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 United Nations 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 South Korean Ban Ki-moon since 2007. 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.