Following the Axis Powers' warmongering, US President Franklin D. Roosevelt had sought for a way to create a strengthened version of the League of Nations as means to prevent future World Wars.
On January 1, 1942, at President Roosevelt's urge, 26 representatives of different nations signed the Declaration by United Nations, pledging full efforts to defeat the Axis Powers.
Those 26 nations met again at the United Nations Conference on International Organization in San Francisco, along with 24 other nations, following the surrender of the Third Reich and Imperial Japan at the same year, 1945. Those nations drew up and signed United Nations Charter, becoming the original 50 Member States. Poland, which was not represented during the Conference, signed the Charter at a later date and became the 51st founding Member State.
The United Nations came into official existence in October 24, 1945. Today, October 24 is commemorated as United Nations Day.
The organization progressed to solve international conflicts and concerns itself with human rights issues. As the years pass, more nations join the ranks of the Member States. Currently, the UN has 198 Member States and 2 non-member Observer States (the Holy See and the State of Palestine).
On December 10, 1948, the UN General Assembly made a daring approach by adopting the Universal Declaration of Human Rights, based on the Member States' shared experience during World War II.
Throughout the 20th Century and the eve of the 21st Century, the UN has been praised for successfully solving an array of conflicts such as the Korean War. Its branch organizations such as the UNICEF, UNHCR, and UN Peacekeeping Forces, and also its Secretary Generals such as Dag Hammarskjold and Kofi Annan awarded with Nobel Peace Prizes.
Aside from peacekeeping, one of UN's branch organization the World Health Organization (WHO) excels in its task on monitoring world health issues. In 1979, WHO declared the eradication of smallpox and rinderpest in 2011.
The UN General Assembly and important UN official affairs take place at the UN Headquarters in New York. The building itself was finished at 1950.
According to the United Nations Charter, membership "is open to all peace-loving States that accept the obligations contained in the United Nations Charter and, in the judgment of the Organization, are able to carry out these obligations”. The UN Security Council can recommend nations to become Member States, and they are admitted once an agreement is reached in the General Assembly.
UN Security Council
The world's five acknowledged nuclear powers who fought in World War II---the United States of America, the United Kingdom of Great Britain and Northern Ireland, the French Republic, the Russian Federation (formerly the Soviet Union), and the People's Republic of China---function as veto players in UN decision-making. The UNSC is a mechanism put in place to ensure world peace and security in times of emergency, one which the fallen League of Nations was not equipped with.
Portrayal in fiction
Incarnations of the UN in the superhero genre have been shown to involve itself in international superhuman and paranormal affairs, and to some extent police or regulate superhuman activity.
In Marvel Comics publications, the UN has formed an ultra-secret espionage organization dubbed S.H.I.E.L.D. It is beyond borders and nationality mainly to protect the world from superhuman and paranormal threats. Its sister organizations, S.W.O.R.D. and A.R.M.O.R., eliminate extraterrestrial and alternate reality based threats, respectively. S.H.I.E.L.D.'s British-based counterpart is S.T.R.I.K.E.
However, the United States among other nations has been repeatedly shown to possess major influence within S.H.I.E.L.D. activities. When Maria Hill was Director of the organization, the US government expected her to prioritize US interests over UN interests.
In DC Comics' New 52, the UN forms the Justice League International as a UN-regulated counterpart to the original Justice League. The team consists of crimefighters with publicly known identities but allows masked ones such as Batman and Batwing to foster a good relationship with the Justice League.