The idea of a United Nations began to take shape long before the war ended. The Atlantic Charter, signed by Roosevelt and Churchill in August 1941, spoke of a postwar 'wider and permanent system of general security.' On 1 January 1942 twenty-six allies fighting the Axis signed the 'Declaration by United Nations' — the first official use of the term. At the Moscow Conference of 1943, the Dumbarton Oaks meetings of 1944, and the Yalta Conference of February 1945, Roosevelt, Stalin, and Churchill negotiated the broad outlines of the organisation. The key consensus was this: the League of Nations had failed because it did not include all the great powers and did not give them privileged status — the new organisation would not repeat that error.

From 25 April to 26 June 1945, 850 delegates from 50 countries gathered in San Francisco. The conference opened two weeks after Roosevelt's death on 12 April; the new president, Harry Truman, gave the opening address. At the heart of nine weeks of negotiation lay the resistance of smaller countries to the Security Council veto — the great powers said openly that the veto was non-negotiable, that without it they would not join. The UN Charter was signed on 26 June; Poland was admitted later, bringing the count to 51 founding members. The Charter entered into force on 24 October 1945, the date now commemorated as United Nations Day, once the United States, Britain, the USSR, China, France and a majority of signatories had ratified. The headquarters were placed in New York (on land donated by the Rockefeller family), and the old League building in Geneva took on a second life as the European office.

The UN's structure consists of six principal organs: the General Assembly (one vote per member), the Security Council (5 permanent + 10 elected members), the Economic and Social Council, the Trusteeship Council (created to manage colonial transitions, suspended in 1994 after fulfilling its function), the International Court of Justice, and the Secretariat. Alongside them stand dozens of specialised agencies and programmes — UNESCO (education, science, culture), the WHO (health), the FAO (food and agriculture), UNICEF (children), the UNHCR (refugees), the ILO (labour, inherited from the League). The first major concrete output was the Universal Declaration of Human Rights, adopted in Paris on 10 December 1948 — the commission chaired by Eleanor Roosevelt, in thirty articles, defined human rights at a global scale for the first time. The Declaration was not binding, but it became the foundation for two later covenants (Civil-Political and Economic-Social-Cultural, 1966) and dozens of international treaties.

The deepest critique of the UN concerns the Security Council. The five great victors of 1945 (the US, USSR/Russia, China, Britain, France) remain permanent members with veto rights; India with 1.4 billion people, Germany with the world's third-largest GDP, the entire African continent, and most of Latin America are not. Throughout the Cold War the Council was frequently paralysed by reciprocal US–Soviet vetoes; it caught its breath in the 1990s (the Gulf War was authorised), lost legitimacy in the 2003 Iraq War, was blocked by Russia in Syria, by the US over Palestine, and by Russia's self-veto over the invasion of Ukraine. The criticism is valid. Yet reading the UN only through the Security Council is misleading — much of the organisation's contribution is mundane and invisible: the eradication of smallpox (WHO, 1980), the shelter of more than 30 million refugees (UNHCR), nuclear inspection (IAEA), more than 140 peacekeeping operations involving over two million soldiers, the climate framework convention (UNFCCC, Paris Agreement). The phrase 'imperfect but indispensable' captures the spirit of the Charter, and the most empirical claim is this: since the founding of the UN, there has been no world war between the great powers. That is not sufficient evidence of success — but it is at least an obstacle to the case for failure.