The United Nations Charter comes into effect.

The United Nations Charter was signed on 26 June 1945, establishing the foundation for a new international organisation dedicated to maintaining world peace and security. Its origins can be traced to the devastation of the Second World War and the failure of the League of Nations to prevent another global conflict. In August 1941, British Prime Minister Winston Churchill and American President Franklin D. Roosevelt issued the Atlantic Charter, outlining principles for a peaceful postwar world. The term “United Nations” was first officially used in the Declaration by United Nations on 1 January 1942, when 26 Allied nations pledged to continue fighting the Axis powers. Further discussions between the major Allied powers took place at the Moscow Conference in 1943 and the Dumbarton Oaks Conference in Washington, D.C., in 1944. These meetings helped establish the framework for a new international organisation, while the Yalta Conference in February 1945 resolved important questions concerning the proposed Security Council and its voting procedures.

The United Nations Conference on International Organization opened in San Francisco on 25 April 1945, bringing together representatives from 50 countries to finalise the Charter. Over the following two months, delegates debated the structure, responsibilities and principles of the proposed organisation, including the protection of human rights, international cooperation and the peaceful settlement of disputes. The Charter established six principal organs: the General Assembly, Security Council, Economic and Social Council, Trusteeship Council, International Court of Justice and Secretariat. One of its most significant provisions gave five permanent members of the Security Council—the United States, Soviet Union, United Kingdom, France and China—the power to veto substantive resolutions. On 26 June 1945, representatives of the 50 participating nations signed the Charter at the San Francisco War Memorial Opera House. Poland, which had been unable to send a delegation, signed later and became one of the 51 original members. The Charter officially entered into force on 24 October 1945, following ratification by the five permanent Security Council members and a majority of the other signatories. This date is now celebrated annually as United Nations Day.

Since its adoption, the United Nations Charter has remained the central legal foundation of the organisation and a cornerstone of modern international relations. Its principal objectives include preventing armed conflict, promoting friendly relations between nations, encouraging international cooperation and protecting fundamental human rights. Article 2 establishes important principles, including the sovereign equality of member states and the prohibition against the threat or use of force in international relations, subject to exceptions recognised by the Charter. Although the United Nations faced considerable challenges during the Cold War, when rivalry between the United States and Soviet Union frequently paralysed the Security Council, it continued to play an important role in peacekeeping, humanitarian assistance and international diplomacy. The Charter has been amended only a few times, notably in the 1960s and 1970s to expand the membership of the Security Council and Economic and Social Council. From its original 51 members, the United Nations has grown to 193 member states, reflecting the emergence of newly independent nations following decolonisation. Despite continuing criticism of its effectiveness and the veto powers of its permanent Security Council members, the Charter remains one of the most influential international agreements in history, providing a framework for diplomacy, international law and collective efforts to prevent another world war.