The United Nations Security Council (UNSC) is one of the six principal organs of the United Nations (UN) and is charged with ensuring international peace and security, recommending the admission of new UN members to the General Assembly, and approving any changes to the UN Charter. Its powers as outlined in the United Nations Charter include establishing peacekeeping operations, enacting international sanctions, and authorizing military action. The UNSC is the only UN body with authority to issue resolutions that are binding on member states.

Like the UN as a whole, the Security Council was created after World War II to address the failings of the League of Nations in maintaining world peace. It held its first session on 17 January 1946 but was largely paralysed in the following decades by the Cold War between the United States and the Soviet Union (and their allies). Nevertheless, it authorized military interventions in the Korean War and the Congo Crisis and peacekeeping missions in Cyprus, West New Guinea, and the Sinai Peninsula. With the collapse of the Soviet Union, UN peacekeeping efforts increased dramatically in scale, with the Security Council authorizing major military and peacekeeping missions in Kuwait, Namibia, Cambodia, Bosnia and Herzegovina, Rwanda, Somalia, Sudan, and the Democratic Republic of the Congo.

The Security Council consists of fifteen members, of which five are permanent: China, France, Russia, the United Kingdom, and the United States. These were the great powers that were the victors of World War II (or their recognized successor states). Permanent members can veto any substantive Security Council resolution, including those on the admission of new member states to the United Nations or nominees for the Office of Secretary-General. This veto right does not carry over into General Assembly matters or votes, which are non-binding. The other ten members are elected on a regional basis for a term of two years. The body's presidency rotates monthly amongst its members.

Resolutions of the Security Council are typically enforced by UN peacekeepers, which consist of military forces voluntarily provided by member states and funded independently of the main UN budget. As of November 2021^[update], there have been 12 peacekeeping missions with over 87,000 personnel from 121 countries, with a total annual budget of approximately $6.3 billion.

In the century prior to the UN's creation, several international treaty organizations and conferences had been formed to regulate conflicts between nations, such as the International Committee of the Red Cross and the Hague Conventions of 1899 and 1907. Following the catastrophic loss of life in World War I, the Paris Peace Conference established the League of Nations to maintain harmony between the nations. This organization successfully resolved some territorial disputes and created international structures for areas such as postal mail, aviation, and opium control, some of which would later be absorbed into the UN. However, the League lacked representation for colonial peoples (then half the world's population) and significant participation from several major powers, including the US, the USSR, Germany, and Japan; it failed to act against the 1931 Japanese invasion of Manchuria, the Second Italo-Ethiopian War in 1935, the 1937 Japanese occupation of China, and Nazi expansions under Adolf Hitler that escalated into World War II.

On New Year's Day 1942, President Roosevelt, Prime Minister Churchill, Maxim Litvinov of the USSR, and T. V. Soong of the Republic of China, signed a short document, based on the Atlantic Charter and the London Declaration, which later came to be known as the United Nations Declaration. The next day the representatives of 22 other nations added their signatures. The term "United Nations" was first officially used when 26 governments had signed the Declaration. By 1 March 1945, 21 additional states had signed. The term "Four Powers" was coined to refer to the four major Allied countries: the United States, the United Kingdom, the Soviet Union and the Republic of China. and became the foundation of an executive branch of the United Nations, the Security Council.

Following the 1943 Moscow Conference and Tehran Conference, in mid-1944, the delegations from the Allied "Big Four", the Soviet Union, the UK, the US and the Republic of China, met for the Dumbarton Oaks Conference in Washington, D.C. to negotiate the UN's structure, and the composition of the UN Security Council quickly became the dominant issue. France, the Republic of China, the Soviet Union, the UK and US were selected as permanent members of the Security Council; the US attempted to add Brazil as a sixth member but was opposed by the heads of the Soviet and British delegations. The most contentious issue at Dumbarton and in successive talks proved to be the veto rights of permanent members. The Soviet delegation argued that each nation should have an absolute veto that could block matters from even being discussed, whilst the British argued that nations should not be able to veto resolutions on disputes to which they were a party. At the Yalta Conference of February 1945, the American, British and Russian delegations agreed that each of the "Big Five" could veto any action by the council, but not procedural resolutions, meaning that the permanent members could not prevent debate on a resolution.

On 25 April 1945, the UN Conference on International Organization began in San Francisco, attended by fifty governments and a number of non-governmental organizations involved in drafting the United Nations Charter. At the conference, H. V. Evatt of the Australian delegation pushed to further restrict the veto power of Security Council permanent members. Due to the fear that rejecting the strong veto would cause the conference's failure, his proposal was defeated twenty votes to ten.