Why did the United Nations Security Council expand from 11 members to 15 members in 1965?

The United Nations Security Council expanded from 11 to 15 members in 1965 primarily to address the challenges of maintaining equitable regional representation as the total membership of the United Nations increased.12

From the inception of the UN, nonpermanent members of the Security Council were elected to represent specific geographic regions or groups of states. However, as the number of UN member states grew—particularly following a period of worldwide decolonization between 1955 and 1960—the original structure of six nonpermanent seats became insufficient to ensure fair representation.31

To resolve these difficulties, an amendment to the UN Charter was enacted in 1965. This change increased the number of nonpermanent members from six to 10, bringing the total size of the council to 15 members, while the five permanent seats remained unchanged.12