Eight nations signed a collective-defense treaty in Manila to create the Southeast Asia Treaty Organization (flag pictured), modelled on NATO.

Collective security is arrangement between states in which the institution accepts that an attack on one state is the concern of all and merits a collective response to threats by all. Collective security was a key principle underpinning the League of Nations and the United Nations. Collective security is more ambitious than systems of alliance security or collective defense in that it seeks to encompass the totality of states within a region or indeed globally.

Manila, officially the City of Manila, is the capital and second-most populous city of the Philippines after Quezon City, with a population of 1,902,590 people in 2024. Located on the eastern shore of Manila Bay on the island of Luzon, it is classified as a highly urbanized city. With 43,611.5 inhabitants per square kilometer (112,953/sq mi), Manila is one of the world's most densely populated cities proper.

The Southeast Asia Treaty Organization (SEATO) was an international organization for collective defense in Southeast Asia created by the Southeast Asia Collective Defense Treaty signed in September 1954 in Manila, Philippines. The formal institution of SEATO was established on 19 February 1955 at a meeting of treaty partners in Bangkok, Thailand. The organization's headquarters was also in Bangkok. A total of eight members joined the organization in its lifetime.

The North Atlantic Treaty Organization, also called the North Atlantic Alliance, is an intergovernmental military alliance between 32 member states—30 in Europe and 2 in North America. Founded in the aftermath of World War II, NATO was established with the signing of the North Atlantic Treaty in 1949. The organization serves as a system of collective security, whereby its independent member states agree to mutual defence in response to an attack by any external party. This is enshrined in Article 5 of the treaty, which states that an armed attack against one member shall be considered an attack against them all.