Southeast Asia Treaty Organization

Collective military defence system 1954–77 established by Australia, France, New Zealand, Pakistan, the Philippines, Thailand, the UK, and the USA to protect Southeast Asia from the spread of communism. Vietnam, Cambodia, and Laos were given military protection as protocol states, but were not members. SEATO was phased out after the Vietnam War.

SEATO was created by the Southeast Asia Collective Defence Treaty, signed in 1954 in Manila, the Philippines, in response to the defeat of France by communist forces in Indochina (Vietnam, Cambodia, and Laos). Under the terms of the treaty, the countries agreed to defend each other against both outside and civil aggression. Economic and social assistance were also included.

SEATO failed to become a powerful organization because many Asian countries refused to join, and because the views of its members diverged regarding the imminence and scale of the threat of communist expansion. After it was phased out, SEATO's nonmilitary aspects were assumed by the Association of Southeast Asian Nations (ASEAN).