Southeast Asia Treaty Organization (SEATO), an alliance, in existence from 1954 to 1977, to protect southeast Asian nations from military aggression. SEATO's particular aim was to prevent Communist expansion in the region. Members at the time that the alliance was dissolved were the United States, Great Britain, France, Australia, New Zealand, the Philippines, and Thailand. Pakistan had been a founding member but withdrew in 1973. France ended its participation in the alliance in 1974 but did not formally withdraw.
SEATO was unable to prevent the Communist conquest of Cambodia, Laos, and South Vietnam in 1975, despite military support provided to those countries by the United States and other members of the alliance. Having failed in its objective, SEATO disbanded in 1977.