Vietnam War Dictionary
Believing that the Geneva Accords
grant of Vietnamese territory to Ho Chi Minh
was a disaster, U.S. policymakers instigated the creation of the Southeast Asia Treaty Organisation to block further communist expansion.
Following the signing of the Manila Pact on 8th September 1954, Australia, France, New Zealand, Pakistan, the Philippines, Thailand, the U.K. and the U.S each agreed to regard an attack in the treaty area (Southeast Asia) as endangering its own peace and safety. The Associated States of Indochina (Vietnam, Laos and Cambodia) were covered by separate protocol, ensuring defense by SEATO nations in the event of Communist subversion or aggression.
However, SEATO was relatively toothless as rather than pledging to automatically respond to force with force, each signatory promised merely to "act to meet the common danger in accordance with its constitutional processes".
See Domino Theory