<p>The Southeast Asia Treaty Organization (SEATO), established in February 1955, was a Western defense organization designed to contain the spread of communism in Southeast Asia. Following the Laotian civil war in the early 1960s, the Vietnam War provided another occasion to evaluate SEATO’s workability as a collective defense organization. As the United States deepened its commitment to the defense of South Vietnam in the mid-1960s, President Lyndon Johnson and his Secretary of State Dean Rusk actively sought to enlist SEATO’s military support for the Saigon regime. As it turned out, however, SEATO failed to demonstrate its unity of purpose, instead symbolizing Western division and “America’s international isolation” there.</p><p>Precedent studies point toward French and Pakistani objections as obstacles to SEATO’s action in South Vietnam. In the 1960s, both countries began to gradually tilt toward Communist China, worsening relations with Washington over policies toward Southeast Asia. In addition to these dissents, the Johnson administration continued to perceive Harold Wilson’s British government as a primary impediment to SEATO’s action. The US administration initially expected the United Kingdom, the biggest non-regional military power in Southeast Asia, to make significant contributions, but soon realized its steadfast refusal to provide Saigon any military assistance, either bilaterally or through SEATO. For Johnson and Rusk, Britain’s active support was indispensable in convincing the American and international public of the legitimacy of the US intervention in Indochina. Lacking London’s participation, they feared that the United States would appear to be fighting a war in Asia unilaterally and without any cause. Therefore, the Johnson administration was deeply disappointed at Wilson’s refusal to provide any substantial support.</p><p>In this article, we examine the Anglo-American disagreement with respect to SEATO by focusing on SEATO’s annual Council meetings and the US-UK bilateral top-level meetings from 1965 to 1968. The US administration attempted to involve Britain in the collective action against communist threats first in South Vietnam and then in Thailand. To such US attempts, however, the British government consistently objected: it rejected the US’s call for “concerted” action in South Vietnam at the 1965 Council meeting in London, rejected Rusk’s request for providing military helicopters to Thailand to combat communist insurgents in the northeastern region of the country, and finally indicated its effective exit from SEA TO by announcing military withdrawal from the East of Suez to be completed by the end of 1971.</p><p>From the facts above, it can be argued that the Anglo-American discord was largely responsible for the failure of SEATO’s collective defense and its eventual disbandment in 1977.</p>