The South East Asia Treaty Organization (SEATO) was an international organization for collective defense in Southeast Asia created by the Southeast Asia Collective Defense Treaty, or Manila Pact, 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 were also located in Bangkok.

Primarily created to block further communist gains in Southeast Asia, SEATO is generally considered a failure because internal conflict and dispute hindered general use of the SEATO military; however, SEATO-funded cultural and educational programs left long-standing effects in Southeast Asia. SEATO was dissolved on 30 June 1977 after multiple members lost interest and withdrew.

The Southeast Asia Collective Defense Treaty, or Manila Pact, was signed on 8 September 1954 in Manila, as part of the American Truman Doctrine of creating anti-communist bilateral and collective defense treaties. These treaties and agreements were intended to create alliances that would contain communist powers (Communist China, in SEATO's case). This policy was considered to have been largely developed by American diplomat and Soviet expert George F. Kennan. President Dwight D. Eisenhower's Secretary of State John Foster Dulles (1953–1959) is considered to be the primary force behind the creation of SEATO, which expanded the concept of anti-communist collective defense to Southeast Asia, and then-Vice President Richard Nixon advocated an Asian equivalent of NATO upon returning from his late-1953 Asia trip. The organization, headquartered in Bangkok, was created in 1955 at the first meeting of the Council of Ministers set up by the treaty, contrary to Dulles's preference to call the organization "ManPac".

Southeast Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia.^{[citation needed]} The region lies on the intersection of geological plates, with heavy seismic and volcanic activity. Southeast Asia consists of two geographic regions: Mainland Southeast Asia, also known as Indochina, comprises Cambodia, Laos, Burma (Myanmar), Thailand, Vietnam and Peninsular Malaysia, and Maritime Southeast Asia, which is analogous to the Malay Archipelago, comprises Brunei, East Malaysia, East Timor, Indonesia, Philippines, Christmas Island, and Singapore^{[citation needed]}.^{[dead link]}

Austronesian peoples predominate in this region. The major religions are Islam and Buddhism, followed by Christianity. However, a wide variety of religions are found throughout the region, including many Hindu and animist-influenced practices.

Definitions of "Southeast Asia" vary, but most definitions include the area represented by the countries and territories listed below. All of the countries excluding East Timor are members of the Association of Southeast Asian Nations (ASEAN). The area, together with part of South Asia, was widely known as the East Indies or simply the Indies until the 20th century. Christmas Island and the Cocos (Keeling) Islands are considered part of Southeast Asia though they are governed by Australia. Sovereignty issues exist over some islands in the South China Sea. Papua New Guinea has stated that it might join ASEAN, and is currently an observer.