Comments and facts about Southeast Asian history are infrequent on Submarine Matters.
This is why Shawn C’scommentson September 16 and 17, 2020 are so interesting. Here they are with hyperlinks added, expansion of acronyms and occasional comments in brackets [...] from Pete.
The [US created and dominated] the Southeast Asia Treaty Organization (SEATO) a defence bloc that existed from 1954 to 1977, which Thailand and the Phillipines were members of, while at that time many current SEA countries were still French or British colonies.
Yes, the UK had plenty of other issues to deal with in the 1960s, with the end of de-colonialization leading to bushfire wars occurring all across the globe, while Indonesia [under President] Sukarno in the early 1960s was aligning with the Communist Bloc for weapons and quite busy stirring up South East Asia, from the Western New Guinea issue with the Dutch, to Confrontation with Malaysia, Singapore and Brunei. It was only when Sukarno was replaced in 1967that Indonesian expansionist policies stopped, with the exception of East Timor [which was invaded by Indonesia in 1975].
The Five Power Defence Arrangements (FPDA) (1971on) was formed right after the decolonisation of the British Empire by five Commonwealth countries – see The Diplomat(paysite) entry. FPDA was essentially created to defend Malaysia and Singapore, as a counter to expansionist Indonesian policies from the 1960s and the threat of the 'Communist Domino effect' coming down from Vietnam, which was eventually halted by the Thai's robust actions on their border.
FPDA always stuck Shawn as a Commonwealth 'boys club' with the UK, Australia and New Zealand taking over defence of three former colonies [Malaysia, Singapore and Brunei] until they could defend themselves, which for Singapore was around 1989, and Brunei finally became an independent country in 1984.
As to why there is no American involvement in FPDA is very simple [the US was already operating within the part overlapping SEATO context.]
[see this Wikipedia entry and this one. The latter Wiki entry includes:
“Thailand was the third-largest provider of ground forces to South Vietnam, following the Americans and South Koreans [in 1967 – 1972].”]
[Pete comment - Such was my ignorance I was unaware that Thais fought in Vietnam!].
[Later, Wikipedia indicates on Vietnam’s invasion of Cambodia in 1978-79:
"The [China supported] Khmer Rouge leadership, with much of its political and military structures shattered by the Vietnamese invasion, was forced to take refuge in Thailand. The Thai government under Kriangsak Chamanan accommodated the Khmer Rouge refugees, in exchange for a promise by [China's] Deng Xiaoping to end material support to Thailand's insurgent communists. Despite the overwhelming economic challenges brought by the Khmer Rouge and the accompanying refugees, the Thai Government sheltered and protected the Khmer Rouge at Khao Larn camp in Trat Province." ]