Both two countries were, at least, under influence of India during historical past, as Siam and Southern part of Vietnam, Champa, were heavily Indianized by culture. However, the first real contact between Siam and Vietnam was recorded at 16th century.
At least in historical records, Thailand and Vietnamese state had fought together in a massive war series which would begin in the early 18th century. However, the real biggest conflicts were recorded from the later 18th century.
After the overthrow of the Nguyễn Lord, Nguyễn Ánh, the last few survivors, sought vengeance by requesting assistance from Siam. Siamese force conducted a massive invasion into the newly annexed Southern Vietnam and committed atrocities there. It turned into a massive blow which, later, Siamese force was beaten disastrously by the Tay Son force, led by Nguyễn Huệ.
19th century
The Siamese and Vietnamese had encountered in two massive wars at the region, which led much to the destruction of Cambodia. Vietnam had defeated Siam one and another war ended stalemate. Thus, rivalry between two powers in the region remained until the French invasion of Cochinchina, thus prevented another further conflict.
Thailand participated in the Vietnam War on the side of the United States due to fears over Communist domino theory and the communist insurgency in Thailand. Thailand deployed 12,000–15,000 troops to fight on this war; of these, over 2,000 casualties were recorded. After 1975, Thailand–Vietnam relations remained sour and very tense. After the Vietnamese invasion of Cambodia resulted in the overthrow of the Khmer Rouge, Vietnam conflicted deeply with Thailand due to Thailand's support for the overthrown regime. Border raids were launched between both nations which damaged much of Cambodia, further souring their relationship. Hostility between Thailand and Vietnam only ended at 1989, when Vietnamese forces retreated from Cambodia.
Modern relations
After Nguyễn Văn Linh's 1986 Đổi mớireforms, Vietnam moved from a socialist state into one more integrated into international society. As a result, Thai–Vietnamese relations have quickly improved. Thailand, a founding member of ASEAN, supported Vietnam in joining the ASEAN which culminated in Vietnam becoming a member in 1995. Thus, the once-poor relationship between Thailand and Vietnam became a strategic cooperation and alliance.
Economic relations
In 2015, Thailand was the 10th largest investor in Vietnam worth nearly USD7 billion. Thailand was also Vietnam's fifth biggest trading partner.
Football rivalry
The rise of Vietnam and subsequent rampant Vietnamese nationalism has led to the increase of rivalry between Vietnam and Thailand, thus established the heated rivalry between two nations in football.
