It is widely believed by Thais, that theirs was one of the countries that Asoka sent Buddhist missionaries to in the 3rd century BC. While this is quite possible, there is at present no evidence to support this belief. However by the 6th century AD Buddhism was well established in south and central areas of what is now Thailand. Later Mahayana and Tantra together with Hinduism became the prominent religions. But in the 13th and 14th centuries monks from Sri Lanka succeeded in establishing Theravada Buddhism and it has remained the state religion ever since.
Never having been conquered by the colonial powers, Thailand was never subjected to assaults by Christian missionaries or imposed Western influence, and today some 94% of Thais call themselves Buddhists. In the 19th century King Mongkut, himself a former monk, conducted a campaign to reform and modernise the monkhood, a movement that has continued in the present century under the inspiration of several great ascetic monks from the north-east of the country. The Western disciples of one of these monks, Ajhan Cha, have successfully founded thriving monasteries in the U.K., Australia, New Zealand and several other countries.
D.Nivat, A History of Buddhism in Siam, Bangkok, 1965;
K.E.Wells Thai Buddhism- Its Rites and Activities, Bangkok, 1975;
K.Kusalasaya, Buddhism in Thailand- Its Past and Present, Kandy, 1983.