According to Thai historical inscriptions found in Vietnam, there are three important waves of migration initiated by the Thai of Yunnan in northwest of Vietnam during the 9th and 11th centuries. This corresponds exactly to the period where Nanzhao was annexed by Dali destroyed, in turn, three centuries later by Kubilai Khan Mongols in China. During this penetration, the Proto-Thaïs were separated into groups: the Thaï of Vietnam, the Thaï in Burma (or Shans), the Thaï in Laos (or Ai Lao in Vietnamese) and the Thaï in Northern Thailand. Each of these groups began to adopt the religion of these host countries. The Thaï of Vietnam do not have the same religion as those of other territories. They continue to keep animism (vạn vật hữu linh) or totemism.
This is not the case of the Thaï living in Northern Thailand, Upper Burma, Laos which were occupied at this time by Indianized and Buddhist theravàda Môn-Khmer kingdoms (Angkorian empire, Môn Dvaravati, Haripunchai, Lavo kingdoms etc …) after the dislocation of Indianized Funan kingdom. The Môn had a key rôle in the transmission of Theravadà Buddhism from Sinhalese tradition for Thai newcomers.
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