The Khmer Empire and the Mon kingdoms. Originally, mainland Southeast Asia civilization consisted of Khmer, Mons, Chams and Viet. The Viets occupied the Eastern mountains next to China, the Chams occupied the eastern sea coast. The Khmers ruled over the gulf and the majority of the lands and the Mon kingdoms are often vassals coexists along Khmer cities.

The Thais migrated to the Southeast Asia during the expansions of the Mongol. It was until the weakening of the Khmer empire in the late 14th century, where a Siamese kingdom broke off in a port city of Ayuthya and initiated more invasions. Many of the traditional arts and music instruments of the Thai are actually Khmer.

Sources

Ceordes, " The Indianized States of Southeast Asia"