Historically the first Great Southeast Asian kingdom, that we know anything about, is the Khmer kingdom of Funan. Funan was a Cambodian kingdom located around the Mekong delta. We know very little about it, except that it was a powerful seafaring and trading state. Top second image from the left: Funan 500 A.D.
This is evidenced by the discovery of Roman, Chinese and Indian goods, found there during archaeological excavations. The capital is thought to initially have been located at Vyadhapura, near modern Phnom Penh, Cambodia. Left: Aspara Thailand 1200 A.D.
The Funanese Empire reached its furthest extent under the rule of one "Fan Shih-man" in the early third century A.D. At this time, Funan extended as far south as Malaysia and as far west as Burma. The Funanese established a strong system of mercantilism and commercial monopolies that would become the pattern for future empires in the region. Fan Shih-man expanded the Funan fleet, and improved the Funanese bureaucracy, establishing a quasi-feudal system that left local customs and identities largely intact, particularly in the empires farthest reaches.
The Funan kingdom is said to have been heavily influenced by Indian culture, Sanskrit was the language at the court, and the Funanese advocated Hinduism after the fifth century, as well as Buddhism. Records show that taxes were paid in silver, gold, pearls, and perfumed wood. Chinese merchants reported that the Funanese practiced slavery and that justice was rendered through trial by ordeal, including such methods as carrying a red-hot iron chain, and retrieving gold rings and eggs from boiling water.
In about 600 A.D, the Champa of Vietnam (more about them later), sacked Funan and brought it under vassalage. This turn of events allowed the "Mon" of Thailand to break away from Funanese vassalage and establish their own kingdoms.
The most important of these Mon kingdoms was Dvaravati, which dates from approximately the 6th to the 11th century A.D. It was centered at the Chao Phraya River valley in modern-day Thailand, with Nakhon Pathom as the capital. The Mon was rarely politically dominant, but rather, almost continually under the shadow of their stronger neighbours. Bottom left: Buddha Thailand 800 A.D. Bottom second image: Thailand-Mon 800 A.D.
Dvaravati was prevented by geo-political barriers from establishing close political ties with other Mon states to the west in southern Myanmar (Burma), and with the Mon state in northern Thailand. Consequently, Dvaravati experienced political domination by neighbouring peoples on at least three separate occasions: in the 10th century, the Burmese conquered the Mon state of Thaton, which was west of the Tenasserim Yoma; from 1100 A.D. to the 1300 A.D, the Khmer (in Cambodia) arose in the east and re-established their Empire; and finally, in the late 13th century, when Dvaravati was absorbed by the current inhabitants of Thailand – the Mongol Thai's.
The Thai were a Mongol people who had already emigrated from southern China during the 9th and 10th century's, and had at first founded only small settlements which were then under Khmer hegemony. In the beginning of the 13th century, they gradually succeeded in becoming free of the reign of the Khmer, who had previously conquered the Mons.
The first Thai kingdom was Sukothai in the Central Plains of Thailand, from which originated the modern Thai culture, which is a blend of Mongol and Mon. Top: Funan 500 A.D.
The Thai alphabet and script were developed during this Sukothai period. Subjugation did not mean immediate extinction for the Mon however; it appears that the Thai allowed the Mon to retain their customs and a relative degree of racial homogeneity for a time.
Of course that could not last very long, naturally there would be cross-breeding. And today, it would be very difficult to find a Thai with purely Mongol blood or one with purely Mon blood.