While the Mon and the Khmer (Cambodians) were still spreading over the southern parts of Indo-China and before they had begun, under the influence of colonists from India, to emerge from a condition of savagery, the tribes which they had left behind them at different points during their southward movement were already being driven back into the mountains and brought into a state of partial subjugation by the members of a third great family of migrants from the north. These were the people now known as Lao-Tai, who, sending out bands from their ancient seat in the valley of the Yangtze, had already, 2,500 years ago, established a powerful state on the banks of the Me Khong in the neighborhood of the modern Wieng Chan (Vientiane).
The Lao-Tai of the Yangtze Valley were evidently very numerous, for not only did they thus early establish kingdoms far from home but also became a power in their own land and for some time bid strongly for the mastery of all China. For centuries they waged successful wars on all their neighbors, but their strong propensity for wandering weakened their state and finally caused its disintegration. The Chinese attacked them repeatedly, each attack producing a fresh exodus until, during the thirteenth century A.D., the Emperor Kublai Khan dealt them a final blow which crushed their power and scattered them in all directions. Fugitives entered Assam, where earlier emigrants had already settled, and became the dominant power in that country; others invaded Burma, where for two centuries a Lao-Tai (Shan) dynasty occupied the throne; while down the Salwin and Me Khong Valleys came band after band of exiles who mingled with their cousins already established in those valleys and, in time fusing with the Mon and the Khmer, produced the race which, since the founding of the city of Ayuthia, has been dominant in Siam.
The principal divisions of the Lao-Tai family now living within the borders of Siam are the Thai ("free men") or Siamese proper; the Lao, who occupy the former seats of those tribes of their own stock that afterward developed into the Thai; and the Shans, a later intrusion of distant cousins, descended from the Lao-Tai tribes that settled in the more eastern districts of Burma in the twelfth century and earlier.