According to various historical sources, the Thais might have originated in western and north-western Szechuan. Like the Chinese, the Thais belong to the Mongolian stock. From the sixth century B.C. onwards, Chinese annals made frequent references to the Thais as the “barbarians” south of the Yang-tse-kiang. The Chinese gradually began to encroach upon them and press them hard. Due to their lack of unity, the Thais could not organize an effective resistance to the Chinese rule and most were eventually absorbed by them, while others made attempts to preserve their independence. In order to attain their objective, they started their southward migrations gradually and intermittently.
The Thais followed the river valleys in their movements towards the south. The western group of Thais descended along the Salween river where they settled down and became Shans or so-called Great Thais. Choosing the Mekong valley as its home, the eastern group spread its influence to Tonking, and constituted the ancestry of the Laotians, while the middle group emigrated into Thailand. These last two groups have been referred to as the Little Thais.