The most recent and accurate theory about the origin of the Tai people stipulates that Guangxi in China is really the Tai motherland instead of Yunnan. A large number of Tai people known as the Zhuang still live in Guangxi today. Around 700 CE, Tai people who did not come under Chinese influence settled in what is now Điện Biên Phủ in modern Vietnam according to the Khun Borom legend. Based on layers of Chinese loanwords in proto-Southwestern Tai and other historical evidence, Pittayawat Pittayaporn (2014) proposed that this migration must have taken place sometime between the eighth–10th centuries.^[23] Tai speaking tribes migrated southwestward along the rivers and over the lower passes into Southeast Asia, perhaps prompted by the Chinese expansion and suppression.

The Simhanavati legend tells us that a Tai chief named Simhanavati drove out the native Wa people and founded the city of Chiang Saen around 800 CE. For the first time, the Tai people made contact with the Theravadin Buddhist kingdoms of Southeast Asia. Through Hariphunchai, the Tais of Chiang Saen embraced Theravada Buddhism and Sanskrit royal names. Wat Phrathat Doi Tong, constructed around 850, signified the piety of Tai people on the Theravada Buddhism. Around 900, major wars were fought between Chiang Saen and Hariphunchaya. Mon forces captured Chiang Saen and its king fled. In 937, Prince Prom the Great took Chiang Saen back from the Mon and inflicted severe defeats on Hariphunchaya. By 1100 CE, the Tai had established themselves as Po Khuns (ruling fathers) at Nan, Phrae, Songkwae, Sawankhalok, and Chakangrao on the upper Chao Phraya River. These southern Tai princes faced Khmer influence from the Lavo Kingdom. Some of them became subordinates to it.