Arrival of the Tais

The most recent and accurate theory about the origin of the Tai people stipulated that Guangxi province in China is really the Tai motherland instead of Yunnan province. A large number of Tai people, known as the Zhuang, still live in Guangxi today. Around 700 AD, Tai people who did not come under Chinese influence settled in what is now Dien Bien Phu in modern Vietnam according to the Khun Borom legend. From there, the Tais began to radiate into the northern highlands and founded the cities of Luang Prabang and Chiang Saen.
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 AD. For the first time, the Tai people made contact with the
Indianized civilizations of Southeast Asia. Through Hariphunchai, the Tais of Chiang Saen adopted Theravada Buddhism and Sanskrit royal names. The Dhatu of Doi Tung, constructed around 850 A.D, signified the piety of the Tai people in the Theravada religion. Around 900 AD, major wars were fought between Chiang Saen and Hariphunchai. The 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 Hariphunchai.

Around 1000 AD, Chiang Saen was destroyed by an earthquake with all the inhabitants killed. A council was established to govern the kingdom for a while, and then a local Wa man known as Lavachakkaraj was elected the King of the new city of Chiang Saen or Ngoenyang. The Lavachakkaraj dynasty would rule over the region for about 500 years.

The overpopulation might have encouraged the Tais to seek their fortune further southwards. By 1100 AD, the Tai had established themselves as Po Khuns (ruling fathers) at Nan, Phrae, Songkwae, Sawankhalok, Chakangrao, etc. in the upper Chao Phraya valley. These southern Tai princes faced Khmer influence from Lavo. Some of them became subordinates to the Lavo-Khmer polity

Hariphunchai

According to the Jamadevivamsa, the city of Hariphunchai (modern Lamphun) was founded by the hermits; Jamadevi, a Lavo princess, was invited to rule the city in around 700 AD. However, the date is considered too early for the foundation of Hariphunchai as Jamadevi brought no Thammachakras to the north. Hariphunchai may be a later (about the 10th century) offshoot of the Lavo kingdom or instead related to the Thaton kingdom.
Hariphunchai was the center of Theravada in the north. The kingdom flourished during the reign of King Attayawong who built the Dhatu of Hariphunchai in 1108. The kingdom had strong relations to another Mon kingdom of Thaton. During the 11th century, Hariphunchai waged lengthy wars with the Tai Ngoenyang kingdom of Chiang Saen. Weakened by Tai invasions, Hariphunchai eventually fell in 1293 to Mangrai the Great, king of Lanna, the successor state of the Ngoenyang kingdom