The migration of the closely related Mon and Khmers from southern China commenced around the ninth century B.C. The Khmer would settle in the Mekong River Valley, while the Mon would gravitate toward the delta region of Burma and the central plains and northern highlands of Thailand. The original inhabitants of the northern highlands were the Lawa but through kingdom expansion by the Mon they were forced further up into the highlands and continue to survive today as one of the major hill tribes. With the decline of the Funan Empire in the sixth century A.D., the Mon would establish their independent kingdoms of Dvaravati and Haripunjaya. The Khmer would take over the heart of the Funan Empire and centre their king- dom in Angkor. The Mon were receptive to the South Asian influences and in the eighth century missionaries from Ceylon would introduce the Mon to Theravada Buddhism. Through the Mon, Buddhism would spread to the Khmer kingdom. Hinduism would continue to be the foundation of society in Southeast Asia with Buddhist religious values and ethical standards. Just as in Burma, the Mon would eventually succumb to the control of their neighbours and by the tenth century Dvaravati was absorbed by the Khmer empire.
On a mountainous plateau south of the Yangtze River in what is now known as Yunnan province live a people known as the Tai. The Chinese of the Tang Dynasty referred to them as the southern barbarians from the state of Nanchao. The Nanchao rebelled against the Chinese, eventually extending their domain into Burma and northern Vietnam. By the thirteenth century the armies of Kublai Khan conquered the Nanchao incorporating their kingdom into the Chinese empire. All this time the Tai people continued migrating south into Southeast Asia. They were referred to by the Khmers as syam, or 'dark brown' people, which is the origin of the term Siam. In Burma they were known as the Shan and in the upper Mekong region as the Lao. The majority would settle in Thailand, which at the time was the northern and western fringe of the Khmer Empire. A Tai chieftain would declare his independence from the Khmer and establish the kingdom of Sukhothai. It was there that the people took the name Thai, meaning 'free' having freed themselves from Khmer rule. Further north another Tai prince would defeat the old Mon kingdom of Haripunjaya and found the kingdom of Lan Na, which was centred around the present day city of Chiang Mai.
Founded in the fourteenth century on the banks of the Chao Phraya River is the Kingdom of Ayuttahya. The Thai would rule there for over 400 years, absorbing most the Khmer empire, the kingdom of Sukhothai, and much of the Malay peninsular region. The kingdom of Lan Na eluded Ayutthayan control in the beginning but they were later conquered by the Burmese Toungoo dynasty in the sixteenth century who would also capture the city of Ayutthaya. The Burmese would be driven from Thailand for the time being until the eighteenth century when Ayutthaya was completely destroyed by the Toungoo kingdom. The Thai made a remarkable recovery and re-establish their capital further south in the city of Bangkok. From there they were able to reunite the entire Thai kingdom.