The Dvaravati Mons and Khmers, who had similar languages, may also have come from the Red (Hong) River Delta in northern Vietnam, in prehistoric times. The Mons likely entered the upper Mekong from western China in the first millennium BC. Soon after that they reached the ChaoPhraya and Irrawaddy. They were powerful from the 6th century to the 11th century AD, but due to geographical barriers, ties between Mon states of lower Burma and those in north Thailand weren't close. The most significant Mon cultural center was at Nakorn Sri Thammarat (one of Thailand's oldest cities, founded during the first millennium, perhaps even the first century, AD, and long known as Ligor). The Mons brought Theravada Buddhism to Southeast Asia, and popularized many characteristics of Indian culture (especially the epic of Rama and Sita). They influenced writing systems, art forms, governmental and religious terminology and the scholarship of all their conquerors - the Khom, the Burmese and the Thai. Their alphabet came from the Devanagari script of southern India, the basis of modern Indian writing.

When King UTong, or Ramatipodi, founded a new capital city, Ayudhaya, in 1351, he called it Dvaravati Sri Ayudhaya. Rama I moved his capital to Bangkok in 1782 and officially named it Krungthep Dvaravati Sri Ayudhaya Mahadilokpop Nopparat Ratchathani Buriromya. The term Dvaravati testifies to concurrent Thai and Dvaravati roots, a joint past greatly influenced by the Indian Ramayana epic, retold Siamese-style as the Ramakien, re-written by King Rama II. That ancient epic was as fundamental to Mon culture as to Siamese.

Although the kingdoms of Sukhotai and Lopburi paid tribute to Kublai Khan's Mongol 'Yuan' dynasty (from about 1280 until 1299), Lanna resisted this. Mongols and their associates invaded several times: in 1301, 20,000 men with 10,000 horses and Mongol archers invaded - unsuccessfully. Support from King Ngam Muang's Phayao and peace with King Ramkamhaeng helped maintain unusual success against one of history's most successful conquering armies. T'ai cohesion enabled Mengrai to effectively harass their supply lines, and thus repeatedly fight them off.

T'ai cohesion was enhanced by influence from Ligor (Nakhon Sri Thammarat). Perhaps most of the marriage and kin relationships were through intermediate royal dynasties, for instance those of Petchburi and Suphanburi, but the three northern T'ai kings were greatly influenced by Buddhist teachings come via Ligor and Mon Haripunchai (Lamphun). Ngam Muang and Phraya Ruang (RamKamhaeng) studied in Lamphun as youths, and all three kings became earnest supporters of Theravada Buddhism. Ramkamhaeng even asked a learned monk from Ligor to use his throne once a week to preach. When that man came north to preach the Dhamma of Buddha, he brought with him sacred images believed to provide great power and protection. Deep respect for the power and prestige of Ligor, its religion and learning, made possible increasing friendship and peace between the three kings, despite their various troubles. The new, purer, Buddhist doctrine was surely helpful towards assimilating, for taxation, conscription and other purposes, people from many, varied tribal backgrounds. Eventually Mengrai became strong enough to send successful raiding parties north into Mongol-held territory.

Mengrai made expeditions into what became Burma, to Pegu and Ava. He brought a wife back from Pegu, and from Ava 500 craftsmen and their families. Goldsmiths he sent to Chiang Tung, and jewelers to Wiang Kum Kam. Bronzesmiths went to the area which became Chiang Saen - perhaps to Mae Chan. Silversmiths and ironsmiths came also, and presumably were similarly sent to specific locations for their guild specialty.

Mengrai's favorite, and middle, son, Khun Kram, defeated Yi-ba's son Phya Berg, and added Lampang to Lanna. Kram was made "Lord of Victory": Chao Chaya Songkram. Eldest son Khun Kruang was killed at 13 on his father's orders, after nobles at Fang plotted to raise him to the top throne. Last son Khun Krua seduced Chai Songkram's queen at his palace at ChiangDao; Kram, old now, just banished Krua to distant Mong Nai (Mone' on the Salween's opposite shore), to where others were banished later.