The Thai are a subgroup of the Tai people, who include the Ahom in India, Dai in China, Shan in Burma, Lao in Laos and others in Vietnam. The Tai appeared historically in the first century CE in the Yangtze River valley. Chinese pressures forced them south.
The ancestors of the Thai entered the central part of the Southeast Asian mainland from Yunnan circa AD 1000.
The most powerful Tai kingdom in Yunnan had been Nanchao or Nanzhao, 729-902. It was followed by the Dali Kingdom, 937-1253, whose founder claimed Han descent, and which was conquered by the Mongols.
Some Tai presumably migrated because of infiltration of Yunnan by Han Chinese. More later fled from the Mongols. It was the Mongols who brought Yunnan definitively into China.
Nanzhao had been influenced by Tantric or Tibetan Buddhism. Its Indian Acharya version as present in the Dali Kingdom. After their migration (earlier?), the Thais became converts to the Theravada or Sinhalese southern Buddhism that had established itself in Burma in 1190.
There have been four main Thai polities in Thailand (capital here means main capital; other cities may have served the function for some of the time):
I Kingdom of Sukhothai, 1238-1438
Capital Sukhothai, 265 miles north of Bangkok
Phra Ruang dynasty, but from 1368 under the suzerainty of the Kingdom of Ayutthaya
II Kingdom of Ayutthaya, 1350-1767
Capital Ayutthaya, 50 miles north of Bangkok
First Uthong dynasty, 1350-70
First Suphannaphum dynasty, 1370-88
Second Uthong dynasty, 1388-1409
Second Suphannaphum dynasty, 1409-1569
Sukhothai dynasty, 1569-1629
Prasat Thong dynasty, 1630-88
Ban Phlu Luang dynasty, 1688-1767
Ayutthaya was brought down by Burmese invaders, who continued to harass Thailand in the coming decades.
III Kingdom of Thonburi, 1768-82
Capital Thonburi, now part of Bangkok
IV Kingdom of Rattanakosin, 1782-present
Rattanakosin comes from Rattanakosin Island in Bangkok, the original site of the capital.
All the Chakri kings have the official name of Rama. Bhumibol is Rama IX. His predecessor Ananda (last post) was Rama VIII.
Thais called the country Mueang Thai. The exonym Siam came from the Portuguese. It has been identified with the Sanskrit śyāma (श्याम), meaning dark or brown. Some Thais are very dark.
Siam was officially adopted under Mongkut or Rama IV (reigned 1851-68). On June 23 1939 the name was changed to Thailand. From 1945 to May 11 1949 it was Siam again. Then it reverted to Thailand.
The distance from the Yangtze basin to the Gulf of Thailand is about 2,500 miles.
Tibetan rivers (old post).