The origin of the Thai people is not clear. The Thai people have characteristic features that are different than that of Vietnamese or most Chinese. There are about three or four Thai ethnic sub-races as can be determined by facial features and skin tones. Those in Isan, for example, look like Cambodians, and many in Northern Thailand look like Burmese. Many think that some of the Thai ancestors came from the region of southern China. Some people think that some of the Thai ancestors came from central China around the Yangtze River. The Thai language is a tonal language, but linguists haven't found a link between the linguistic family group of the Thai language and other tonal language groups such as that in central China. This doesn't necessarily mean that some of the Thai ancestors didn't come from central China because perhaps they adopted the Thai regional language. The Thai people are probably mainly a mixture of people from around southern China and Southeast Asia.
It is thought that there was a big migration of the Nanzhao and Dali people of Yunnan into the region from about 700 to about 1300 AD. During the 700s and 800s, an empire called the Nanzhao Empire expanded in Yunnan and Southeast Asia. It is thought that the Nanzhao people were mainly a people called the Tai who are thought to be the ancestors of modern Thai people. About the year 738 AD, Taihe near Dali in Yunnan became the capital of the Kingdom of Nanzhao. After this, the Nanzhao Empire controlled the important trade routes in Southeast Asia and Yunnan. So under their empire, the people called the Tai migrated in the region and mixed with the inhabitants who were called Mon and Khmer. After about 200 years, in 937 the Duan clan conquered the Nanzhao Empire. They established the Dali Kingdom. This kingdom lasted until 1253 when the Mongols invaded. It is thought that there was a wave of migration of Dali people into Thailand when the Mongols invaded Yunnan.
In 1238, near the end of the Dali Kingdom, the first big Thai kingdom called Sukhothai was established in Northern Thailand. It is thought that the kingdom was strengthened by the migration of Dali people when their kingdom was attacked in 1253. Then in 1350, King Ramathibodi founded the Ayutthaya Kingdom at a location about 50 kilometers from Bangkok. The empire became powerful and rich, and they subjugated the Kingdom of Sukhothai to the north in 1378. During the 1400s, the empire expanded and defeated Angkor. Thai settlements spread in the region, but they were not able to conquer the Malays or the Burmese. Instead, the Burmese attacked them.
In 1568, the Burmese took the city of Ayutthaya. King Naresuan recaptured it. But it fell again. After this time, the Qing Empire started to try to expand their control southwards at the same time that Europeans arrived in the area. In the centuries afterwards, the Thai rulers succeeded in keeping Thailand unconquered by either the Qing or the Europeans. They traded with the Europeans and learned from them and modernized. But in 1766, the Burmese attacked again and destroyed Ayutthaya.
The Ayutthaya general named Taksin fled southwards, and he and his followers established a new Siamese capital at Bangkok. This started the Kingdom of Siam. He became king in 1768. He succeeded in driving off the Burmese, and there was peace for 15 years, but he may have gotten corrupted by power because he proclaimed himself to be the Buddha, and his supporters killed him in 1782. A general near Chakri became the new king. After this, the English, French and Dutch established colonies all around them, but the Kingdom of Siam remained independent.
During the 1800s and early 1900s, Siam had kings who not only engaged in trade and diplomatic relations with powerful countries around the world, but also traveled the world and gained a Western education. They started to modernize the country and started to establish modern education and transportation such as railroads. During the early 1900s, a Siamese king who was educated in Oxford University continued to modernize Siam.
During WWII, the Japanese tried to colonize Asia, but the Thai government made an alliance with the Japanese and agreed to assist them. In this way, they kept themselves from being invaded by the Japanese. But when Japan was defeated, they became allies of the US. During the Cold War struggle following this, there were military coups and the Vietnam War. The Thai government was allied with the US during the war. Thailand faced invasion by the Vietnamese or other Communist governments. In the 1980s, Thailand became a stable republic and started to rapidly develop economically. There was strong economic growth during most of the early and middle years of the decade of the 2000s. But from 2004 onwards, there was a Muslim insurgency in the south. In 2006, the Thai government was overthrown in a coup. After this, there has been a political crisis as well as an economic slump.