For centuries before Thai-speaking people began to arrive in the Northeast area, the Khorat Plateau was within the Angor Empire (khmer). After Thai-speaking people began to occupy the area, the Khmers started to feel their pressure.
The shans, the Laotians and the Siamese are all descended from a parent racial group, cognate to the Chinese, which is thought to have made its first historical appearance in the 6th century B.C.
Around the thirteenth century, the Thai-speaking people overcame a Khmer outpost and established the first capital of Thai autonomous state, Sukhothai, which had formerly been occupied by Mon and Khmers. Shortly afterwards, Sukhothai fell and two new capitals were established, Ayuthaya, the Siamese kingdom in the central region of the peninsula, and Lan Chang (or Lan-Xang) of Laos, in the mid-fourteenth century. Later the Laos kingdom tried to expanded its territory over the northern part of Northeast Thailand and the Khmer empire continued to share the territory of the Northeast with Laos.
In 1350, however, one of the Laos' great kings, King Fa Ngum, married a Khmer princess after he was forced into exile in Cambodia. He returned to unite Laos with the help of Khmer troops. King Fa Ngum was able to take some parts of the Northeastern region which were still in hostile hands. It was the first mass migration of Laos to the Northeast region.
When King Fa Ngum established Laos, he also introduced Buddhism to the Laos people. However, Maha Sila Viravong said that the main reason that the Khmer supported King Fa Ngum was that the Khmer emperor wanted him to stop Thai's (Siamese) expansion. On the other hand, it was because of the weakness of the Khmer kingdom to protect itself against Ayuthaya that it gave military support to Lan Chang.
Before the seventeenth century King Narai (1656-1688) ordered the two of Khmer towns of Muang Senao and Muang Khorabura to be outposts for Ayuthaya. These outposts were renamed Nakhon Ratchasima (or Khorat). This was the first clear evidence of Thai strength in the Northeast.
To prevent confrontations between Ayuthaya and Lan Chang, the two kingdoms recognized all Khorat Plateau as a Thai(Siamese) boundary region. A large number of Laotian began to migrate to the Northeast during King Fa Ngum's reign. Later on, a large number of Laotian people around Vientiane again moved into the area extensively from Roi-Et to Champasak to escape one of the usurper kings of Lan Chang. Another migration to Kalasin took place later in the eighteenth century.
The Laos people brought with them both their own culture and languages. However, they also absorbed some Khmer influence. This is seen today in the fact that a large number of Khmer-speaking people are still left in the Northeast at Surin, Buriram and some part of the Sisaket provinces. But as significant as some of the Khmer influences were, the Northeast was becoming influenced even more by Laotian cultural, social and political ideas. Ayuthaya and even Chiang Mai, another autonomous state up north, shared with them the common enemy of the Burmese troops from the west.
Prior to the beginning of the eighteenth century, after the reign of King Suriya Wongsa (1633-1690 or 1695), the Laos kingdom broke into three small kingdoms: Luang Prabang, Vientiane and Champasak. Ayuthaya now became most powerful. The Thai completely expanded her power into the Northeast much more than had been done previously. The Northeast area became a region of interest to at least three kingdoms: Ayuthaya, Champasak and Vientiane. Champasak was located on the left bank of the Mekong River, and her kingdom's territory lay in the area of the Mun (Moon) and Chi Rivers which today is in Roi-Et, Ubon Ratchathani, and Kalasin provinces. This gradually disintegrated the Northeast into five smaller regions.
A new force, that of Burma, was now entering the scene. In 1767, Burmese troops from the west completely destroyed Ayuthaya, and Vientiane was forced to join Burma. Champasak, at the same time, attempted to expand it's territories into the Northeast. In due course, the Burmese occupiers met with increasing resistance, and under General Phraya Taksin's leadership they were able to reorganize their troops and drive the Burmese out of the country. In 1768 General Phraya Taksin, who was half Chinese and half Thai, established a new Thai capital at Thonburi and proclaimed himself the new king. Fortunately, Luang Prabang was saved because she aligned herself with the Thai kingdom. Afterwards, the three states of Vientiane, Luang Prabang and Champasak became Thai vassals. Since that time the Northeast, or Khorat Plateau, has remained an outer region within the Thai kingdom.
In 1782 a new Chakkri dynasty, ruled by King Rama I, was established in the Thai kingdom. Thailand moved her new capital from Thonburi, which was on the left bank of the Chao Phraya River, to the present Bangkok (Glung Thayp in Thai) location. In 1804, King Rama II of Bangkok placed Chao Anu of Vientiane, his personal friend, as the new king of Vientiane. In 1827, however, when King Rama III ascended to the throne, King Chao Anu of Vientiane attempted to regain the independence of Vientiane. With the combined support of two groups of vassal troops, King Chao Anu moved toward Bangkok for battle. He pretended that he was going to help Thailand, which was being threatened by British gunboats. The Laos troops were able to reach the area of Saraburi province in the Central plain of Thailand.
At first the Thai troops were surprised, but quickly organized themselves to fight against the Laos troops. King Rama III ordered Vientiane completely destroyed, and deported some of the Laos people to the Central plains. To this day, these Laos-speaking groups still remain in the area of Lopburi and Ratchaburi provinces in the Central plain. Later on, King Chao Anu and his family were arrested, and for punishment were placed in an iron cage and subjected to public ridicule. They died four days later. At that time both Vientiane and Champasak were reduced in status and became vassals. Luang Prabang also remained a Thai vassal.
But Thailand was to have its own troubles not leastly from other aggressive Southeast Asian states or kingdoms.
With the nineteenth century, strong, new pressures developed from the European colonial powers, notably the French and the British. The Franco-Siamese Treaty of 1893, signed under threat of a French ultimatum, allowed the French and British to expand their territorial influence into Southeast Asia, thus halting Thai expansion. It established the present borders of Thailand. This treaty also transferred the entire area on the left bank of the Mekong River, or what is Laos today, to France. Later, by the treaty of 1904, both the area on the right bank of the Mekong River, Sayaboury province (opposite Luang Prabang in Laos) and Champasak (it is also called Bassac by the French) were also conceded to France. Since that time the present borders between Laos and Thailand have remained unchanged as they are today.