The Khmer Empire was divided up between Thailand, Cambodia, Laos and Vietnam by
Angkor was the capital of Khmer Empire and was the biggest city in the world at the time.
Angkor’s population reached over a million.
The Khmer Empire had its beginnings in 790 when King Jayavarman conquered the Kingdom of Kambuja, until then under the Joke of the Javanese. The consecration rites of Jayavarman on Mount Julen [ now in Cambodia ] in 802 formalised the freeing of Kambuja from Java and the introduction of the Hindu cult of the Devaraja [ God King ] Hariharalaya was the name of the first settlement of the new Angkor Kingdom and by 889 this Khmer Empire held control as far as Ubon in Thailand.
By 944 after battles with the Cham people, the Empire expanded to the Annamite chain in the East [ now Vietnam ], Burma to the West and the Gulf of Siam in the South. In the early 14th Century this Empire expanded and levied tribute from South Thailand. During the reign of Jayavarman 7th from 1181-1220 the Khmer Empire expanded further to include the Korat Plateau in Isan, the Menan Valley, Southern Malaysia, Northern Laos and the Kingdom of Champa. It was not until 1430 when the Ayutthayan Empire of the Tai -Siamese sacked Angkor did the decline of the Khmer commence.
The Khmer had a presence in Central, East, some parts of the North and the Northeast of Thailand [Isan ] since the second half of 6th Century. This presence lasted until the mid 13thCentury. However the influences of Khmer styles, inspiration and iconography continued long after. Where the Khmer were, and when, can be traced from looking at the many stone inscriptions found throughout Thailand. These are dated by the alphabet used, which was either ancient Khmer, Pallava, Later Pallava, or Khmer and the language used which was either Pali, Sanskrit [ both from India ] and Khmer.