Chao Phraya Basin under the Tai

There are several legends about the Tai coming to rule mandalas of Central Thailand including Lavo and Suphannabhum. The Suphannabhum mandala had been independent of Khmer influence and composed originally of the Western Provinces of modern Thailand including Suphanburi, Ratchaburi, and Phetchaburi. In the 12th century, the Tai Po Khuns of the upper Chao Phraya coalesced around the Po Khun of Sukhothai - an important Khmer outpost. With the weakened Khmer authority, Po Khun Si Nau Namthom
of Sukhothai gained the autonomy of the upper Chao Phraya valley. However, during the reign of Jayavarman VII, the Khmer power was strengthened, and the Tai Po Khuns were subdued and brought under Khmer suzerainty. Suphannabhum was also weakened by Khmer invasions and only the city of Suphanburi itself was left.