In Roi Et, Fa Ngum directly challenged Ayutthaya, which acknowledged Lan Xang's control over the Khorat Plateau. Uthong sent 100 elephants, gold, silver, over 1,000 pieces of ivory and betrothed his daughter Nang Keo Lot Fa to be a second wife to Fa Ngum. By 1357 Fa Ngum had established the mandala for the Kingdom of Lan Xang which extended from the borders of the Sipsong Panna with China south to Sambor below the Mekong rapids at Khong Island, and from the Vietnamese border along the Annamite Range to the western escarpment of the Khorat Plateau. It was thus one of the largest kingdoms in Southeast Asia.