Nan-Chao kingdom was a comparatively powerful state with a high level of culture. It lasted for some seven centuries until it fell in 1253 A.D. "A history of South -East Asia" says that the Tai (or Thai) never ceased to be on the move. (from the earlier days of Nan-Chao) , and slowly that infiltrated along the rivers and down the valleys of Central Indo-China. Small groups settled among the Khmers, the Mons and the Burmese, and, long before that, they had been crossing into the Menam valley (in central Thailand) from the river Mekong and undoubtedly from the river Salween too.