Perhaps the most enduring legacy of the Mongol invasions was the emergence of Tai @-@ Shan states in mainland Southeast Asia . The Tai @-@ Shan people who came down with the Mongol invasions stayed . By the early 14th century , several Tai @-@ Shan states had come to dominate a vast region from present @-@ day Assam to northern and eastern Myanmar to northern and central Thailand and Laos . Their rise was encouraged by the Mongols , who viewed the states as a useful buffer between Yunnan and the rest of Southeast Asia . The Mongols , who were still trying to incorporate Yunnan into the central administration , were unwilling or unable to make necessary sustained investments to bring the vast regions south of Yunnan into the fold . ( The integration of Yunnan itself into “ China Proper ” was to take several more centuries , and continues to today . ) As such , from the newly formed Tai @-@ Shan states in western and central Southeast Asia to Dai Viet and Champa in eastern Southeast Asia , the Mongols elected to receive nominal tribute . Though the rulers of these states were technically governors of the Yuan government , they were the native chieftains , “ who would have ruled there in any case , and they did as they pleased . ”