Excerpts from Books and Wikipedia
"The young Tran Thái Tông centralized the monarchy, organized the civil examination on the Chinese model, built Royal Academy and Confucian Temple, constructed and repaired the delta dikes during his reign. In 1257, the Mongol Empire under Möngke Khan who was waging a war to conquer the Song Empire, sent envoys to Tran Thái Tông, demanded the Emperor of Dai Viet to present himself to the Mongol Khan in Peking. The envoys were imprisoned and the demand was rejected, about 25,000 Mongol–Dali troops led by general Uriyangqadaï to invade Dai Viet from Yunnan, and then to attack the Song from Dai Viet. Unprepared, Tran Thái Tông’s army was overwhelmed at battle of Bình Le Nguyên on 17 January 1258. Five days later they captured and sacked Thang Long. The Mongols retreated to Yunnan fourteen days later, as Tran Thái Tông had submitted and sent tribute to Möngke. Tran Thái Tông’s successors Tran Thánh Tông (r. 1258–1278) and Tran Nhân Tông (r. 1278–1293) continued to send tribute to the new Mongol-led Yuan dynasty." [Wikipedia] The other two invasions of Vietnam occurred in 1285 and 1287. The third invasion was a decisive Vietnam victory. "To avoid further conflict, Dai Viet agreed to a tributary relationship with the Yuan dynasty." [Wikipedia: Timeline of Middle Ages]