Tông, demanded the Emperor of Đại Việt 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 Đại Việt from Yunnan, and then to attack the Song from Đại Việt. Unprepared, Trần Thái Tông's army was overwhelmed at battle of Bình Lệ Nguyên on 17 January 1258. Five days later they captured and sacked Thăng Long. The Mongols retreated to Yunnan fourteen days later, as Trần Thái Tông had submitted and sent tribute to Möngke. Trần Thái Tông's successors Trần Thánh Tông (r. 1258–1278) and Trần Nhân Tông (r. 1278–1293) continued to send tribute to the new Mongol-led Yuan dynasty. In 1283, Yuan emperor Kublai Khan launched the invasion of Champa. In early 1285 he commissioned prince Toghon to led the second invasion of Đại Việt to punish the Vietnamese Emperor Trần Nhân Tông for not helping the Yuan campaign in Champa and refusing to send tribute. Kublai also appointed Trần Ích Tắc, a Trần prince dissent as the puppet Emperor of Đại Việt. Yuan forces though initially captured Thăng Long, however, were defeated by Cham–Vietnamese alliance in June. In 1288 they decided to launch the third and also the largest invasion of Đại Việt but were repelled. Prince Trần Hưng Đạo ended the Mongol yokes through a decisive naval victory in the battle of Bạch Đằng River in April 1288. Đại Việt continued to flourish under the reigns of Trẩn Nhân Tông and Trần Anh Tông (r. 1293–1314). Crisis of the Fourteenth century By the 14th century, Đại Việt kingdom began experiencing a long decline. The transitional decade (1326–36) from the end of the Medieval Warm Period to the Mini-ice age period affected the climate of the Red River Delta into extremes. Weather phenomena such as drought, violent flooding, storms frequently occurred, weakened the irrigation system that damaged agriculture production, generating famines, together with widespread non-bubonic plagues, impoverished the peasantry, unleashing robbery and chaos. The estimated population could have been grown from 1.2 million in 1200 to perhaps 2.4 million in 1340. Trần Anh Tông seized northern Champa in 1307, intervening in Champa's politics through the marriage of Cham king Jaya Simhavarman III with Trần Anh Tông's sister Queen Paramecvariin. Trần Minh Tông (r. 1314–1329) went into conflict with Tai peoples in Laos and Sukhothai from the 1320s to 1330s. During