 Complete the following phrase:  During the Trần dynasty, the armies of the Mongol Empire under Möngke Khan and Kublai Khan invaded Annam in 1258, 1285, and 1287-88. Annam repelled all attacks of the Yuan Mongols during the reign of Kublai Khan. Three Mongol armies said to have numbered from 300,000 to 500,000 men were defeated. The key to Annams successes was to avoid the Mongols strength in open field battles and city siegesthe Trần court abandoned the capital and the cities. The Mongols were then countered decisively at their weak points, which were battles in swampy areas such as Chương Dương, Hàm Tử, Vạn Kiếp and on rivers such as Vân Đồn and Bạch Đằng. The Mongols also suffered from tropical diseases and loss of supplies to Trần armys raids. The Yuan-Trần war reached its climax when the retreating Yuan fleet was decimated at the 