 Three Mongol armies said to have numbered from 300,000 to 500,000 men were defeated.The key to Annam's successes was to avoid the Mongols' strength in open field battles and city sieges—the 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.