The invasion began on 22 September 1283 . Prince Sangqudar was the commander @-@ in @-@ chief of the invasion force ; his deputies were Vice Governor Taipn , and commander Yagan Tegin . The Mongol armies marched to the border in two columns . One column advanced along the Taping River using over 200 boats ; the other proceeded by land and joined the first column at the Burmese fort at Ngasaunggyan . The Burmese chronicles report an overwhelming number of Mongol forces laying siege to the fort although their numbers are greatly exaggerated . ( The chronicles say that the Burmese army numbered 400 @,@ 000 men while the Mongol army numbered 20 million men and 6 million horses . ) The Burmese withstood the siege for over two months but the fort fell on 3 December 1283 .