The defeat at Ngasaunggyan broke the back of Burmese defenses . The Burmese army lost several thousand men as well as senior commanders . Kaungsin , the next fort inline , fell just six days later on 9 December 1283 . The Mongol sources say that the Burmese lost 10 @,@ 000 men at Kaungsin . The Mongol armies pushed farther south into the Irrawaddy valley . They took the ancient Burmese capital of Tagaung , about 380 km north of Pagan on 5 February 1284 . There , the invaders paused their advance . They found the heat of the searing Irrawaddy valley excessive , and evacuated Tagaung , allowing the Burmese to return to Tagaung on 10 May 1284 . But the Mongol army renewed their offensive in the following dry season . They retook Tagaung , and defeated another Burmese stand south of Tagaung , probably near Hanlin , on 26 January 1285 , opening the way to Pagan , about 270 km south . After the defeat , the king panicked , and fled to Lower Burma . The evacuation proved premature . The Mongol forces did not advance on Pagan as it was not part of their invasion plan .