The Mongol army pressed on after the monsoon season . In the following dry season of 1277 – 78 , c . December 1277 , a Mongol army of 3800 led by Nasr al @-@ Din , son of Gov. Sayyid Ajjal , advanced to Kaungsin , which defended the Bhamo Pass . They occupied the fort and destroyed a large number of abandoned stockades . But they found the heat excessive and returned .