 After the death of the Retired Emperor Trần Thái Tông on April 1 of Lunar calendar, 1277, the Emperor Trần Thánh Tông officially began to reign as the sole ruler of Đại Việt, but only one year later, he decided to pass the throne to his crown prince Trần Khâm, now Trần Nhân Tông, took up the post of Retired Emperor on the 22nd of the tenth lunar month (November 8) 1278.In 1279, the Yuan dynasty had the decisive victory over the Song dynasty in the Battle of Yamen which marked the end of the Song dynasty and the total control of Kublai Khan over China.As a result, Kublai Khan began to expose his attempt to take over the southern countries like Đại Việt or Champa.