The Đại Việt kingdom, or Annam, emerged in 960s as the Vietnamese had carved up their territories in northern Vietnam (the Red River Delta) from the local Tang remnant regime since the fall of the Tang empire in 907. The kingdom had gone through four dynasties, all of which had kept a regulated peaceful tributary relationship with the Chinese Song empire. In the autumn of 1257, Uriyangkhadai sent two envoys to the Vietnamese ruler Trần Thái Tông (known as Trần Nhật Cảnh by the Mongols) demanding submission and a passage to attack the Song from the south. Trần Thái Tông opposed the encroachment of a foreign army across his territory to attack their ally, therefore the envoys were imprisoned, and soldiers on elephants were prepared to deter the Mongol troops. After the three successive envoys were imprisoned in the capital Thang Long (modern-day Hanoi) of Đại Việt, Uriyangkhadai invaded Đại Việt with generals Trechecdu and Aju in the rear.
