 In the autumn of 1257, Uriyangkhadai addressed three letters to Dai Viet emperor Trần Thái Tông demanding passage through to southern China.Trần Dụ Tông opposed the encroachment of a foreign army across his territory to attack their ally, and so prepared soldiers on elephants to deter the Mongol troops.After the three successive envoys were imprisoned in the capital Thang Long (modern-day Hanoi) of Dai Viet, Uriyangkhadai invaded Dai Viet with generals Trechecdu and Aju in the rear.