Trần Thái Tông
In the autumn of 1257, Uriyangkhadai addressed three letters to Vietnamese ruler Trần Thái Tông (known as Trần Nhật Cảnh by the Mongol) demanding passage through to southern China. Trần Thái 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 Đại Việt, Uriyangkhadai invaded Đại Việt with generals Trechecdu and Aju in the rear.