In the autumn of 1257, Mongol general Uriyangkhadai addressed three letters to Trần Thái Tông demanding passage through to southern China in order to attack the Song dynasty. After the three successive envoys were imprisoned in the capital Thang Long (modern-day Hanoi) of Dai Viet, Uriyangkhadai invaded Dai Viet. A battle was fought in which the Vietnamese used war elephants: the emperor even led his army from atop an elephant. Aju ordered his troops to fire arrows at the elephants' feet. The animals turned in panic and caused disorder in the Đại Việt army, which was routed. The Vietnamese senior leaders were able to escape on pre-prepared boats while part of their army was destroyed at No Nguyen (modern Viet Tri on the Hong River). The remainder of the royal army again suffered a major defeat in a fierce battle at the Phu Lo bridge the day after. This led the Tran leadership to evacuate the capital. The Vietnamese annals report that the evacuation was "in an orderly manner;" however this is viewed as a embellishment because the Vietnamese must have retreated in disarray to leave their weapons behind in the capital. While Chinese source material incorrectly stated that Uriyangkhadai withdrew from Vietnam due to poor climate, Uriyangkhadai left Thang Long after nine days to invade the Song dynasty. After the Mongol departure, Trần Thái Tông agreed to send tribute every 3 years to the court of the Mongol Empire.