In 1257, the first Mongol invasion of Đại Việt was launched with the purpose of opening a southern front against the Song dynasty, whom they were fighting for more than two decades. The Mongols had already conquered parts of modern-day Sichuan and the Dali kingdom in modern-day Yunnan in order to besiege the Southern Song from the west. Trần Thái Tông opposed the encroachment of a foreign army across his territory to attack their ally, and so dispatched soldiers on elephants to deter the Mongol troops. The elephant-mounted troops were routed from sharpshooters targeting the elephants. At the beginning of the war, the Đại Việt army suffered several defeats by an overwhelming force that had already conquered a vast area in Asia. Several high-ranking officials of the Trần dynasty were so fearful that Prince Trần Nhật Hiệu, the younger brother of Thái Tông, even suggested to the Emperor that they might escape from Đại Việt to the Song dynasty. Trần Thái Tông submitted to the Mongols after the loss of a prince and the capital. In 1258, the Trần commenced regular diplomatic relations and a tributary relationship with the Mongol court, treating them as equals to the embattled Southern Song dynasty without renouncing their ties to the Song.