 In 1257, Crown Prince Trần Hoảng witnessed the first Mongol invasion in Vietnam.In the beginning, Đại Việt's army suffered several defeats by the Mongols who had already conquered a vast area in Asia.Several high-ranking officials of Trần dynasty were so frightened that Prince Khâm Thiên Trần Nhật Hiệu, younger brother of Thái Tông, even suggested the Emperor that they might escape from Đại Việt to the Song dynasty.