The ancestors of the Trần clan originated from the province of Fujian before they migrated under Trần Kính to Đại Việt. Trần Thái Tông's given name was "Trần Cảnh". He was born in 1218 during the last years of the Lý Dynasty. Trần Thủ Độ, his uncle, prepared the way for his marriage to Queen Lý Chiêu Hoàng, the last queen of the Lý Dynasty, who later abdicated to make him the founder of the Trần Dynasty in 1226. His progress to the throne in particular and the replacement of the Trần Dynasty over the Lý Dynasty in general were mostly thanks to the efforts of Trần Thủ Độ, Trần Cảnh's uncle. At that time, Trần Thủ Độ was the front commander of citadels of the Lý Dynasty. Trần Cảnh's father, Trần Thừa, was also an official under the Lý Dynasty, like Trần Thủ Độ. He had been "Nội thị khán thủ", one of the most important officials in the Lý Dynasty.
Reign
During his reign Trần Thái Tông used three era names: Kiến Trung, Thiên Ứng Chính Bình and Nguyên Phong. In the autumn of 1257, Mongol general Uriyangkhadai addressed three letters to Trần 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 of Dai Viet, Uriyangkhadai invaded Dai Viet. A battle was fought in which the Vietnamese used war elephants: their 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 Dai Viet senior leaders were able to escape on pre-prepared boats while part of their army was destroyed at No Nguyen. The remainder of the Dai Viet 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 Dai Viet annals report that the evacuation was "in an orderly manner;" however this is viewed as a embellishment because the Dai Viet 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 agreed to send tribute every 3 years to the court of the Mongol Empire. Learned in both Confucianism and Buddhism, he ruled the country wisely and authored several profound works on Buddhism, the most famous of which is Khoa Hu Luc, a Zen manual. A prodigious writer, he left behind a substantial number of works, of which only a small number survive. A boy student was given money in exchange for becoming a eunuch by Tran Canh in 1254 since many men castrated themselves to become eunuchs during the Tran and Ly dynasties. In 1258 he abdicated the throne in favor of his son, crown prince Trần Hoảng.
was Trần Thái Tông's elder brother. In 1237, Trần Thái Tông and Empress Chiêu Thành still did not have any son to maintain the continuation of his dynasty, due to Trần Trịnh's premature death. At that time, Princess Thuận Thiên, Trần Liễu's wife, had been pregnant with Quốc Khang for 3 months. Trần Thủ Độ and his wife princess Thiên Cực advised the emperor to arrogate the pregnancy to himself to maintain the continuity of the dynasty. Taking their advice, the emperor gave injunction to appoint princess Thuận Thiên the status of empress, and demote Chiêu Hoàng to princess. In response, Trần Liễu took his army to Cai River to rebel. This incident embarrassed Trần Thái Tông and he left the capital for Yên Tử mountain. Only after taking advice from the Buddhist priests Trần Thủ Độ and Phù Vân, did he return to the capital. Two weeks later, Trần Liễu surrendered. Trần Thủ Độ intended to behead him, but Trần Thái Tông intervened by covering him with his body, so that Trần Thủ Độ could not do anything. Afterwards, he gave him his territory, consisting of Yên Phụ, Yên Dưỡng, Yên Sinh, Yên Hưng, and Yên Bang. Due to the name of his territory, Liễu is also called "Yên Sinh Vương."
