Emperor Wu of the Han Dynasty had six sons, among whom Yi rebelled because of the "witchcraft incident" and failed to commit suicide. Liu Hong, the king of Qi, died early, and Liu Dan, the prince of Yan, was the longest living son of Emperor Wu. According to the feudal patriarchal clan system, the heir of the emperor should be the eldest son of his first wife (queen). If there is no eldest son, it can be inherited by other sons; If there is no son, it can also be inherited by the middle-aged elders born to concubines. The prince is the longest and should be made a prince. However, Emperor Wu did not like him. On one occasion, the prince wrote a letter asking to stay in the palace, which meant to compete for the position of the crown prince. Emperor Wu not only refused, but killed the messenger sent by the prince. On another occasion, the prince committed the crime of hiding a fugitive and was cut off to three counties. These two things made Emperor Wu extremely disgusted. As a result, the youngest son Fuling was made a prince.