Emperor Taizong of song Zhao Guangyi (赵炅), the second emperor of the Northern Song Dynasty, devoted his life to recovering the "Sixteen Prefectures of Yanyun" occupied by the Liao State and completing the great cause of unifying the world. However, in his "unremitting efforts", the Northern Song Dynasty suffered heavy setbacks in succession at the Battle of the Sorghum River and the Yongxi Northern Expedition, and the military superiority was lost, and it never posed a substantial threat to the Liao state from then on.
The defeat on the battlefield made Zhao Guangyi understand that the mission of recovering the "Yanyun Sixteen Prefectures" was very arduous, and it was likely that his own generation would not be able to complete it. Therefore, for him, choosing an ideal successor to complete his unfinished business has become a top priority.
In the early years, Zhao Guangyi took the throne from his brother Zhao Kuangyin, who suddenly died violently, on the grounds that he did not know whether it was true or false. According to the agreement of the "Alliance of The Golden Daggers", Zhao Guangyi should pass the throne to his younger brother Zhao Tingmei after a hundred years, and then pass the throne back to Zhao Kuangyin's son Zhao Dezhao after a hundred years.
However, with the increasing stabilization of imperial power, Zhao Guangyi had already thrown the "Alliance of The Golden Dagger" out of the cloud of nine clouds. For him, the only one who was qualified to inherit his throne was his own son. As for which son it was passed on, it bothered him a lot.
Zhao Guangyi had eight sons, and according to the ancient patriarchal system of "having a concubine and not having a concubine", his eldest son Zhao Yuanzuo was undoubtedly the first choice. Zhao Yuanzuo has been intelligent since childhood, complete in both literature and martial arts, and is deeply loved by Zhao Guangyi. Logically, Zhao Yuanzuo had no problem becoming the heir at all. However, Zhao Yuanzuo was pure in nature and attached importance to family affection. When he learned that his father had "worn small shoes" for his uncle Zhao Tingmei, he immediately protested, which caused a rift between Zhao Guangyi's father and son. Later, Zhao Yuanzuo heard that Zhao Tingmei died of grief after being demoted to Fangzhou, which caused him a bigger blow. After that, Zhao Yuanzuo's behavior gradually went crazy, and he angered Zhao Guangyi several times, so he was completely isolated from the throne.
After Zhao Yuanzuo went crazy, Zhao Guangyi put all his expectations into his second son Zhao Yuanzuo. In the third year of Yongxi (986), Zhao Yuanxian was appointed as the Kaifeng Yin and Shizhong (侍中), the Prince of Xu, and the post of Zhongshu Ling (中書令). The most striking of this long list of titles is the position of Kaifeng Yin, which seems to be the least important, because from the Fifth Dynasty to the Northern Song Dynasty, Kaifeng Yin was the standard position before the emperor ascended the throne, which was also a strong signal that Zhao Guangyi intended to pass the throne to Zhao Yuanyi.
However, Zhao Guangyi could not have imagined that Zhao Yuanyi, who had placed great expectations on him, only lived to the age of 27 and died. According to the "History of The Song Dynasty", after zhao Yuanyi's death, Zhao Guangyi "remembered endlessly, wept bitterly and did not sleep, and wrote the "Poem of Thinking of the Dead Son" to show his close subjects." However, people cannot be resurrected from the dead, and Zhao Guangyi has to face the question of who will inherit the throne again.
Since Zhao Guangyi did not have a concubine, according to the order of the eldest and youngest, the third son of the emperor, Zhao Yuankan, became the heir to the throne. Zhao Yuankan was of mediocre talent and wisdom, and he knew at a glance that he was not the material of the Ming Emperor, but as the King of Shoucheng, it was not a problem, and the other surviving princes did not seem to see who was particularly excellent. Therefore, in the absence of any competitors, Zhao Yuankan was made crown prince step by step and changed his name to Zhao Heng.
In the third year of the Northern Song Dynasty (997), Emperor Taizong of Song died, and Zhao Heng succeeded to the throne as crown prince, known as Emperor Zhenzong of Song.
References: History of the Song Dynasty, Continuation of the Zhizhi TongjianChang Edition