Coup at Xuanwu Gate
In 618 Li Yuan proclaimed himself Emperor Gaozu of the Tang Dynasty (618-907). In 626, his second son Li Shimin killed Crown Prince Li Jiancheng and his younger brother Li Yuanji in a coup at Xuanwu, the north gate of the imperial palace. The coup ended in a forced transfer of sovereignty from Li Yuan to his son Li Shimin.
Li Yuan had four sons. Three of them survived into the Tang Dynasty. After Li Yuan ascended the throne, he named his oldest son Li Jiancheng as his heir, although his second son Li Shimin had proven to be wiser and more capable as a leader.
Since Li Shimin possessed wisdom and bravery, people gathered around him. This made Li Jiancheng jealous and insecure. Together with his youngest brother Li Yuanji, he spread damaging rumors about Li Shimin to their father, trying to malign him. Li Yuan believed what his sons said and Li Shimin fell out of his favor.
In 626, emergency border situations gave Li Ji'ancheng and Li Yuanji the excuse to propose that forces under Li Shimin's command be moved out of the capital city. On hearing this, Li Shimin decided to take pre-emptive action. He set up an elite squad for an ambush inside Xuanwu Gate. While passing through the gate on their way to the royal palace, Li Jiancheng and Li Yuanji suspected that something was anuss. Then Li Shiman suddenly appeared on a horse. Li Jiancheng tried to kill his brother with an arrow. But he was too frightened to pull open his bow. It was too late to escape either. Li Shimin shot Li Jiancheng with an arrow, while his warrior Yuchi Gong killed Li Yuanji. Li Shimin sent Yuchi Gong, spear in hand, into the palace, informing his father that Li Jiancheng and Li Yuanji had been plotting treason and had been killed. Emperor Gaozu was shocked when he learnt about the killing between his sons. His Prime Minister Xiao Deng told him he had to face the truth: "Li Jiancheng and Li Yuanji had not really deserved what you bestowed on them. They were the ones who had kindled and nurtured hatred between your sons. Everything shall be fine if you pass sovereignty on to Prince Li Shimin."
The old emperor knew there was little else for him to do. He confirmed the offenses Li Shimin had charged his brothers with, and appointed Li Shimin, now his only surviving son, in command of all royal forces. Two months later, Li Yuan abdicated. Li Shimin was crowned as Emperor Taizong (r. 627-650). The period of his reign was known as Zhenguan.
* Scramble for the Throne
When the first emperor of the Tang Dynasty Li Yuan took the throne he conferred the Crown Prince on his first son Li Jiancheng. However, Li Jiancheng hadn't led army in any battles for the dynasty's establishment, whereas Li Shimin, the second son of Li Yuan, had fought battles himself and made many battle achievements. Therefore the conflict between them was inevitable. In the ninth year of the Yuande period (626) the battle between the brothers finally happened, that is the Coup at Xuanwu Gate.