Xuanwu Gate Incident

The Xuanwu Gate Incident was a palace coup for the throne of the Tang dynasty on 2 July 626. During the incident, Prince Li Shimin (Prince of Qin) and his followers assassinated his brothers Crown Prince Li Jiancheng and Prince Li Yuanji (Prince of Qi).

Li Shimin took control and set up an ambush at Xuanwu Gate, the northern gate leading to the Palace City within the imperial capital Chang'an. Near that area, Li Jiancheng and Li Yuanji were killed by Li Shimin and his men. Within three days after the coup, Li Shimin was installed as the crown prince. About two months later, Emperor Gaozu abdicated and passed the throne to Li Shimin, who would become known as Emperor Taizong.

After the founding of the Tang dynasty by Emperor Gaozu, the Emperor's eldest son, Li Jiancheng, was established as the heir apparent, but he was often overshadowed by his younger brother Li Shimin. For instance, Li Shimin was instrumental in defeating several of Tang's major rivals. He had led the attack on Dou Jiande and Wang Shichong, whom he defeated in battle, which gained him prestige among his contemporaries. Meanwhile, Li Jiancheng was stationed along the northern frontier to guard it against the Tujue, which left him unable to build up a similar reputation. Eventually, Emperor Gaozu elevated Li Shimin's position above all other nobles. He placed Li Shimin in charge of the civil and military administration of the eastern plain, with Luoyang as its headquarters. There, Li Shimin established himself and appointed about fifty civil and military officials, which made it possible for him to challenge the heir apparent's pre-eminence. In 621 AD, he established the College of Literary Studies comprising a staff of eighteen scholars, who served as his advisors on state affairs. This may have insinuated in Li Jiancheng's mind that Li Shimin could harbor ambition to ascend to the throne himself. Henceforth, Li Jiancheng attempted to undermine Li Shimin by getting his staff members removed and reassigned to other posts.

Prince Li Shimin found himself unable to gain support in the capital Chang'an or inside the imperial palaces, as he was often away on military expeditions. He mostly relied on Luoyang where he could successfully build strong support among military and civil officials. In Chang'an, Crown Prince Li Jiancheng was increasing his power by recruiting more than two thousand men to serve in the Changlin troops, which he stationed at the East Palace near the Changlin Gate. He was allied with his second younger brother Prince Li Yuanji. They had the support of Emperor Gaozu's consorts, who often interceded with court affairs on behalf of the two princes.

There were allegations that Yang Wengan was raising troops for Li Jiancheng, who was left in charge of the capital while Emperor Gaozu was away in his summer palace, to stage a coup for the throne. Whether Li Jiancheng was actually involved is still disputed among historians. Yang Wengan was the regional commander of Qingzhou in Gansu and a former guard of Li Jiancheng at the East Palace. The alleged plot was disclosed to the authorities, so Li Jiancheng was summoned from Chang'an and Yang Wengan was summoned from his garrison post. Li Jiancheng went to seek forgiveness against the advice of a subordinate to seize the throne. In contrast, Yang Wengan raised his troops and rebelled in the sixth month of 624. Emperor Gaozu sent Li Shimin to put down the rebellion, but Yang Wengan's own subordinates killed him after the imperial forces arrived at the scene. Emperor Gaozu initially offered Li Shimin the position of heir apparent in light of Yang Wengan's rebellion. However, Li Jiancheng's supporters, Li Yuanji, the palace ladies, and Minister Feng Deyi interceded to clear Li Jiancheng from the affair. Thus, Emperor Gaozu allowed Li Jiancheng to remain the heir apparent, but exiled some of Li Jiancheng's advisors and at least one member of Li Shimin's staff.

There are many questions with this rebellion, including why Yang Wengan rebelled, whether Li Jiancheng actually intended to rebel, what happened between Yang Wengan and Yuwen Ying (an emissary sent by the Emperor to Yang before the rebellion), why Yuwen Ying joined the rebel army, whether Li Shimin influenced some of the events or people involved, why Li Shimin's subordinate Du Yan was one of those who were exiled, and why the emperor did not install Li Shimin as the new heir as was promised.

Some texts state that, prior to the coup, Li Shimin survived a poisoning attempt by his two brothers. According to the Jiu Tangshu, it happened prior to 626 AD, while the Zizhi Tongjian dates it to the sixth lunar month of 626 AD, placing it within three days of the coup. Bingham (1950a) states that the latter interpretation is most probably the one to be incorrect, since the poisoning had rendered Li Shimin seriously ill. It is still disputed when or whether this poisoning actually happened. Li Jiancheng and Li Yuanji successfully plotted the dismissal of Fang Xuanling and Du Ruhui, who were Li Shimin's principal advisors, from service. Yuchi Jingde, who was a general of Li Shimin, escaped an assassination attempt ordered by the two princes, but he was later slandered by the two princes at court and came near execution if it had not been for Li Shimin's intercession. By 626 AD, Li Shimin became increasingly worried by his brothers' successful machinations in turning Emperor Gaozu against him and in removing his staff members.

In early 626 AD, the Tujue attacked the frontier of the Tang empire. At Li Jiancheng's recommendation, Li Yuanji was commissioned for a military campaign against these hostile forces, thus Li Shimin's best generals and crack troops were transferred to Li Yuanji. Thereafter, Li Shimin received word from his men that Li Jiancheng and Li Yuanji had taken preparations to assassinate him when he would see off Li Yuanji as was the custom during the onset of a military campaign.