Prince of Qin Li Shimin sets an ambush at Xuanwu Gate in the Taiji Palace of Chang'an, targeting Li Jiancheng and forcing Emperor Gaozu of Tang, Li Yuan, to surrender his power. After the establishment of the Tang Dynasty, Crown Prince Li Jiancheng governs from the Eastern Palace, while Prince of Qin Li Shimin, having earned outstanding military achievements during the wars of unification, establishes the Tiance Mansion and independently appoints officials, creating a rival power that stands against the Eastern Palace. Feeling that his position as crown prince is under threat, Li Jiancheng joins forces with Li Yuanji to marginalize and suppress Li Shimin, even attempting to strip him of his military command under the pretext of sending him on an expedition against the Turks. Their mutual trust completely collapses, and an armed confrontation becomes imminent.