In 620 AD, in July of the third year of Tang Wude, Li Shimin, the second son of Li Yuan, the founding emperor of the Tang Dynasty, was ordered by his father to march east to Luoyang. Luoyang was the eastern capital of the previous Sui Dynasty, and it was of great significance in politics, military and economy. The Battle of Luoyang became the most important battle for the Tang Dynasty to unify the Central Plains. After months of siege by Li Shimin's army, Luoyang City no longer had the power to fight against the Tang army, and Wang Shichong, the leader of Luoyang, had to pin his hopes on reinforcements. Dou Jiande of Hebei led hundreds of thousands of troops to march towards Luoyang to lift the siege of Luoyang. Li Shimin ordered his younger brother, Li Yuanji, the Prince of Qi, to stay at the base camp, and he personally led his troops to meet Dou Jiande's army. Li Shimin made full use of the advantages of the terrain, defeated Dou Jiande's army with a few thousand people, and captured Dou Jiande himself. Wang Shichong saw that the situation was hopeless, so he had to open the city and surrender. Li Shimin defeated the two most powerful enemies of the Tang Dynasty in one fell swoop, and his merits spread all over the world. In order to commend Li Shimin's achievements, Li Yuan appointed him as General Tiance and granted him various privileges. His status was almost equal to that of the crown prince.