In spring 645, Li Shiji reached You Prefecture (, roughly modern Beijing), and then headed into Goguryeo territory. Together with Li Daozong, he captured Gaemo (, in modern Fushun, Liaoning), and then put the important city of Liaodong/Yodong (, in modern Liaoyang, Liaoning) under siege. After Emperor Taizong himself arrived, Liaodong fell. Tang forces then continued heading southeast, toward Yalu River, putting Ansi (, in modern Anshan, Liaoning) under siege. When a major Goguryeo army, commanded by the generals Go Yeonsu () and Go Hyezin () arrived, Emperor Taizong had Li Shiji command 15,000 men to serve as decoy, and when Goguryeo forces attacked Li Shiji, Zhangsun Wuji attacked them from behind with 11,000 men, and Li Shiji and Zhangsun, as well as Emperor Taizong himself, defeated Goguryeo forces, forcing their surrender. He then considered directly attacking Pyongyang itself, but Li Shiji believed that if Ansi was not captured first, the general in command of Ansi (a capable general known in Korean popular legends as Yang Manchun, although whether that was his real name is unknown), might attack Tang forces from the rear. Emperor Taizong agreed, and therefore put Ansi under siege again.