李勣
Later in 644, when Emperor Taizong launched a major attack against Goguryeo, he had the generals Li Shiji and Li Daozong lead the forward forces, while commanding the main forces himself, assisted by Zhangsun, Cen, and Yang Shidao. In summer 645, at a major battle against the main Goguryeo forces commanded by the generals Go Yeonsu () and Go Hyezin () engaged Tang forces, 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 Goguryeo's capital Pyongyang itself, but Li Shiji believed that if Ansi (, in modern Anshan, Liaoning) 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. However, the commander at Ansi was a capable defender, and the resolve of the defenders was strengthened when Li Shiji, in anger, declared that after the fall of the city all residents would be slaughtered. With the Tang forces bogged down in sieging Ansi, a number of officials suggested skipping past Ansi and attacking Ogol (, in modern Dandong, Liaoning) and then head toward Pyongyang. Zhangsun opposed, believing the strategy to be too risky without first capturing Ansi and Geonan (, in modern Yingkou, Liaoning) first. Emperor Taizong agreed and continued sieging Ansi, but still could not capture it. In fall 645, with winter approaching, Emperor Taizong was forced to withdraw. (The Song Dynasty historian Hu Sanxing, the commentator to the "Zizhi Tongjian", commented that overcautiousness was what cost Emperor Taizong the victory, indirectly blaming Zhangsun for opposing the strategy to attack Pyongyang directly.) As the army retreated, Zhangsun was responsible for building a temporary bridge over the Liao River to allow the army to cross.