Battle of Ansi

On 20 July, the two sides descended into battle. The Tang Dynasty sent Li Shiji leading 15,000 infantry and cavalry to fight the Goguryeo army directly. But Tang general Zhangsun Wuji led 11,000 elite cavalry across the canyon from the north of the mountain to hit the rear of Goguryeo forces. In the battle, Taizong personally led 4,000 infantry and cavalry to fight. The Tang army came out victorious in the end, decimating the Goguryeo forces. At least 20,000 Goguryeo soldiers were killed and 36,800 Goguryeo soldiers, including their generals Go Yeonsu and Go Hyezin, surrendered. The Tang army captured 50,000 horses, over 50,000 cattle, and over 10,000 iron armors. After the battle, Tang had succeeded in isolating Ansi fortress from other Goguryeo territory.[6]

The Tang first attacked Ansi Fortress with several siege weapons including catapults and battering rams. However Goguryeo repelled the attacks and repaired the ramparts each time. As a result, Taizong was furious and Li Shi asked permission to massacre all the men and women if the fortress was captured, which was granted. After Anshi people heard this, they defended the fortress more tenaciously, the Tang army's assaults were repelled several times. When the Ansi fortress attack came to a standstill, Tang was in a hurry so the Tang Army attacked the west side of the fortress as many as six or seven times per day.[7] One night, hundreds of Goguryeo soldiers climbed out of the fortress and attempted to attack the Tang army. When Taizong heard about it, he called up soldiers to make an emergency joint attack which killed dozens of Goguryeo soldiers, and the rest fled back to the fortress.[8]

The Tang forces grew weary and considered the idea of bypassing Ansi fortress to attack other parts of Goguryeo, which was rejected due to threat Ansi posed to the advancing Tang forces.[6] Under the leadership of Tang's prince Li Daozong, Tang forces attempted to build a rampart in the southeastern corner of the fortress, gradually approaching the wall. Meanwhile, the external wall was constantly raised by the garrison. Li Daozong was injured in the battle. The Tang used soldiers and unknown number of laborers (possibly war captives) to build the rampart and the top of the rampart was only a few feet away from the fortress.[8] It overlooked the city. Fu Fuai, one officer of the Tang Army, stationed his troops on the top of the rampart. However, Fu Fuai left the camp privately and the rampart suddenly fell, and the Goguryeo army occupied it. Taizong was very angry and put Fu Fuai to death. After that, Tang tried to regain the rampart for 3 days but failed. On the 3rd day, the Goguryeo army reinforcements arrived and Taizong had still not captured the Ansi Fortress. In addition, the weather was getting colder and food ran out, so the Tang unavoidably ordered retreat.[9] Taizong's retreat was difficult and many of his soldiers died.[10] Taizong himself tended to the injuries of his Turkic generals Qibi Heli and Ashina Simo, who were both wounded during the campaign against Goguryeo.[11]