This, along with the cold winter approaching as well as arriving Goguryeo reinforcements and the lack of supplies, forced Tang troops into a retreat.

On 1 April 645, under the pretext of marching to camp Huaiyuan, Tang forces led by Li Shiji suddenly turned to invade Goguryeo.

Simultaneously, naval forces led by Zhang Liang had landed in Liaodong Peninsula and went on to capture Bisa on 2 May.

[4] In response to the Tang invasion, Yeon Gaesomun, Generalissimo of Goguryeo, sent about 150,000 forces with Go yeon-su and Go hye-jin to rescue Ansi.

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.

As a result, Taizong was furious and Li Shiji asked permission to massacre all the men and women if the fortress was captured, which was granted.

After the people of Ansi heard this, they defended the fortress more tenaciously; the Tang army's assaults were repelled several times.

However, Fu Fuai left the camp privately and the rampart suddenly fell, and the Goguryeo army occupied it.

[7] Taizong himself tended to the injuries of his Turkic generals Qibi Heli and Ashina Simo, who were both wounded during the campaign against Goguryeo.

[9][10][11][12] Emperor Taizong prepared another invasion in 648, but died, possibly due to an illness he contracted during his Goguryeo campaigns.