The siege of Ansi took place between Goguryeo and Tang forces in Ansi, a fortress in the Liaodong Peninsula, and the culmination of the First campaign in the Goguryeo–Tang War. The confrontation lasted for about three months from 20 June 645 to 18 September 645.

The first conflict of the Goguryeo–Tang War started when Emperor Taizong of the Tang dynasty led a military campaign against Goguryeo in 645 to protect Silla and punish Generalissimo Yŏn Kaesomun for the killing of King Yeongnyu. The Tang forces were commanded by Emperor Taizong himself, and generals Li Shiji, Li Daozong, and Zhangsun Wuji.

The Battle of Mount Jupil, also known as Battle of Jupil-san and Battle of Mount Zhubi was a battle between Tang Chinese and Goguryeo Korean forces that took place in Mount Jupil at the south of the fortress city of Ansi. It took place on July 11, 645 AD. The small but elite Tang army wiped out the Goguryeo relief force trying to come to bolster the defenses of Ansi, achieving a major victory.