The first conflict of the Goguryeo–Tang War started when Emperor Taizong (r. 626–649) of the Tang dynasty led a military campaign against Goguryeo in 645 to protect Silla and punish Generalissimo Yeon Gaesomun for the killing of King Yeongnyu. The Tang forces were commanded by Emperor Taizong himself, generals Li Shiji, Li Daozong, and Zhangsun Wuji. In 645, after capturing multiple Goguryeo fortresses and defeating large armies in his path, Emperor Taizong appeared poised to march on the capital Pyongyang and conquer Goguryeo, but could not overcome the strong defenses at Ansi Fortress, which was commanded by Yang Manchun at the time. Emperor Taizong withdrew after more than 60 days of battle and unsuccessful siege.