First campaign in the Goguryeo – Tang War The Goguryeo – Tang War started when Emperor Taizong ( r . 626 - 649 ) , the Tang Dynasty emperor , initiated a military campaign against Goguryeo to protect Silla , an ally , and punish Generalissimo Yeon Gaesomun for killing King Yeongnyu of Goguryeo . The campaign against Goguryeo was launched in 645 . It was commanded by Emperor Taizong , General Li Shiji , General Li Daozong , and General Zhangsun Wuji . In 645 , after defeating the main Goguryeo army , Emperor Taizong appeared poised to march on Goguryeo ' s capital Pyongyang and conquer Goguryeo , but became bogged down by the strong defenses put up by the defender of Ansi , traditionally believed to be Yang Manchun , before withdrawing for winter . Background . King Yeongnyu planned to have Yeon Gaesomun killed , because he considered Yeon to be violent and criminal . However , Yeon received news of it and killed the king . Afterwards , he held actual control of the government and the military through the puppet King Bojang of Goguryeo . In 643 , Silla ' s Queen Seondeok of Silla submitted a report claiming that her state was under heavy attack by Goguryeo and Baekje . She requested aid from Tang . Emperor Taizong sent the official Xiangli Xuanjiang ( 相里玄獎 ) to demand Goguryeo and Baekje to cease attacking Silla . In the Goguryeo capital Pyongyang , Yeon Gaesomun refused to follow Emperor Taizong ' s order , given by with Xiangli . Course . Emperor Taizong used as the pretext for an invasion Yeon Gaesomun ' s murder of the Goguryeo king . The preparations for an invasion began in 644 . General Li Shiji commanded an army of 60 , 000 Tang soldiers and an undisclosed number of tribal forces . They would gather at Youzhou . Emperor Taizong commanded an armored cavalry of 10 , 000 strong . His cavalry would eventually meet up and join General Li Shiji ' s army during the expedition . A fleet of 500 ships would also transport an additional 40 , 000 conscripted soldiers and 3 , 000 military gentlemen ( volunteers from the elite of Chang ' an and Luoyang ) . This fleet would sail from the Liaodong Peninsula to the Korean Peninsula . In April 645 , General Li Shiji ' s army departed from Yincheng ( present - day Chaoyang ) . On 1 May , they crossed the Liao River into Goguryeo territory . On 16 May , they laid siege to Gaimou ( Kaemo ) . It fell after only 11 days . They captured 20 , 000 people . They also confiscated 100 , 000 " shi " ( 6 million liter ) of grain . Afterwards , General Li Shiji ' s army advanced to Yodong ( Liaodong in Chinese ) . On 7 June 645 , they crushed a Goguryeo army of 40 , 000 troops strong , who were sent to the city to relieve the city from the Tang siege . A few days later , Emperor Taizong ' s cavalry arrived at Liaodong . On 16 June , the Tang army successfully set Liaodong ablaze with incendiary projectiles and breached its defensive walls , resulting in the fall of Liaodong to the Tang forces . The Tang army marched further to Baegam ( Baiyan in Chinese ) and arrived there on 27 June 645 . However , the Goguryeo commanders surrendered the city to the Tang army . Afterwards , Emperor Taizong ordered that the city must not be looted and its citizens must not be enslaved . On 18 July 645 , the Tang army arrived at Ansi ( Anshi in Chinese ) . A Goguryeo army , including Mohe troops , were sent to relieve the city . The reinforcing Goguryeo army totaled 150 , 000 troops . However , Emperor Taizong sent General Li Shiji with 15 , 000 troops to lure the Goguryeo forces . Meanwhile , another Tang force would secretly flank the enemy troops from behind . On 20 July , the two sides descended into battle and the Tang army came out victorious . Most of the Goguryeo troops dispersed after their defeat . The remaining Goguryeo troops fled to a nearby hill , but they surrendered the next day after a Tang encirclement . The Tang forces took 36 , 800 troops captive . Of these prisoners , the Tang forces sent 3500 officers and chieftains to China , executed 3300 Mohe troops , and eventually released the rest of the ordinary Goguryeo soldiers . However , the Tang army could not breach into the city of Ansi . This city was defended by the forces of Yang Manchun . Emperor Taizong considered abandoning the siege of Anshi to advance deeper into Goguryeo , but Anshi was deemed to pose too great of a threat to abandon during the expedition . In mid - September 645 , the harsh winter worsened the conditions for the Tang army , which compelled Emperor Taizong to withdraw his forces from Goguryeo . Aftermath . The campaign caused Goguryeo to fall into famine . In 647 and 648 , Emperor Taizong decided to harass Goguryeo ' s frontier regions . It was said that both of these forces were successful in inflicting serious losses on Goguryeo forces . Believing that Goguryeo had been weakened by these attacks , Emperor Taizong prepared for the resumption of the invasion in 649 . However , Emperor Taizong died in the summer of 649 . Li Zhi succeeded him as Emperor Gaozong , who would eventually continue the military conflict .