Goguryeo – Tang War The Goguryeo – Tang War occurred from 645 to 668 and was initially fought between the Goguryeo kingdom and Tang empire . During the course of the war the two sides allied with various other states . Exhausted from numerous attacks , the kingdoms of Goguryeo and Baekje succumbed to a two front attack by the Tang empire and Silla kingdom . Onset . In 627 , Li Shimin ( Emperor Taizong ) succeeded the Tang throne after the Xuanwu Gate Incident , killing his two brothers — Li Jiancheng and Li Yuanji — and forcing his father , Li Yuan ( Emperor Gaozu ) , to abdicate . Soon thereafter , he set his mind to conquering Goguryeo . The Silla kingdom had made numerous requests to the Tang court for military assistance against Goguryeo , which the Tang court began to consider not long after they had decisively defeated the Göktürks in 628 . At the same time , however , Silla was also engaged in open hostilities with Baekje in 642 . A year before ( 641 ) , King Uija had assumed the throne of Baekje . While , in 642 , the military dictator Yeon Gaesomun murdered over 180 Goguryeo aristocrats and seized the Goguryeo throne . He placed a puppet king onto the throne after killing the king in 642 . These newly formed governments in Baekje and Goguryeo were preparing for war and had established an mutual alliance against Tang and Silla . Course of the war . Conflict in 648 . 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 Liaodong ( Ryotong ) . 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 Baiyan ( Paekam ) 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 Anshi ( Ansi ) . 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 Anshi . 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 . Conflict in 654 - 668 and fall of Goguryeo . After Emperor Taizong ' s death in 649 , the conquest of Goguryeo and the personal rivalry with Yeon Gaesomun became an obsession with Emperor Gaozong . Under Emperor Gaozong ' s reign , the Tang empire formed a military alliance with the Silla kingdom . When Goguryeo and Baekje attacked Silla from the north and west respectively , Queen Seondeok of Silla sent an emissary to the Tang empire to desperately request military assistance . In 650 , Emperor Gaozong received a poem , written by Queen Seondeok , from the princely emissary Kim Chunchu ( who would later accede the Silla throne as King Muyeol ) . In 653 , Baekje allied with Yamato Wa . Even though Baekje was allied with Goguryeo , the Han River valley separated the two states and was a hindrance in coming to each other ' s aid in time of war . King Muyeol assumed the Silla throne in 654 . Between 655 and 659 , the border of Silla was harassed by Baekje and Goguryeo ; Silla therefore requested assistance from Tang . In 658 , Emperor Gaozong sent an army to attack Goguryeo . In 660 , the Tang empire and the Silla kingdom sent their allied armies to conquer Baekje . The Baekje capital Sabi fell to the forces of Tang and Silla . Baekje was conquered in 18 July 660 , when King Uija of Baekje surrendered at Ungjin . The Tang army took the king , crown prince , 93 officials , and 20 , 000 troops as prisoner . The king and crown prince were sent as hostages to the Tang empire . The Tang empire annexed the territory and established five military administrations to control the region instead of Silla , which they painfully accepted . In a final effort , General Gwisil Boksin led the resistance against Tang occupation of Baekje . He requested military assistance from their Yamato allies . The Tang fleet , comprising 170 ships , advanced towards Chuyu and encircled the city at the Baekgang River . As the Yamato fleet engaged the Tang fleet , they were attacked upon by the Tang fleet and were destroyed . In 663 , the Baekje resistance and Yamato forces were annihilated by the Tang and Silla forces at the Battle of Baekgang . Subsequently , Prince Buyeo Pung of Baekje and his remaining men fled to Goguryeo . After the conquest of Baekje in 660 , the Tang and Silla forces planned to invade Goguryeo . In 661 , the Tang forces set off to Goguryeo . As the Tang army advanced with 350 , 000 troops , Silla was requested to provide only supplies during this expedition . The Tang army besieged Pyongyang , Goguryeo ' s capital , for several months until February 662 , when it had to withdraw from the campaign due to the harsh winter conditions . In 666 , the Goguryeo dictator Yeon Gaesomun died and an internal struggle between his sons for power broke loose . This paved the way for an invasion by Tang and Silla . In 668 , the Tang had invaded Goguryeo and laid siege to Pyongyang . Soon thereafter , the Silla forces joined the battle against Goguryeo . That year ( 668 ) , the Tang and Silla forces conquered Pyongyang , which led to the conquest of Goguryeo . Over 200 , 000 prisoners were taken by the Tang forces and sent to Chang ' an . Aftermath . In 669 , the Tang government established the Protectorate General to Pacify the East to control the former territories of Goguryeo . A subordinate office was in Baekje . By the end of the war , the Tang empire had taken control over the former territories of Baekje and Goguryeo and tried to assert dominion over Silla . Large parts of the Korean Peninsula were occupied by the Tang forces for about a decade . However , the Tang occupation of the Korean Peninsula proved to be logistical difficult due to shortage of supplies , which Silla used to provide previously . Furthermore , Emperor Gaozong was ailing , Empress Wu took a pacifist policy , and the Tang empire was diverting resources towards other priorities . This situation was in the advantage of Silla , because soon Silla would have to forcibly resist the imposition of Chinese rule over the entire peninsula . War was imminent between Silla and Tang .