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.
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.
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 logistically difficult due to shortage of supplies, which Silla used to provide previously. Furthermore, Emperor Gaozong was ailing, so Empress Wu took a pacifist policy, and the Tang empire was diverting resources towards other priorities. This situation favored 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.
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