Battle of Mount Jupil

After the fall of Baekam Fortress on 1 June (7 July CE), the combined main force of Tang led by the Emperor Li Shimin began marching to Ansi Fortress. However, the march was slowed down for undescribed reason. When Tang forces finally reached the outskirt of Ansi Fortress on 20 June (21 July CE), they were challenged by the reinforcements of Goguryeo dispatched by the Supreme Commander (Dae-Makriji) Yeon Gaesomun. According to Tang's records, 150,000 soldiers of Goguryeo and Malgal filled area of 100 li² (≈ 10.5 km²) like red ants, and Li Shimin was frightened by the sight.[3][10][7]

Goguryeo commander Go Jeongeui, the prime minister of Goguryeo, ordered his assistant generals to stay defensive to maximize the attrition of Tang forces, and constantly send out troops to cut off Tang's supply line, which the action Li Shimin described as the worst scenario for Tang army. However, a young sub commander Go Yeonsu and his lieutenant Go Hyejin defied the commander's order and moved commanding troops closer, 40 li (≈ 13 km) away from Ansi Fortress. On 21 June (22 July CE), witnessing the opportunity, Li Shimin ordered Ashina She'er, the Duke of Bi, in hope to lure Goguryeo troops deeper. Ashina She'er and his 1,000 Göktürks cavalries clashed with Go Yeonsu in attempt for luring, but instead was defeated by Malgal cavalries and began to run away. However, the strategy remained in progress as Go Yeonsu chased down Ashina She'er and his remaining troops, and arrived at 8 li (≈ 2.5 km) southeast of the fortress. There, Goguryeo soldiers began the construction of base using the mountain as a natural fortification, facing Tang forces located at the north mountain.[3][1][7][15][2][16]

During the night, generals were summoned for meeting. Zhangsun Wuji reported high morale of Tang forces, and everyone was optimistic of the situation. On the other hand, Li Daozong, the Prince of Jiangxia, suggested to avoid direct contact and dispatch 5,000 strike troops straight to Pyeongyang, the capital of Goguryeo, which Li Shimin did not reply. Instead, the emperor ordered Li Ji and 15,000 soldiers and cavalries to set base on the west hill, while Zhangsun Wuji and Niu Jinda and their 11,000 elite troops will position at the northside of the mountain; they will advance once the fight begin, and move through valleys so they will be undetected during attempt to surround Goguryeo forces. The emperor and his 4,000 soldiers and cavalries will remain at the mountain. Tang will launch a joint attack upon signals from drums and flute.[1][2][17][18]

On the morning of 22 June (23 July CE), Go Yeonsu noticed Li Ji's army was lonely positioned, thus hurried massing the troops for fight. Once ready, Goguryeo divided the army into two group, and charged at Tang forces commanded by Li Shimin and Li Ji.

The battle was joined on 23 June when one of the Goguryeo generals, Go Yeon-su, sighted that the Tang army was seemingly smaller and had its lines thinly held. With this, he decided to attack with the cavalry and attempt to dislodge the large Tang army in front of his army. What he did not know is that it was actually a ploy by the Emperor Taizong to lure the Goguryeo army into a trap as earlier planned by his subordinates. The Goguryeo cavalry led by Go Yeon-su charged across the plain dominating the battlefield at the foot of Jupil Mountain just south of Ansi, towards the Tang infantry that was awaiting them in a shield wall formation. The other general, Go Hye-jin, followed suit and also joined the attack. Despite mass projectile shots from archers, crossbowmen and catapults just within the Tang formation, the cavalry charge kept up its momentum until it finally impacted the shield wall with many Tang soldiers dead in the process as the cavalry charged across. But it soon came as a shock for both generals that behind the long shield wall of the Tang army were massive and dense amounts of infantry with Emperor Taizhong and his entourage far behind observing the scene, and yet despite it they continued the assault on the Tang army. The massive Tang battle line thus slowly developed from a straight line into a U-shaped formation as its army purposefully gave way to the Goguryeo attack, giving the Goguryeo a false sense of victory at this stage.

When Emperor Taizhong finally deemed the time right to spring his trap, he ordered to sound the drums and horns to signal the flanking attack by Zhangsun Wuji and other generals with a 11,000-strong cavalry detachment from behind hidden in a hill north of Mount Jupil until now. In what seemed to be a Cannae-like event, the Tang cavalry finally closed in into the rear of the Goguryeo cavalry, hitting it through its rear. The Tang army started pushing and tightening the encirclement even as the two Goguryeo generals tried to rally their men to fight on, but the one-sided mass slaughter continued. Seeing that they could not salvage the situation, the two Goguryeo generals tried as best as they could to get what was left of their force out of the encirclement to rejoin the main body of their army across the river to the south, but 30,000 of their men died trying to do so. Worse came when they found out that the bridges in the river that leads them back to the main body of their army were already torn down by the Tang commander Zhangsun Wuji under orders from Emperor Taizhong who already foresaw the event. Finding no other way to rejoin the main body of their army, the two generals rallied whatever remained of their force northwards to the peak of Mount Jupil with the Tang army in hot pursuit. The Tang army then caught up and encircled the remaining Goguryeo force atop Mount Jupil once again.[17] In an attempt to save their beleaguered comrades atop Mount Jupil, the main body of the Goguryeo army finally joined the battle by fording the river and attacked the Tang army. However, soon they soon found themselves counterattacked on three sides by the Tang army, having failed to reach their comrades atop the mountain.

Seeing that all was lost, Go Yeon-su and Go Hye-jin surrendered what remained of their command that was stranded atop the mountain, now reduced to just 3,800 wounded and famished men. What remained of the Goguryeo army that had not yet surrendered, that is, the force that tried to save their comrades atop Mount Jupil but failed, is now also subjected by attacks of the Tang army on three sides, pummeling it into a pulp until they finally retreated and dispersed southwards,[8] only to be chased down by the pursuing Tang army with most of them dead during the chase and captured 33,000 Goguryeo soldiers prisoner.[17] Among these, the Tang forces sent 3,500 officers and chieftains back to China, executed 3,300 Mohe troops, and eventually released the rest of the ordinary Goguryeo soldiers.[17] As part of the war spoils taken from the enemy, the Tang army captured from the vanquished Goguryeo army 50,000 horses, 50,000 cows, and 10,000 metal body armor.

Releasing of the prisoner in the middle of the war is unlikely to happen. It is more likely to be used for exaggeration of Li Shimin's victory for propaganda purpose. Goryeo military casualty was much lower than what Tang described, and continued its fight throughout the Battle of Ansi. This can be supported by historical records.

• The commander of Goryeo Army was Go Jeongeui, whose title was Dae-Daero (Great Daero), 1st class government figure similar to the prime minister.[19]
• The title of Go Yeonsu and Go Hyejin was Yoksal, regional governors and sub-commanders of Goryeo Army who were recorded because they were captured.[20]
• Seized armor count is much lower than surrendered troops.
• Both Old & New Book of Tang skips all event occurred between the Battle of Mount Jupil to the Siege of Ansi Fortress. Li Shimin likely deleted his defeats on the field.
• In July, Tang army relocated its position to the east of Ansi for undescribed reason, and Li Shimin ordered to mark dead Tang soldiers to carry back to the mainland—indicating heavy fight occurred after the Battle of Mount Jupil, or Tang actually suffered significant casualty at the Battle of Mount Jupil.[5]
• Zhangsun Wuji mentions that there are 100,000 Goryeo troops remaining in the Ansi Fortress prior to the siege.[21]
• Tang Army finally relocated its position near the fortress, and began the siege in August 645.[22] If there was a decisive victory per Li Shimin claim, then there is absolutely no reason for halting the attack on the fortress for nearly two months and waste supplies.

• Li Shimin exaggerated his victories. Since 10,000 killed and 5,000 armors are from Old Book of Tang, which was edited by Li Shimin himself, the actual dead is likely less than 10,000.

The two Goguryeo generals, Go Hye-jin and Go Yeon-su, then defected to the Tang and were thus given imperial Chinese royal titles by Emperor Taizhong, and they later on gave advice to the Emperor on his campaign to subjugate Goguryeo.

After the battle, the Emperor officially began the siege of the city that would go on for three months before being lifted due to the onset of bad weather and dwindling of supplies on part of the besiegers. Although the siege was abandoned and hence the city was saved, this disaster of a battle greatly consumed the already very limited manpower pool of Goguryeo so much that it would have a negative effects on its conduct of war for the rest of the conflict, which after years of battles and sieges would eventually result to the end of its existence and the conquest of most of its territory including its capital Pyongyang by the Tang with the rest going to another Korean kingdom, Silla, in 668 AD.

The 2018 South Korean film The Great Battle featured this battle as its first scene.[23]