[Lee Sang-hoon's Korean History] The Mystery of the Battle of Bukhansanseong Fortress

Professor Sang-Hoon Lee, Korea Military Academy

After the fall of Baekje in 660 and the rise of the Baekje restoration movement, Silla focused on suppressing this movement. As a result, a power vacuum emerged along the northern border of Silla.

Goguryeo did not miss this opportunity. Leading Malgal troops, they launched an attack on Bukhan Fortress (北漢山城), a strategic stronghold of Silla in the Han River basin. This is known as the Battle of Bukhan Fortress in 661. Goguryeo General Noe Eumshin (惱音信) attacked Bukhan Fortress alongside Malgal General Saenghae (生偕). Using siege engines called pocha (抛車), they broke down the fortress walls and buildings, pushing the defenders to the brink of collapse. However, surprisingly, Goguryeo withdrew citing a natural disaster.

The Samguk Sagi records the situation as follows:

"At that time, there were only about 2,800 men and women inside the fortress. The fortress lord Dong Tacheon (冬陀川) encouraged the children and elderly to bravely resist the powerful enemy for about twenty days. However, as food ran out and strength waned, they earnestly prayed to heaven. Suddenly, a great star fell into the enemy camp, followed by thunder, lightning, rain, and the earth shaking. The enemy, suspicious and fearful, lifted the siege and retreated."

Goguryeo troops were stationed to the west of Bukhan Fortress, and Malgal troops to the east, surrounding the fortress from both sides. At that time, Silla was not in a position to send reinforcements. Bukhan Fortress had to withstand the attack from the Goguryeo-Malgal allied forces on its own.

The Goguryeo-Malgal allied forces used catapults to break the fortress walls. Food inside the fortress was depleted, and the fortress was on the verge of falling. Those inside could only pray to heaven.

At that moment, a great star suddenly appeared and fell into the Goguryeo-Malgal camp, accompanied by thunder, lightning, and a heavy rainstorm. Due to this unexpected natural disaster, the Goguryeo-Malgal allied forces lifted the siege and withdrew.

According to the Samguk Sagi record...

Bukhan Fortress, Silla’s strategic stronghold in the Han River basin, surrounded west by Goguryeo troops and east by Malgal troops

Allied forces breaking walls with catapults, withdrawing hastily due to natural disaster just before fortress fell

If we follow the Samguk Sagi record literally, it can be said that despite being on the verge of collapse, Goguryeo forces retreated simply because of a natural disaster.

Bukhan Fortress is identified as Achasan Fortress (峨嵯山城) in Gwangjin-gu, located on the northern bank of the Han River. At that time, about 2,800 men and women, led by fortress lord Dong Tacheon, were inside. The mention of men and women suggests that a significant number of civilians were included. Also, since Dong Tacheon was promoted from Dae-sa (大舍) to Dae-nama (大奈麻) as a reward for successful defense, it seems there was no large-scale Silla relief force.

The most puzzling aspect of the Battle of Bukhan Fortress is the outcome. The Goguryeo-Malgal allied forces supposedly lifted the siege and withdrew solely due to a natural disaster. The battle took place around late May in the lunar calendar, which is around the monsoon season. Therefore, heavy rain showers are not surprising but rather expected.

It is hard to understand logically that the allied forces would retreat just because of a natural disaster without Silla reinforcements arriving. Considering the strategic importance of Bukhan Fortress, this cannot be overlooked. What was the real reason for the Goguryeo-Malgal allied forces’ withdrawal? What did they suspect and fear?

Bukhansanseong Fortress [Photo by Yonhap News]

Regarding this, the Samguk Yusa reveals a slightly different circumstance.

"From May 11 to June 22, the Silla army was in great peril. The king heard the news and consulted with his ministers. 'Is there any strategy?' The king hesitated and could not decide. At this time, Kim Yushin rushed in and said, 'The situation is critical and human strength alone cannot prevail; only divine arts (神術) can save us.' He then built an altar on Seongbusan (星浮山) and practiced divine arts. Suddenly, a light the size of a large jar appeared on the altar and flew north like a star."

The following passage describes how a light suddenly shone from the southern sky of Bukhan Fortress and broke about 30 siege stones (砲石). Although quite legendary, this account is presumed to be based somewhat on fact. Gyeongju is located south of Bukhan Fortress, and Bukhan Fortress lies north of Gyeongju. In other words, the light flying north from Seongbusan in Gyeongju appeared from the south of Bukhan Fortress. The direction of the light matches perfectly. This suggests a connection between Kim Yushin’s divine arts performed at Seongbusan in Gyeongju and the event at Bukhan Fortress.

The light that suddenly appeared at Bukhan Fortress broke about 30 siege stones. Siege stones were siege weapons used to attack fortress walls or towers, also called catapults or balsokcha (發石車). The sudden light destroyed over 30 Goguryeo catapults, and it is said that Goguryeo soldiers collapsed to the ground and only woke up much later.

Based on the Samguk Yusa...

Kim Yushin’s divine arts strategy in a crisis... dispatching catapult units to turn the tide

Long-range siege weapons flying like stars destroyed over 30 Goguryeo catapults

The Samguk Sagi states, "Suddenly a great star appeared and fell into the enemy camp, followed by thunder, lightning, rain, and the earth shaking." This can be understood as Goguryeo forces being physically shocked by the sudden light. In other words, this light clearly had the power to deliver a physical impact to a specific target. Moreover, since over 30 catapults were destroyed, it is natural to interpret this not as a single light but as multiple instances of light striking repeatedly.

Is it possible to view the sudden light not as divine arts but as an actual weapon? At that time, Goguryeo’s catapults would have been positioned at a certain distance from Bukhan Fortress walls. Therefore, a weapon capable of attacking Goguryeo’s catapults from a corresponding distance would have been a similar type of long-range projectile weapon.

The name "Seongbusan" itself means "mountain where stars float." This suggests that a long-range projectile weapon flying like a star was tested there.

During the reign of King Munmu of Silla, weapon performance improved significantly. In this process, excellent weapons such as large crossbows (노, 弩) were developed. Just before the Silla-Tang War, Silla could produce large crossbows with a range of 1,000 bo (步). Moreover, Silla had a special siege unit called Saseoldang (四設幢), consisting of Nodang (弩幢, crossbow unit), Unjedang (雲梯幢, ladder unit), Chungdang (衝幢, battering ram unit), and Tuseokdang (石投幢, stone-throwing unit).

In this context, it is presumed that Silla’s catapult development level at that time was also considerable. If over 30 Goguryeo catapults were destroyed and Goguryeo soldiers collapsed from shock, it is likely that Silla newly deployed large catapults specifically to attack the enemy’s catapults.

"Divine arts" means a miraculous stratagem. Kim Yushin seems to have devised a plan that ordinary people would not have thought of while Bukhan Fortress was under siege. Also, the phrase "the light flew north like a star (乃星飛而北去)" appearing above the altar on Seongbusan indicates that it was not an actual star.

The long-range weapon capable of destroying the enemy’s catapults is presumed to be a catapult as well. Kim Yushin, unable to dispatch a large relief force, devised a plan to break the siege of Bukhan Fortress by sending a small catapult unit. At that time, it was nighttime and raining, making it difficult for Goguryeo to accurately assess the movement or scale of Silla’s reinforcements.

The Goguryeo-Malgal allied forces’ food supplies were also poor due to the prolonged siege. Goguryeo had previously attacked Bukhan Fortress in 603 but withdrew due to Silla’s relief forces.

Since it was the monsoon season and raining at night, it was easier to avoid enemy surveillance. However, there was no capacity to send a large relief force. Under these circumstances, selecting and dispatching a balsokcha (siege engine) unit to Bukhan Fortress seems to have been proposed and led by Kim Yushin.

Ultimately, Goguryeo appears to have mistaken the arrival of Silla’s catapult unit in bad weather for a large relief force and withdrew.

It is quite plausible that a thunderstorm accompanied by lightning and heavy rain occurred during the Battle of Bukhan Fortress. However, the sudden appearance of a great star or light from the south that directly attacked the Goguryeo-Malgal allied forces should be interpreted differently. This can be seen as the deployment of Silla’s catapult unit, and the idea was conceived by Kim Yushin.

Thus, although Kim Yushin did not personally participate in the Battle of Bukhan Fortress in 661, related anecdotes were exaggerated and glorified, leaving records in historical texts.

Professor Sang-Hoon Lee, Department of Military History, Korea Military Academy