In the autumn of 645, one of the most powerful rulers in the world turned his army away from a fortress he could not conquer.
He was Li Shimin, known to history as Emperor Taizong of Tang. A veteran commander who had helped establish his dynasty, he had defeated formidable rivals and extended his empire’s influence across much of Asia. He had marched against Goguryeo expecting another triumph.
Instead, Goguryeo remained standing.
The fortress was Ansi. Its commander’s name was not preserved in the principal historical accounts. Yet what he and his people accomplished became one of the most enduring stories in Korean history: a defending army and community holding their ground until an emperor’s invasion could go no farther.
For readers unfamiliar with Korea’s ancient past, the story begins beyond the borders of the modern Korean Peninsula.
Goguryeo was a major kingdom extending across northern Korea and substantial parts of what is now northeastern China. Its capital was Pyongyang, but its western defenses lay far beyond the Yalu River, in Liaodong. To its inhabitants, this region was home—a landscape of farms, settlements, military strongholds, and routes connecting generations of Goguryeo life.
For centuries, the kingdom had maintained its own political identity while competing and negotiating with Chinese dynasties, northern peoples, and neighboring Korean states. Its independence faced a particular danger whenever a powerful empire united China and turned its attention eastward.
That danger had arrived before.
In the early seventh century, the Sui dynasty repeatedly invaded Goguryeo. In 612, the Goguryeo commander Eulji Mundeok inflicted a devastating defeat on a Sui expedition, remembered in Korea as the victory at the Salsu River. When Tang later prepared its own invasion, Goguryeo still carried the experience of those earlier wars.
Its people knew what imperial conquest meant. They also knew that an invading empire could be resisted.
Relations with the newly established Tang dynasty initially allowed room for diplomacy. But as Tang power expanded, the pressure on Goguryeo increased. The kingdom strengthened its western defenses, while the Tang court gathered information about its territory and military preparations.
In 642, the powerful Goguryeo statesman and military leader Yeon Gaesomun seized power in a violent coup. Taizong invoked the killing of Goguryeo’s king as a justification for intervention. Appeals for assistance from Silla, Goguryeo’s rival on the peninsula, added another dimension to the conflict.
For the people defending Goguryeo’s frontier, however, the immediate issue was clear: a foreign emperor was preparing to enter their country and impose his will by force. National Institute of Korean History, “The Goguryeo–Tang War”
Taizong committed an enormous military effort.
Tang armies approached by land and sea. Experienced generals commanded the expedition, supported by siege equipment, transport ships, and supply arrangements. The emperor himself joined the campaign.
The exact number of troops remains debated. A Korean historical overview estimates total mobilization at more than 300,000. A 2022 study published in the Korean military-history journal Gunsa, using recorded grain supplies to estimate troop strength, proposed a minimum of approximately 403,000 to 422,000 soldiers, including naval forces.
These are scholarly estimates, not a surviving, complete military roll. Nor should the soldiers alone be confused with everyone whose labor sustained the expedition. Shipbuilding, transport, construction, and provisioning extended the demands of war far beyond the battlefield. Choi Jin-yeol, “Tang Taizong’s Expedition against Goguryeo and the Strength of the Tang Army”
Goguryeo therefore faced a formidable concentration of imperial power.
The opening campaign demonstrated how dangerous that power was.
Tang forces crossed the Liao River and attacked Goguryeo’s western strongholds. Their greatest early success was the capture of Liaodong Fortress, a central pillar of the kingdom’s defenses.
The Samguk Sagi, the Korean history compiled in the twelfth century, describes how Tang troops exploited a strong wind to set a tower ablaze. Fire spread through the fortress, and the attackers assaulted its walls. The defenders fought but could not hold.
The record lists heavy losses among soldiers and civilians, along with the seizure of a vast quantity of grain. Other strongholds also fell.
Goguryeo’s victory in 645 did not emerge from an uninterrupted sequence of successes. It came after burning fortresses, lost supplies, captured inhabitants, and severe military reverses. That is what gives the resistance at Ansi its significance. Samguk Sagi, “The Fall of Liaodong Fortress”
As Tang forces approached Ansi, Goguryeo dispatched a large relief army under Go Yeonsu and Go Hyejin. The historical accounts put its strength at 150,000, including Malgal troops.
The relief effort ended in disaster.
According to the Samguk Sagi, the experienced Go Jeongui advised against a direct confrontation. He urged the commanders to prolong the campaign and disrupt the invaders’ supplies. The relief army instead advanced into battle, where Tang maneuvering and attacks from multiple directions broke its resistance. Its commanders surrendered.
Ansi’s defenders now faced the imperial army after the force sent to rescue them had been defeated. Samguk Sagi, “Go Yeonsu Advances”
They continued to fight.
Ansi mattered because an invading army could not safely treat every unconquered fortress as an obstacle to be walked around. A defending garrison left behind could threaten supply convoys and routes of retreat. Advancing farther into Goguryeo while leaving hostile forces in the rear carried serious risks, especially with the emperor present.
The resistance of one fortress could therefore affect the prospects of an entire expedition.
Tang troops brought siege engines against the walls. Stones battered defensive structures. Repeated assaults tested the garrison.
The defenders repaired the damage. Where walls or towers were broken, they erected wooden barriers. As the attackers changed their methods, the defenders adjusted their positions.
These brief entries in the historical record reveal sustained organization. A fortress could endure such fighting only through the combined efforts of its commanders, soldiers, and supporting population. Combat and repair had to continue together.
Most of those people left no names in history. Their work survives in the record of a wall repeatedly damaged and repeatedly defended.
As the siege continued, Tang forces undertook an immense construction project: an earthen mound beside the fortress.
The mound would give the attackers an elevated position overlooking the defenses and a means of approaching the walls. The Samguk Sagi describes approximately sixty days of work.
Then part of the mound collapsed, damaging a section of the fortress wall.
It was a dangerous moment for Ansi. Yet the defenders seized the opportunity. Goguryeo troops emerged, occupied the mound, and established a defensive position on it. Tang attempts to recover it failed.
The attackers had spent weeks constructing an advantage that the defenders took from them.
Ansi’s resistance involved more than endurance. Its troops responded decisively when an opportunity appeared, turning a crisis at their own walls into a setback for the besieging army. Samguk Sagi, “Tang Constructs an Earthen Mound”; National Institute of Korean History, Ansi campaign records
Meanwhile, the season was changing.
The army that had entered Liaodong in spring was still campaigning as autumn advanced. Soldiers needed food. Horses needed fodder. Transport demanded labor and protection. Every additional day before Ansi consumed resources and narrowed the time available for further operations.
On the eighteenth day of the ninth lunar month, Taizong ordered the withdrawal.
Ansi had held. Goguryeo’s government remained intact. Pyongyang had not fallen. The emperor had captured important strongholds and defeated a major relief army, but he had failed to turn those achievements into the submission of the kingdom.
Goguryeo had won the strategic contest of 645. National Institute of Korean History, “The War between Goguryeo and Tang”
The journey home exposed the expedition to further hardship.
The Samguk Sagi records that mud in the Liao marshlands obstructed horses and wagons. Thousands of men were assigned to cut vegetation and fill a passage; wagons were used to bridge deeper water. Cold and snow compounded the difficulties of withdrawal.
Exactly how many soldiers returned alive cannot be established from the surviving evidence. The transmitted campaign summary gives approximately 2,000 Tang combat deaths, a figure that cannot simply be subtracted from a modern estimate of the expedition’s total strength to produce a reliable survivor count. Korean historical commentary has questioned whether the recorded losses adequately reflect the damage suffered.
The same transmitted record reports the death of seven or eight out of every ten warhorses—an immense loss to an army dependent on mobility and transport. That figure concerns horses, not the proportion of soldiers killed. Samguk Sagi, the retreat through the Liao marshes; Academy of Korean Studies, “The Battle of Ansi”; Korean History Database, campaign-loss record
Taizong did not immediately abandon his ambition.
He ordered further attacks and prepared another major expedition. Goguryeo continued to resist. When he died in 649, he had still not conquered the kingdom.
The Samguk Sagi preserves a final instruction: he ordered the Liaodong campaign discontinued in his testament.
The emperor who had personally led the invasion thus ended his life without achieving its objective, leaving an order to halt the renewed campaign. His successor would eventually resume the war, but Taizong’s own struggle with Goguryeo had ended in failure. Samguk Sagi, “The Death of Tang Taizong”
Goguryeo would fall in 668, under a different Tang emperor and profoundly changed circumstances. Prolonged warfare, cooperation between Tang and Silla, and division within Goguryeo’s leadership would combine to destroy the kingdom.
That later defeat does not diminish what its people accomplished in 645.
They had faced an emperor at the height of his military prestige. They had suffered grievous losses and continued to resist. At Ansi, disciplined defense, relentless repair, and timely counterattack brought the imperial advance to a halt.
The defenders did not need to destroy every invading soldier to win. They needed to deny the emperor the conquest he had come to achieve.
They did.
In the autumn of 645, Taizong went home.
The people of Ansi remained on their own ground.
