The Sui-Tang Invasions of Goguryeo

The kingdom of Goguryeo was one of the three great Korean kingdoms of that country’s Three Kingdoms period (57 BC – AD 668) that first arose in the 1st Century BC. The state expanded well beyond the Korean peninsula and soon encountered several Chinese dynasties in succession. A series of wars erupted between them even as China splintered in AD 220, ending the 400-year Han dynasty. The period that followed, known as the Age of Fragmentation, with interludes, lasted until 589 when the new Sui dynasty reunified the broken country.

The Sui quickly flexed its muscles on the neighboring powers, including Goguryeo. The first conflict between the two states occurred because of a limited invasion by Goguryeo in 597. This first war ended a year later in embarrassment for the Chinese. However, a diplomatic arrangement saved face for both sides. Years later a new emperor, Yang Guang, took the throne as Emperor Yang and initially did not think of Goguryeo. When Sui diplomats, and the emperor himself found diplomats of Goguryeo at the court of the Eastern Turks in 607, this changed.

Judged unacceptable by the Sui they demanded the King of Goguryeo, Yeongyang, come to the Chinese court to give account. When the king refused, the Sui began preparing for an invasion. On February 8th, 612 the Sui army, numbering over 600,000 men, invaded Goguryeo.

Both Emperor Yang and King Yeongyang had been aware that siege warfare would be inevitable in the coming conflict. The Sui had great siege experience, and the Chinese siege arsenal was formidable. However, Goguryeo also had siege experience, with the fortress line in what is now Manchuria was the kingdom’s greatest defense against external threats. After crossing the Liao River, the Chinese realized they would be forced to break the fortress line before being able to strike at the capital of Goguryeo at modern Pyongyang.

The fortress of Ryotongsong or Liaodong on the banks of the river was the primary target of the Sui siege effort. Ryotongsong was the key to the entire Liao River Valley and was judged ‘must take’ regardless of whether the other fortresses were bypassed or not. However, Emperor Yang discovered that other key fortresses would have to fall as well and so decided to settle down and prepare to take down the fortress line. He would command the siege of Ryotongsong personally while leaving the other sieges to his lieutenants.

The Chinese had several siege weapons at their disposal, for our purposes we will cover two in this article and another two in the next part. One of the most wide-spread of China’s siege machines was the so-called ’whirlwind’ traction catapult (xuanfeng pao in pinyin) which gained its name from its ability to swivel 360 degrees. The xuanfeng has been called a ‘sniper rifle of a catapult’ because it was incredibly accurate. They were used mainly to take out other catapults or even enemy generals. The xuanfeng was also adaptable and could be modified to suit the situation. Typical variations include two leg base, four leg base, swivel battery, and cart-mounted.

The other siege weapon we will cover is the cloud ladder. Called yunti che in pinyin the cloud ladder was a cart with a triangular base on which a series of ladders, curved so as to clamp into each other, rested. They operated the device by use of a wheel that could extend the ladder and propping levers to control the angle. A counterweight dropped as the ladder extended, increasing the height. The yunti che used a combination of wooden wheels and iron ‘teeth’ to latch onto its target as it was extended. The weapon gained its name from the perception there was no limit to how high it could extend.

For comparison, we will also cover the Korean fortresses and their garrisons. Goguryeo had come to depend on its fortress line in Manchuria, and so great lengths were taken to ensure their defense. The fortresses themselves came in several types, but we will cover the most common type here.

This was the ‘half-moon’ fortress type, typically anchored between the banks of a major river and a nearby tributary. Ryotongsong was one such fortress. Part of what made the ‘half-moon’ fortresses so hard to crack was the presence of multiple walls outside the central citadel, which could reach as high as 6 meters, with a thickness of 3 meters. A system of ditches dotted by a line of towers, each fortified by stone protected the walls in turn. Siege countermeasures varied though a common defense against catapults was the use of makeshift bastions and wooden cages draped over the walls.

The garrisons of the fortress line were a mixture of dedicated local forces and other forces. The military system of Goguryeo allowed for the formation of strong regional bands of mainly professional soldiers grouped around powerful local aristocrats empowered by the king. The fortress garrisons were similar but tied to their respective strongholds. They were supported by bands of young men, in their early 20s, organized into religious-military ‘clubs’ called kyongdang. The leader of a garrison was called the songiu, fortress lord.

Emperor Yang had started his invasion from the beginning of the year on purpose, to give himself time to get through Manchuria before the rainy season would make warfare impossible. The Sui needed to reduce or starve the garrisons, or they would have to retreat.

At first they made good progress (even expanding operations), and one key fortress, Yodong, began to make overtures of surrender. However, when the offer came it had to be relayed to the emperor at Ryotongsong, and he had to approve it first. By the time word got back to Yodong, the garrison had resupplied and reinforced. Word spread quickly and using this flaw in the Sui command structure the fortress line was able to hold out.

As June began the Sui sent out a detached force to race ahead to Pyongyang to force an end to the war by surprise. Emperor Yang decided to continue siege operations in the meantime, scaling back his attacks to small scale skirmishes. However, on August 27th word arrived the Premier of Goguryeo, Eulji Mundeok, had destroyed the Sui forces at the Salsu River around the 15th of that month. With his last hope destroyed and the first rains beginning to fall the emperor ordered a withdrawal. Several sieges ended in confusion and chaos; Premier Eulji perished in one such withdrawal at Sinsong fortress.

However, this was not the end of the matter. Emperor Yang had become obsessed with Goguryeo and sought to bring the kingdom down once and for all. Even as flood, drought, and disease sparked revolts across the empire the Sui war machine prepared to return the following year.

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Sui Yang di, second emperor of the Sui dynasty. Yen Li-pen [Public domain], via Wikimedia Commons.

The events of the previous invasion had barely fazed the Sui monarch. Unwilling to accept his defeat, Emperor Yang announced a fresh attack in January, 613. The Chinese took precautions to avoid the problems that had plagued them in 612: extensive measures were chosen to make sure the army remained in supply across Manchuria. Emperor Yang also granted his commanders almost total autonomy, so the Korean garrisons would not be able to exploit command delays. However ominous rumblings of rebellion, as well as a horse shortage, cast a pall over the expedition.

On March 30th, the third invasion of Goguryeo was launched. On the Korean side, King Yeongyang remained confident the fortress line would be able to hold out, but still took added precautions across the fortress line and in the mountain fortresses near Pyongyang.

Sui invasion forces crossed into the Liao River Valley in Late May. The army then broke off into three columns with the emperor mandating that Ryotongsong and Sinsong must fall. The third column was directed to make an overland strike at Pyongyang in conjunction with naval forces coming up from the Shandong Peninsula.

The best-known siege of this war is the siege of Ryotongsong. Commanded by the Emperor in person the besiegers were prepared to use the full force of their arsenal against the garrison. For our purposes, we will cover another two siege machines leading on from last week.

In addition to the already mentioned xuanfeng catapult and yunti che ladder, another weapon employed by the Chinese was the ‘four-footed’ traction catapult. Called sijiao pao in pinyin it gained its name from the four large wooden columns that formed its legs and provided the corners of the frame. The sijiao was largest of the Chinese traction catapults, standing a good four times the height of the average adult man. Its primary purpose was the reduction of walls, a task it did well. Unlike the xuanfeng, the sijiao was one-directional. However, it could be made mobile through being mounted on a cart (though it was never fired while mounted). The weapon could be adapted by increasing or decreasing the number of bundled rods that comprised its throwing arm. Typical variations were: Five-rod arm, nine-rod arm, ten-rod arm, and thirteen-rod arm.

The other siege weapon we will cover is the assault cart. Called chong che in pinyin the assault cart was the Chinese version of the well-known siege tower. Like any siege tower, the chong che was a makeshift tower built on a wheeled cart. There was no typical chong che because each was constructed to fit the circumstances of its specific siege, thus allowing for much adaptation and flexibility. One famous chong che type was the so-called ‘cloud bridge’, a massive 100-meter tall weapon armored by layers of cowhide on the sides. The top level contained many bags of water to douse fires.

In the time between the sieges of 612 and 613, no significant changes took place on the Korean side of the war. However, we will cover in brief the importance of natural obstacles in Goguryeo defense strategy. As already mentioned the Koreans built their most common fortress type anchored between the banks of a river and a nearby tributary, which provided a natural defense line. They built other fortifications on the sides of or anchored onto a mountain. A network of such mountain fortresses protected Pyongyang, which made the city nearly impenetrable.

Historical records also point out that sometimes the garrisons of a particular citadel would try to extend the walls to take advantage of a nearby natural defense. Ryotongsong was one such fortress. While already anchored on the Liao River the garrison successfully reached the walls to a nearby mountain (probably between 612 and 613), causing the Chinese endless frustration.

From the beginning, the Chinese assaulted the Manchurian fortress line with far greater force than the previous year. Sui military engineers unrolled a much greater part of their siege arsenal in the third invasion then they had in the second. They hit Ryotongsong the hardest of all. Emperor Yang knew the fortresses’ importance and ordered attacks on it from all four directions at once unceasingly.

However, the Korean garrison held on grimly. Each move made by the Chinese was countered by the garrison successfully. One celebrated episode was a mass assault on the walls by the Sui using yunti che assault ladders and battering rams. The attack was driven off by the Goguryeo garrison through the use of fire, burning them as the machines got caught in the ditch system in front of Ryotongsong. Another incident involved a different tack. Emperor Yang decided to attempt to take the fortress through tunneling, but the garrison had been expecting such a move. As the Chinese siege engineers dug toward the walls, the defenders flooded the tunnels with water from the nearby river, drowning them out.

However, the Sui Emperor had another card to play. After the siege had been going on for a month, the emperor tried another ploy. He ordered the construction of an enormous earthen ramp along one corner of the fortress wall. To protect the engineers and laborers as they worked eight chong che manned by the best archers in the Sui forces were built and rolled out near the walls. This time the Chinese met with success. The defenders were forced to abandon the section of the wall threatened by the ramp under withering arrow fire. On July 20th as the Sui prepared to storm the walls of Ryotongsong disastrous news arrived from China. Yang Xuangan, the son of the late famous Premier Yang Su, had risen in revolt and threatened the imperial capital of Luoyang itself.

The situation had drastically changed. Emperor Yang suddenly found himself facing the possibility of being caught between a rapidly growing rebellion at home and the forces of Goguryeo at the front. He acted immediately, recalling the advance column and the naval arm and directing them to make all haste to the heartland. The entire war was called off, and the Sui withdrew with barely disguised hurry. King Yeongyang of Goguryeo and his generals could barely believe it, especially as false retreats were a well-known Chinese trick. However, the hurry in which the Sui evacuated and several defections led them to infer it was real.

Eventually, while the Sui would mount one more invasion the following year (614), the third attack on Goguryeo was the last major offensive. Yang Xuangan’s revolt had lit the powder keg of discontent, leading the Sui Empire to unravel and fall in 618. Goguryeo survived but just barely, and would go on to face the successors of Sui: Tang.


Armed and armored statue from the tomb of King Dongmyeong. yeowatzup [CC BY 2.0 (], via Flickr. 

Events had changed dramatically since 613. The Sui dynasty, ravaged by rebellion, had begun to shatter in 615 and by the following year it became apparent the Sui could not regain control (at least with their current leadership). In late 617 a measure of peace returned when Emperor Yang’s first cousin, Li Yuan, suddenly seized control of the capital and moved the emperor into retirement. He took the title Prince of Tang and sought to quell the chaos as regent for a puppet child-ruler. However, the former emperor was assassinated soon after. Li Yuan then took the imperial throne for himself, becoming Gaozu of Tang in 618. Meanwhile in Goguryeo, King Yeongyang died and was succeeded by his younger half-brother as King Yeongnyu.

The two empires would spend the following years in cordial relations with each other. As the Tang sought to unify China under their rule, they tried to reconcile with the Korean kingdom on an equal basis, exchanging niceties and such forth.

In 626 an internal coup within the Tang imperial family resulted in the abdication of Gaozu in favor of his most able son, the Prince of Qin, Li Shimin. As Emperor Taizong, he embarked on a whirlwind campaign of military conquest. By 640 Tang power had become so great that even the Turkic khaganates had been humbled, and Taizong recognized as Great Khan.

For Goguryeo, the years saw the ascendancy of a new leader. Tang excursions had resulted in the construction of a new defense line in Manchuria, the Cheolli Jangseong (Thousand-Li Wall). The task of overseeing construction fell to a rising young military star, Yeon Gaesomun, who came from a prestigious background. Because of internal friction in Goguryeo between the civil and military officials Yeon launched a coup in 642 and killed the king. He placed a puppet ruler in power, King Bojang and assumed dictatorial powers as Grand Premier.

Yeon’s actions played into Chinese ambitions. Emperor Taizong wanted to succeed where Yang had failed, including against Goguryeo. The assassination of Yeongnyu gave him the perfect pretense for war. In April, 645 the Tang army invaded Goguryeo by land and sea.

Both the Chinese and the Koreans had learned valuable lessons from the Sui invasions. The Tang were well aware that Emperor Yang’s primary problem had always been the issue of keeping his massive armies supplied. Taizong launched a much smaller force of 113,000 men in total, a much easier number to manage logistically. The Chinese also had a far larger naval force than before, including river ships that could navigate the great rivers of Manchuria. Naval support not only gave them more flexibility tactically, but it also gave the Tang a steady source of supply for the first phase of operations (before moving too far inland).

Perhaps the most significant difference between the Tang and the Sui in approach was that Yang meant only to ‘chastise’ Goguryeo. Taizong intended to annex it outright. On May 1st, the Tang vanguard crossed the Liao River trailed by the naval force and Taizong’s personal cavalry squadrons.

Before moving on, we will say a few words about another two Chinese siege machines. The ‘crouching tiger’ traction catapult or hudun pao in pinyin was a medium catapult that roughly fell between the xuanfeng and sijiao in size and firepower. The weapon gained its name from the resemblance of its frame to that a tiger in a crouch. Like the sijiao, it was one-directional, but like the xuanfeng it was also mobile and could be fired on the move. One purpose of the hudun was to throw incendiary projectiles, which could disperse the enemy in a pinch. However, the hudun was adaptable. Typical variations include stationary, cart-mounted, three-rod arm, and seven-rod arm.

The other weapon we will cover is the chao che. Meaning ‘nest cart’ in English the chao che was one of the oddest Chinese siege machines. Put simply, the chao che was an eight-wheeled cart on which a tall pole, or a pair of tall poles was fixed. By use of a pulley wheel a small house-like box went up and down the pole (or poles). What the use of this device was is unknown, but its most probable use was as a mobile look-out tower, an artillery spotting platform, or a command platform.

On the Goguryeo side, things had changed as well. Yeon Gaesomun had been preparing for a Tang invasion since 643, once it became apparent that Taizong was planning an attack. Yeon placed much of his faith in the hardiness of his soldiers and garrisons and the impregnability of the Cheolli Jangseong. However the ‘thousand-li wall’ was still unfinished in 645. Yeon had realized this earlier, and so had focused on completing what he judged to be the most relevant sections of the wall, large fortress cities that formed the cornerstones of the network.

For our purposes in this article, we will focus on these fortress cities, Ansisong in particular. Much like the Liao River Valley line used by Eulji Mundeok and his successors during the wars with the Sui, the fortress cities of the Cheolli Jangseong anchored on natural features. Mountains were most common of all, given the most of the wall crossed. However for Ansisong and other fortresses like it the walls played a greater role. The walls of the Cheolli Jangseong were much larger than the walls of the old system and made from stone and packed earth reinforced with clay. Extra defensive measures similar to the ones used by the river fortresses were also used, such as ditches and secondary walls. Permanent stone bastions and forts built into the wall were all well-known. Protecting the city itself was smaller stone walls dividing it into sections.

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Map and prominent actions of the First Goguryeo-Tang War. By Historiographer at en.wikipedia [Public domain], from Wikimedia Commons

The Korean forces found themselves caught off guard almost as soon as the invasion began. The Tang vanguard had crossed the Liao River further north than predicted and put the fortress of Gaemosong under siege on May 16th. It fell only eleven days later. On June 7th Ryotongsong, the fortress that had defied the Sui twice, was put under siege. Emperor Taizong arrived a few days later. To the shock of Goguryeo, the great citadel fell on June 16th when the Chinese carried out a massive fire attack on the interior of the fortress. The advance kept up at a quick pace, leaving Yeon Gaesomun having to play a game of catch up. On June 27th, the fortress of Baegamsong fell without a fight, making the remaining citadels in the Liao River Valley inconsequential. Earlier the Chinese naval forces had landed a sizable contingent at the mouth of the Yalu River and captured Bisasong, the southernmost fortress-city of the Cheolli Jangseong. On July 18th, the main body of Tang troops arrived before the fortress-city of Ansisong, which was to become the most memorable siege of the war.

However, siege preparations were interrupted by a Goguryeo attempt to relieve the city. The Grand Premier had posted two governors, Go Yeonsu and Go Hyezin, in that region and ordered them to prevent the Tang from capturing Ansisong. Realizing the fall of that city would leave the interior open to invasion. However, neither of the two generals were skilled at war. Taizong routed the Koreans at Mt. Zhubi (as the Chinese dubbed it) and their nomadic allies (the Mohe, ancestors of the Manchus) in a two-day battle (July 20th-21st).

The siege itself now began in earnest. Much to Taizong’s dismay, and the cheer of Goguryeo, the walls and garrison of Ansisong held out against the first assault. A stalemate resulted within a month, and the emperor became impatient. He desired a quick campaign, and the longer Tang forces were held up at Ansisong, the less likely it looked that they would be able to clear Manchuria before the rains. The idea was floated to abandon the siege of Ansisong and take another fortress. However, Goguryeo resistance had stiffened considerably, and Taizong was aware the garrison commander, a Mohe dubbed Yang Manchen by tradition, at Ansisong could cut his supply lines if left unhindered.

The stalemate continued. In early October the Tang, realizing that time was running out, gambled everything on a large earthen mound that had been under construction for the past two months. However on the day of the assault, October 10th, the hill collapsed, and the Tang inexplicitly withdrew. Yang Manchen then took control of the hill, using it to reinforce the walls. In anger, Taizong ordered much of his army into the breach but on October 13th he ordered a withdrawal of all Tang forces. The first Tang invasion was at an end.

Eventually, the Tang would return. This victory significantly strengthened Yeon Gaesomun and his regime, and Taizong planned a second attack on Goguryeo. Before he could, the Tang Emperor passed away in 649, and invasion plans were called off until 660.

The failure of the first attack was the trigger for future conflict. Tang Taizong had never accepted his defeat at Ansisong and laid plans for future conquest. Border skirmishes and deep raids became the norm all along the northeast. Taizong’s death in 649 and the ascension of his son Li Zhi to the throne as Tang Gaozong had brought a respite. In Goguryeo the unyielding Grand Premier, Yeon Gaesomun, planned to take advantage of the lull. In 654 it was broken when a Goguryeo expedition threatened Tang client states on the steppe.

The Tang response, when it came, struck from an unusual direction. Emperor Gaozong knew that to take down Goguryeo would require an attack from more than one direction. To this end, he had continued his father’s diplomatic offensive in Korea, cultivating a relationship with the kingdom of Silla, Goguryeo’s most powerful rival on the peninsula. In 660 the King of Silla called for aid against Baekje, Goguryeo’s ally on the peninsula. Gaozong responded by sending an expeditionary force. Together the allied forces conquered Baekje, turning it into a Tang province. In an instant, the entire strategic balance of power in Northeast Asia had changed.

In 661 the Tang court moved against Goguryeo proper. For this campaign the leading commanders of the Baekje campaign, led by Su Dingfang, took command. In August 661 these forces were sent across the sea to Goguryeo, bypassing the Cheolli Jangseong, and landed. The emperor accompanied the expedition to oversee the war.

Tang and Goguryeo had both been preparing for this inevitability. During the years between 645 and 660 both the Chinese and the Koreans had gained much experience and fighting the other. These lessons proved invaluable. Territorial advances along the Liao River line had pushed the border further into Manchuria. An attack directly on Pyongyang thus became possible, even though the fortress cities remained a concern. Chinese naval forces continued to exercise almost total control of the waves. Logistical concerns had already been taken care of earlier as the supplies gathered for 649 were now used to supply the expedition. Also, Silla promised military and logistics support to the Chinese.

As before, the aim of the war was to annex Goguryeo. By capturing Pyongyang, the Tang hoped to force the surrender of the Grand Premier, thus ending the war. Coordination with Silla, and their leading general Gim Yusin, in particular, played a role in this. While the Chinese handled affairs in the north, it was the job of Silla to keep them tied down in the south and give support to the forces in former Baekje territory.

Before moving on, we will cover our last Chinese siege weapon. Perhaps one of the most famous siege machines in East Asia was the siege crossbow, or chuangzi nu in pinyin (meaning ‘little bed crossbow’). These weapons were initially oversized crossbows mounted on a table frame, thus gaining its name. By Tang times, they had become far more sophisticated and powerful. Tang chuangzi nu used double bows (facing in opposite directions), increasing the drawing power significantly. The uses of the chuangzi nu were varied and used to good effect both for offense and defense. Mobility was an important feature of these weapons and functioned in a similar role to later horse artillery in that way. Typical variations include single bow, double bow, stationary, mobile, and battery (multiple weapons mounted on the same frame).

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Yeon Gaesomun, dictator of Goguryeo. See page for author [Public domain], via Wikimedia Commons.

On the Goguryeo side of the war, preparations had been underway for some time. Yeon Gaesomun was no fool and knew that his army would be unable to deal with a protected war. Now they would be forced to deal with two theaters, north, and south. The garrisons had been tired out by the long watch on the border zone and were demoralized. However, the Grand Premier had not given up. He focused his considerable energy on ensuring the defense of Pyongyang, taking measures to ensure the integrity of the city walls and the mountain fortresses that surrounded the capital. Yeon did not take command the garrison in person, however, and instead intended to remain mobile, commanding the capital armies and his personal militia. He also committed other troops to the field, both to support him and deal with Silla.

The much-vaunted Cheolli Jangseong would not play a role this war. While completed in 647 to great fanfare in Goguryeo the nature of the Tang invasion, an oversea surgical strike, meant that it would not come into play in any significant fashion. While Chinese armies did operate in that region, they did not engage the wall.

The initial landing went without trouble. The Tang quickly established a base on the coastline and Gaozong set up headquarters. However misgivings within the court over yet another invasion of Goguryeo, as well as the objections of the emperor’s strong willed consort, Wu Zhao, forced him to withdraw. He left affairs in the hands of his chief commanders: Su Dingfang and Qibi Heli (a Tiele or Siberian chieftain serving Tang). Within weeks, Su had crossed the Taedong River and by late August/early September he had put Pyongyang under siege.

Now the siege began. Besieging Pyongyang was no easy matter and was complicated by the mountain fortresses surrounding the city. An attack direct on the city walls was not probable as long as those strongholds remained intact. From these vantage points, the garrisons could rain down fire on the Tang besiegers, catching them in a crossfire between the walls and the mountains. The mountain holdfasts would have to reduced first before mounting an attack on Pyongyang proper. As the Sui engineers of that dynasty’s third invasion would have testified, trying to crack a fortress built onto the sides of a mountain was difficult.

However, outside developments appeared to aid the besiegers. Yeon Gaesomun knew the Chinese would attempt to link with their allies in Silla (which he was struggling to contain) and put a garrison on the Yalu River to stop the second column from crossing. The attempt failed spectacularly (around October, 661) and one of his sons, Yeon Namsaeng, was almost killed. Morale plummeted drastically. However, Su Dingfang recalled the victorious troops before they could cross the Yalu to take part in a troop rotation, giving the defenders time to recover. Winter proved to be the Tang’s greatest enemy as it proved unusually harsh, even for northeastern Korea. However, Su did not suspend operations but was preparing for a big push in the coming spring.

The following year, 662, the Chinese tried to reinforce their positions. As winter ended Emperor Gaozong dispatched additional forces from northeastern China into Manchuria to aid Su Dingfang in his siege of Pyongyang. Yeon Gaesomun heard of this and set a trap for the relief column at the Sasu River, near the capital. In the battle that followed the Koreans destroyed the Tang column and slew its commander. Su Dingfang now endured a morale crushing defeat of his own but hung on grimly until a spring blizzard wrecked the siege camps. Su ordered a general withdrawal of all Tang forces and suggested that Silla do the same. The second Tang invasion was at an end.

Goguryeo had managed to defeat the fifth major Chinese attempt against it, but the kingdom’s luck was running out. Despite the failure of the besiegers to take Pyongyang or cross the Yalu, the conflict proved the deadly effectiveness of the Tang-Silla alliance. The northern-most of the Korean Three Kingdoms now stood alone. Goguryeo was only able to hold out by virtue of the strength of Yeon Gaesomun and the effect he had on his soldiers. When the Grand Premier died in 666, it all fell apart.

In 663 a major uprising in former Baekje territory occurred to try and eject the Tang from the Korean peninsula as the latest of a series of rebellions over the last three years. This attempt, which was backed militarily by Yamato Japan, failed. This event marked the end of Baekje resistance and solidified the Tang-Silla alliance, increasing their ability to work together.

By now strategists in both Tang and Silla had realized that as long as Yeon Gaesomun lived the possibility of conquering Goguryeo was slim. Instead, they focused on externally weakening their common enemy. Over the next several years, both powers steadily built up along their border with Goguryeo. As a result border clashes intensified, especially in the Liao River Valley.

Then in 666 the situation changed. Yeon Gaesomun died toward the end of spring that year after abdicating his former position in favor the ceremonial post of Supreme Chancellor. Technically his eldest son, Yeon Namsaeng, was supposed to succeed him, but the two younger sons (Yeon Namgeon and Yeon Namsan) rose against him. The court split between the brothers.

The coup was too good an opportunity to pass up. The Tang moved quickly to take advantage of the chaos in Goguryeo, and sheltered Yeon Namsaeng. Military preparations in both China and Silla, which had been keeping pace with the situation, were ramped up. In fall, 667, Tang armies commanded by Chancellor Li Shiji crossed into Goguryeo. The armies of Silla did the same.

On this occasion, the Tang used Goguryeo’s infighting to their advantage. By co-opting the faction loyal to Yeon Namsaeng the Tang leadership hoped to ensure two things:

First they wanted to keep their army supplied. Li Shiji had brought a far larger army than in the previous invasions and knew that naval superiority and Silla would not be able to keep his men in supply. Those in Goguryeo loyal to Namsaeng would be more than willing to help a Tang ‘intervention’ if it meant the end of the chaos.

Second the Tang wanted to keep Goguryeo divided. A Goguryeo divided between court factions would be far easier to destroy than one united under a single ruler’s ambition, as had been the case under Yeon Gaesomun. By manipulating one faction into an alliance with them and setting it against the others, the Tang secured their victory by ensuring that no one would be strong enough to oppose them. It also kept their chosen faction beholden to China’s interests.

Before we move on we will cover what exactly made Goguryeo’s defense strategy work so well and why it failed. In the past, Goguryeo was able to mount such an excellent defense against sieges due to their use of ‘active defense’. As was displayed so well by Yeon Gaesomun and by Eulji Mundeok the generals of Goguryeo preferred to remain mobile rather then command a garrison under siege. Goguryeo had an extensive system of field armies formed chiefly of cavalry. These mobile forces allowed Goguryeo’s commanders to keep ahead of their enemies.

A favored ploy was one of scorched earth and quick raids. In 612 Goguryeo forces under the overall command of Eulji Mundeok had allowed the Sui forces to place their fortresses under siege but denied them supplies through the extensive use of scorched earth strategy. When Eulji defeated Emperor Yang’s vaunted detached force, he had weakened them through hit-and-run tactics in their rear before the battle at the Salsu River. Yeon Gaesomun had also used those same hit-and-run raids to great effect against the Tang in 662, attacking the siege camps of Su Dingfang shortly before the onslaught of that final blizzard.

However, this strategy was eventually defeated. In the first Tang invasion, Emperor Taizong recognized Goguryeo’s system for what it was and thus developed a counter strategy. He deployed his cavalry to fight with Goguryeo’s and avoided being caught between the ‘anvil’ of the fortresses and the ‘hammer’ of the mounted columns. Taizong also recognized the tactical weaknesses of Goguryeo’s field armies ( excessively oriented to heavy cavalry) and exploited it to excellent effect. In the third invasion, Li Shiji kept these lessons in mind and utilized a third. Goguryeo’s defensive garrisons had held off the armies of Sui and Tang through several invasions over the course of half a century. The protracted conflict had worn on them in more than one way. On top of these issues was the near self-destruction of Goguryeo’s royal court and military command. Without a strong, charismatic personality to inspire them the garrisons would not be able to hold out.

The Tang advance went quickly. Li Shiji swept away Goguryeo’s military presence on the Chinese side of the Liao River Valley before crossing over to the eastern side. On October 6th Sinsong, the key to the Cheolli Jangseong’s western flank fell after a short siege because of defectors inside the fortress-city. Li Shiji then continued down the ‘thousand-li wall’ and captured sixteen fortresses in all by the year’s end. Meanwhile, Tang mobile columns had moved to link with the armies of Yeon Namsaeng and were forced into a series of field battles with Yeon Namgeon, which ended in Tang victory. The younger Yeon then moved to set up a defensive perimeter at the Yalu River.

Here on the river the Chinese advance met its first setback. As a result of taunting from a member of Li Shiji’s staff the defensive measures at the Yalu were strengthened and the Tang advance halted at the riverbanks. More bad news arrived from the north with the defeat of the Tang force besieging Ansisong. Li Shiji then declared winter quarters and intended to pick up the following spring. However, despite these setbacks the Tang advance had reached further into Goguryeo then before. Also, as planned, the soldiers loyal to Namsaeng provided much-needed supplies to both Tang’s land and sea forces.

Conflict resumed in the spring of 668. Li Shiji turned away from the Yalu and instead decided to focus on reducing Goguryeo’s northern flank first. In March-April the north end of the Cheolli Janseong, Buyeosong, fell, and the entire northeast was laid bare. When he attempted to recapture the fortress, Yeon Namgeon was defeated handily. The Tang forces then divided into columns and advanced from the north down to the Yalu.

That fall it all came together. The various columns reformed into a central army at the banks of the Yalu after the fall of the last of the northern fortresses. They then forced their way across the Yalu and put Pyongyang under siege. Forces from Silla, victorious in the south, marched up to join them, and the attack commenced around September, 668. On October 17th, King Bojang and Yeon Namsan attempted to surrender, only to be prevented from doing so by Yeon Namgeon, who refused to accept the inevitable. On October 22nd, the city fell for real when his generals betrayed Namgeon. Li Shiji occupied the city in triumph soon after, and Goguryeo officially annexed that winter. The kingdom became a Tang province like Baekje, and the long wars between China and Goguryeo were at an end.

Tang’s preeminence in Korea was not destined to last long. Almost as soon as the kingdom fell a slew of revival movements rose, with the greatest one led by Prince Anseung (a relative of King Bojang) and Geom Mojam. To make matters worse Silla was covertly backing these movements as a result of a falling-out with the Tang. This falling-out was because of the differing aims of the two states: While both sought the end of Goguryeo, Tang intended to bring Korea under its control while Silla sought unification.

The uneasy peace continued until the outbreak of war in 670. The Silla-Tang War lasted for six years with Tang unofficially admitting defeat by moving the headquarters of its ‘Eastern Protectorate’ into Liaodong. The region between Liaodong and the Taedong River, marking the border of ‘Unified Silla’, remained a frontier zone until 698 when a Goguryeo revival movement led by Dae Joyeong created Balhae.