Northeast Asian History Network

By the reign of King Pyeongwon of Koguryo in the late 6th century, the pluralistic international order in East Asia that had been ongoing since the 4th century had changed rapidly. The Sui dynasty was founded and emerged as a superpower in East Asia after dominating the Göktürks in the north and unifying China in 589 CE. The Sui dynasty desired to establish unitary international order centered in its homeland. Thus, the Sui dynasty did not recognize the Koguryo's supremacy in Northeast Asia and marched toward the Liao River region in defiance. In response, some of the forces in Northeast Asia, including the Khitan and Mohe tribes, broke away from Koguryo and allied with the Sui.
During this time of change, King Yeongyang of Koguryo (reign: 590-618^{[note 001]} attacked the western Liao River region of the Sui dynasty. Emperor Wen (reign: 581-604^{[note 002]} of the Sui dynasty immediately went on the counterattack. An army and navy comprised of over 300,000 soldiers were mobilized to battle Koguryo. However, the Sui supply lines were severely disrupted due to a rainy season, and the navy was crippled by a typhoon. The Sui counterattack in 598 CE had failed before it even began.
The Sui attempted another attack on Koguryo in 612 CE. To attack Koguryo, Emperor Yang of Sui (reign: 604-618^{[note 003]} mobilized over one million troops for his campaign. It was the single largest army ever mobilized in history. Nevertheless, Koguryo was able to thwart the Sui army. Koguryo's success was primarily due to the consolidated efforts of the major aristocracies including King Yeongyang and General Eulji Mundeok^{[note 004]}, and the nation's defenses were strong enough that no city in the Liaodong region fell to the Sui. The Battle of Salsu^{[note 005]}, led by General Eulji Mundeok, was one of the decisive battles of the war.
Additional campaigns were launched in 613 and 614 CE by the Sui dynasty. By this time, however, the people of Sui had already grown restless over the war against Koguryo. Peasant rebellions had sprung up throughout the country, and major aristocrats such as Yang Xuangan^{[note 006]} had revolted against the Sui. For these reasons, the attempted conquest of Koguryo could not continue. Caught in a vortex of rebellions, the Sui dynasty finally collapsed in 618 CE.
Following the Sui dynasty, the Tang dynasty secured the central regions of China. Initially, the Tang dynasty desired friendly relations with Koguryo. Koguryo had become significantly weakened in the war against the Sui dynasty. Thus, the two countries shared friendly relations for a short period. In the late 620s, however, the Tang dynasty resolved domestic and international turmoil to emerge as a leading powerhouse in East Asia. Soon after, the Tang dynasty exerted diplomatic pressure on Koguryo. Like the Sui, the Tang dynasty wanted to establish international order with its nation at the center. In response, Koguryo built the Cheolli Jangseong (literally "Thousand-Li Wall^{[note 007]} to defend against an attack by the Tang dynasty. At this time, however, the Koguryo aristocracy divided into two major parties, the hardliners and moderates.
As indicated by the above excerpt from the "New Book of Tang," Yeon Gaesomun^{[note 008]} faced elimination from the many great ministers who opposed him. In 642 CE, Yeon Gaesomun instigated a coup d'état. Through this action, he eliminated the major aristocratic forces who opposed him as well as King Yeongnyu^{[note 009]}. Yeon Gaesomun subsequently placed Jang (King Bojang of Koguryo^{[note 010]} on the throne. Thus, Yeon Gaesomun, a hardliner, seized political control of Koguryo.
The aftermath of Yeon Gaesomun's coup reached far throughout Koguryo. It is said that Ansi Fortress was involved in armed conflict due to their opposition of Yeon Gaesomun. The signs of infighting had emerged. From the perspective of the Tang dynasty, Yeon Gaesomun's political upheaval was a great opportunity to attack Koguryo. On the surface, Yeon Gaesomun's successful assassination of the King was a justifiable cause for conquest. And so in 645 CE, Emperor Taizong of Tang (reign: 626-649^{[note 011]} led an attack on Koguryo.
Unlike the war with Sui, however, several Koguryo cities in the Liaodong region fell to the Tang in 645 CE. The Tang forces broke through defenses in the Liaodong area and marched toward Pyongyang through Gungnae Fortress. In this critical situation, Ansi Fortress put forth a formidable resistance. For several months, Ansi Fortress repelled the 150,000-strong army led personally by Emperor Taizong of Tang. Emperor Taizong eventually withdrew. On several occasions, Emperor Taizong sent armies to attack Koguryo without much success.
While Koguryo was busy fighting wars against the Sui and Tang dynasties, Baekje and Silla stayed active. The two countries, through the Sui and Tang dynasties, attempted to contain Koguryo even while at constant war with each other. In the midst of such complex international relations between the Three Kingdoms, Silla was most disadvantaged. In 648 CE, Kim Chunchu of Silla (King Taejong Muyeol^{[note 012]} formed a military alliance with the Tang dynasty. The Silla-Tang alliance was formed. The Silla-Tang alliance first attacked and conquered Baekje in 660 CE. The alliance then continued their offensive to attack Koguryo.
Koguryo was initially successful in thwarting attacks by the Silla-Tang alliance. In addition, as the revival of Baekje was being actively developed, the Tang dynasty was hesitant to continue their efforts in attacking Koguryo. However, the situation changed rapidly after the death of Yeon Gaesomun in 665 CE. Yeon Gaesomun's first son Namsaeng^{[note 013]}, who had succeeded his father as ruler, was in conflict with his two younger brothers Namgeon^{[note 014]} and Namsan^{[note 015]}. Due to this fact, Koguryo was embroiled in infighting. After losing power to his two younger brothers, Namsaeng surrendered to the Tang dynasty and joined forces to attack Koguryo in an attempt to regain control.
As such, Koguryo was no longer able to counter the enemy's attacks due to the infighting. Eventually, the capital Pyongyang fell to the Silla-Tang alliance in 668 CE. King Bojang and the major aristocracies of Koguryo were taken prisoner and transported to the Tang capital Chang'an (modern-day Xi'an). Subsequently, the displaced people of Koguryo attempted to revive the state through the efforts of Geom Mojam^{[note 016]}, Anseung^{[note 017]}, and GoYeonmu^{[note 018]} among others but were ultimately unsuccessful in rebuilding the dynasty.