in 661 AD, General Su Dingfang withdrew the main body of his land army from the mouth of the Kum River and sailed his fleet north to the Taedong River to launch a frontal attack against Koguryo's capital in Pyongyang. Koguryo armies under Yon Kaesomun stiffly resisted the Chinese and, with a bit of help from a fierce winter blizzard, forced them to withdraw.
Throughout its seven hundred year existence, Koguryo fought the Chinese Han, Sui and Tang dynasties all along its western and northern frontiers. Koguryo's population of 3.5 million people and its 176 walled cities sustained massive invasions and fought off the best the Chinese could offer for nearly 70 years, a feat unmatched by any other Korean state. Chinese legions assailed Koguryo four times during his twelve year campaign against the peninsula, all to no avail. Along its southern frontier, Koguryo battled the two rival kingdoms of Silla and Paekche. Now, with a family feud tearing it apart from the inside, Koguryo plunged headlong toward impending doom. Neither Tang China nor Silla, each for its own reasons, was willing to let such a golden opportunity slip through its hands.
Years of almost continuous warfare and the disaffection created by Yon Kaesomun's dictatorial rule generated deeply felt internal dissention in Koguryo. A serious alienation of loyalties developed within Koguryo's ruling elite that weakened the kingdom's power to resist external aggression. The festering turmoil erupted into open conflict during the summer of 666 AD, following Yon Kaesomun's death. The contentious beliefs held by Yon's two sons, Namgon and Namsaeng, and his younger brother Yon Chongt'o quickly developed into a major power struggle within the aristocracy. Namgon gained the upper hand and forced his older brother to flee the city. Namsaeng, still hopeful of one day returning to power, fled to the old capital at Kungnae-song and surrendered to the Tang armies garrisoned there. Yon Chongt'o defected to Silla.
In 667 AD, Tang Emperor Gao Zong appointed General Li Ji, the Duke of Ying, to marshal an army to attack Koguryo and sent a request to Silla for military assistance. King Munmu appointed Humsun, Kim Yu-sin's younger brother, and Kim Inmun, the son of Kim Yu-sin's sister, as generals to command the army against Koguryo. General Humsun was reluctant to fight without his famous older brother on the battlefield, but the king reminded him that keeping General Kim Yu-sin in reserve to defend against a possible Chinese attack would be "as if there is a great hidden wall" to protect the country. Silla launched its major offensive against Koguryo from the south under the command of General Kim Inmun. Although severely weakened by internal factionalism, Koguryo still managed to hold out for another year. In late autumn of 668 AD however, the Tang-Silla forces settled down for a month-long siege against the capital at Pyongyang. Trapped between two powerful allied armies that practically destroyed the city, Koguryo finally succumbed that winter. In light of its many past failures trying to conquer Koguryo, the fall of Koguryo was seen as an epic Chinese victory.