King Yongnyu of Koguryo opened his early relations with the new Tang Dynasty on a friendly note by ordering the repatriation of thousands of Chinese soldiers taken prisoner during campaigns against the Sui Dynasty. In addition, he accepted investiture from Emperor Tai Zong and accepted the Tang calendar as a symbol of Koguryo's tributary status. Yongnyu even sent his son, the crown prince, to enter the Confucian academy in the Tang capital. Despite his friendly relations with Tang China however, King Yongnyu had no illusions about the potential for a future disaster emerging beyond his northern frontier. He soon embarked on a ten year program to strengthen his defenses centered on the construction of a massive and elaborately fortified defensive wall that stretched for nearly 250 miles along the Liao River.
Protected from external threats, Koguryo began to suffer the chronic bane of all Asian monarchies, the internal threat from friction within the ruling class. As the frontier wall neared completion, an internal split developed within the Koguryo aristocracy that prompted a coup d'état in 642. Yon Kaesomun, a domineering military officer in charge of constructing fortifications along the Liao River, emerged victorious in the bloody grab for power that resulted in the wholesale slaughter of all opposition, including King Yongnyu. Assuming the position of a military dictator, Yon Kaesomun soon dominated the Koguryo government. His absolute political control of the kingdom and his aggressive foreign relations policies had the immediate effect of setting Koguryo on a collision course with both Tang China and Silla.
Facts and Figures :
In an effort to help mediate a negotiated end to the continuing disputes among Koguryo, Silla, and Paekche, the Tang court sent its envoys to P'yong'yang as part of their regular tributary exchange of relations with Koguryo. Yon Kaesomun defiantly rejected the Tang envoys' assistance. His aggressive temperament not only fanned a rising anger in China, it opened the possibility of a second front against Koguryo. That possibility became reality in 645, when Emperor Tai Zong dispatched a massive army in a large-scale invasion that swept across the Liao River into Koguryo. Five hundred ships sailed from the port of Donglai on the Shandong Peninsula carrying a nearly equal-sized force. Chinese troops eventually overran numerous towns in Koguryo and laid siege to the mighty Liaodong Fortress, which they promptly reduced to rubble along with a number of other fortresses in the area.
The An-shih Fortress, a minor link in Koguryo's line of defensive strongholds stood defiantly in the hills overlooking the Yinma River near the modern city of Jiutai, Manchuria. Under the inspired command of Yang Man-ch'un, An-shih held firm despite repeated attempts to breach the fortress walls. For over sixty days, the stubborn defenders at An-shih withstood a fierce siege by the Tang army, fighting off as many as six or seven assaults by the entire Chinese force in a single day. As fall turned to winter and with icy winds sweeping out of northern Manchuria, the Chinese finally withdrew from the battlefield taking thousands of prisoners with them back to China proper. The battle for An-shih represented a massive defeat for Emperor Tai Zong.
Koguryo's victories over the invading armies of the Sui and Tang dynasties hold a special place in Korea's history because they highlight the peninsula's resistance to foreign aggression. Had the Tang army defeated Koguryo, it would have been but a small part of China's grand imperial design to dominate all of East Asia. Had Korea been less mountainous it might have suffered Manchuria's fate and been absorbed into the expanding Chinese empire. Such a conquest would have opened the floodgates onto the peninsula and led to Chinese subjugation of both Paekche and Silla. That never happened. Instead, the Liao River valley frontier defense line held firm and Koguryo held on to its independence.
Tang China's failed military campaign against Koguryo in 645 did not diminish its interest in the Korean kingdom, and the Chinese launched three more unsuccessful assaults against Koguryo in 647, 648, and 655. China's military commanders decided to change tactics and flank Koguryo by first conquering Paekche, then attacking the kingdom in a north-south pincer movement. By the time Chinese troops landed at the mouth of the Kum River, General Kim Yu-sin, the Guardian Protector of Silla, had already marched his troops through T'anhyon Pass east of Taejon, driving eastward in a coordinated assault against Paekche.