Northeast Asian History Network

Koguryo's territorial expansion was at its height after the 4th century. From the early 4th century, nomads in the northern plains of East Asia, including the Xiongnu^{[note 001]} Jie^{[note 002]}, Xianbei^{[note 003]}, Di (Wu Hu^{[note 004]}, and Qiang^{[note 005]}, migrated south and established a number of states in the northern part of China’s Central Plains. These states were collectively referred to as the Sixteen Kingdoms. The Sixteen Kingdoms were in constant conflict. At this time, East Asian international politics were changing from moment to moment. This turbulent political landscape was actually beneficial for Koguryo's territorial expansion in Northeast Asia.
In 313 CE, King Micheon of Koguryo (reign: 300-331 CE) ^{[note 006]}united the Lelang and Daifang Commanderies to the south. These regions are now the northwestern part of the Korean peninsula. In addition, Koguryo extended its control in the Liaodong region through intense competition with Former Yan^{[note 007]} (Murong Huang^{[note 008]}) and its Xianbei people. Koguryo's pushed into Buyeo^{[note 009]} in the north and advanced toward the Songhua River area. King Micheon's territorial expansion efforts continued under King Gogugwon's command (reign: 331-371 CE^{[note 010]}.
However, Koguryo received strong resistance from the Former Yan and Baekje peoples during his reign. In the 12th year of King Gogugwon (342 CE), Koguryo was attacked by member of the Former Yan and was devastated enough for its capital to fall^{[note 011]} Notably, in 371 CE, King Gogugwon was killed during combat while fighting against the Baekje army in Pyongyang Fortress, the capital of Gojoseon^{[note 012]} Needless to say, Koguryo experienced difficulty with territorial expansion, and the country faced a national crisis at this time. At this critical time, King Sosurim (reign: 371-384 CE^{[note 013]} succeeded King Gogugwon of Koguryo. To overcome the state's internal and external crises, King Sosurim of Koguryo devised to reorganize the national infrastructure.
As recorded in the "History of the Three Kingdoms," King Sosurim of Koguryo officially recognized Buddhism and established a Taehack^{[note 014]} in 372 CE. In addition, Yeul-lyeong^{[note 015]} was declared shortly after in the 3rd year of King Sosurim of Koguryo (373 CE).
Buddhism was introduced to Koguryo by Former Qin, and at the time, Buddhism practiced in Former Qin as well as Northern Dynasties had a strong bias toward Nationalist Buddhism. Nationalist Buddhism was based on the primary concept of Wangjifu, or literally, "the King is Buddha." Therefore, the acceptance of Buddhism at this time helped to establish a centralized state system with a focus on the king of Koguryo.
The Taehack was an educational institution for training officials with Confucian literature. Zhongxiao (literally "filial devotion") was one of the important virtues of Confucianism. The concept emphasized devotion to one's parents and the king. Thus, government officials trained in Confucian literature were extremely loyal to the king. By establishing a Taehack, Koguryo trained all government officials to pledge allegiance to the king. In addition, Koguryo also trained administrative officials required to operate the established state system.
Currently available information regarding Koguryo does not describe specifics about operating the state system. However, Yeul-lyeong contained the operating principles for a centralized state system with legislation established and developed by Chinese dynasties. Therefore, it is understood that the centralized state system only reflected the ideas found in Yeul-lyeong.
Koguryo in the King Sosurim period was equipped with a centralized state system with the king at its center. Based on these aforementioned ideas, Koguryo achieved rapid expansion of its territories after the 5th century. In particular, Koguryo territories were significantly expanded during the reign of King Gwanggaeto of Koguryo (reign: 391-413 CE^{[note 016]}. King Gwanggaeto of Koguryo occupied the Liaodong region through a successful war campaign against Later Yan^{[note 017]} to the west. A part of Khitan^{[note 018]} territory beyond the Liao River was seized and placed under Koguryo control. Furthermore, the offensive against Baekje to the south was strengthened, thus increasing its area of influence from Imjin River basin to the Han River basin. The influence over Buyeo to the north was strengthened as well. According to inscriptions on the "Moduru Epitaph^{[note 019]}", a 5th century epitaph for Koguryo aristocracy, a local government office referred to as Susa was installed in the Buyeo area.
The above passage from the Stele of Gwanggaeto^{[note 020]} the Great is a summary of the King's accomplishments. The above excerpt states that King Gwanggaeto's greatness was well-known throughout the four seas, or Northeast Asia with Koguryo at its center, praising him for bringing peace to the people of Koguryo. As such, King Gwanggaeto created a view of the world with Koguryo at its center, and the country's territorial expansion was at its prime during his reign.
Koguryo's prosperity continued with King Jangsu of Koguryo (reign: 413-491 CE) following King Gwanggaeto. In 427 CE, King Jangsu of Koguryo moved the capital from Gungnae Fortress to Pyongyang and actively promoted territorial expansion to the south.
As recorded in the "History of the Three Kingdoms," Koguryo fell to Baekje in 475 CE and executed King Gaero. Centered on the Han River region, Koguryo expanded its influence throughout modern-day Gyeonggi Province, northern Chungcheong Province, and south Han River basin. The Chungju Koguryo Monument^{[note 021]}, estimated to have been built during the King Jangsu period, is a testament to Koguryo's southern expansion policy.
At this time in southern China, the Liu Song, Southern Qi, Liang, and Chen dynasties rose and fell, respectively (Southern Dynasty), and in northern China, the Sixteen Kingdoms rose and fell (Northern Dynasty). These dynasties are collectively referred to as the Southern and Northern Dynasties. During this era, the two states of nomadic Mongolic tribes in the north and west known as Rouran^{[note 022]} and Tuyuhan^{[note 023]}, respectively, were powerful forces in addition to the Southern and Northern Dynasties. As such, not any one state was able to dominate East Asia, and a balance of control was maintained with the major powers. Koguryo was one of these major powers. In the struggle for balance of power in East Asia, Koguryo was recognized as a disruptive force in Northeast Asia.