King Jangsu is remembered as a great ruler, just like his father, Gwanggaeto the Great. Based on his father’s vast territorial expansion, King Jangsu built a well-established state system and advanced culture. Historians agree that the Goguryeo kingdom reached its height politically, socially and culturally and enjoyed its status as a major power in East Asia during the king’s reign in the fifth century.
King Jangsu became king at the age of 19 when his father died in 413. One of the most notable policies of the king was the relocation of the capital from Gungnae Fortress, which is modern-day Jian on the China-North Korean border, to what is now Pyongyang in 427. After moving the capital southward, the king pushed for his extensive southward expansion policy in a muscular fashion. The king carefully watched for a chance to invade the southern Korean kingdoms of Baekje and Silla, while the Chinese states of Northern Wei and the Song Dynasty were fighting each other.
With the purpose of invading the southwestern kingdom of Baekje, the king sent a Buddhist monk named Dorim, who had a secret mission to corrupt the Baekje court ruled by King Gaero. King Jangsu’s strategy proved successful. The Baekje king trusted the monk deeply and played a board game every day with him. The monk also coaxed the king into spending a huge amount of money on construction projects, which eventually depleted the state coffers. Baekje became increasingly unstable politically, and King Jangsu never missed the opportunity. In 475, he launched a full-scale attack on Baekje. Goguryeo forces easily occupied the unprepared capital city and killed King Gaero, while the Goguryeo monk safely escaped the city after fulfilling his mission successfully. As a result, Baekje abandoned the Han River basin and moved its capital southward to present-day Gongju. It was a fatal loss for Baekje, as it was essential for any state to occupy the Han River area in today’s Seoul in order to maintain military and commercial dominance on the Korean Peninsula at the time. Baekje remained a strong power on the peninsula for hundreds of years because it was able to take control of the region. However, with the emergence of a new ruler, namely Goguryeo, Baekje lost its hegemony over the region and its national power decreased.
After successfully defeating Baekje, King Jangsu turned his eyes to another Korean kingdom of Silla in the southeast.