Relations between Koguryo and Silla during the Reigns of King Gwanggaeto and King Jangsu : Regarding the Historical Background of Koguryo Cultural Elements Confirmed in the Northern Gyeongbuk Region

According to the Gwanggaeto Stele, Silla was a Sokmin of Koguryo, paying tribute to Koguryo. The Chungju Koguryo Stele describes Silla's relationship with Koguryo as ‘like brothers’showing a changing picture. Maegeumto, a territory directly under Maegeum, and Sillao, a territory not under Maegeum. Koguryo had established a system of vassal rule encompassing all of these, but this relationship was no longer sustainable. Koguryo directly ruled Koguryo territory around the time of its capital's relocation to Pyongyang. King Nulji viewed this as a signal that he intended to dominate all of Silla and, in response, accepted Baekje's request for peace and prosperity. King Jangsu, seeking to maintain Silla under Koguryo influence, held a ceremony in May 449 to bestow clothing upon Silla. King Jangsu's efforts and King Nulji's immediate apology for the assassination of Koguryo by a disguised Koguryo general led to a breakup. However, soon after, the Koguryo forces in the Silla capital were annihilated, ultimately leading to a breakup. Subsequently, the Great Silla invasion began, and in 481, Koguryo withdrew north of the Sobaek Mountains after a surprise attack on the Silla capital was crushingly defeated by the allied forces. The northern Gyeongbuk region, ceded to Koguryo by Silla in return for reinforcements sent in 400, remained under Koguryo rule until the late 5th century, making it familiar with its culture. Because cultural influences far outweigh political and military influences, Koguryo cultural elements have been found in this region for a long time. The mural tombs in Sunheung and the Buddhist statues in Bonghwa and Yeongju likely involved the creation of local powers familiar with Koguryo culture due to their historical ties to the region, as well as Koguryo people who strategically migrated to Silla and their descendants. Monk Hyeryang and the Koguryo people who entered Silla for various reasons would likely fall into this category.