Silla's Acquisition of the Han River Basin

Overview

In the mid-6th century, Silla occupied the entire Han River basin, changing the balance of the Three Kingdoms rivalry. In 551 (12th year of King Jinheung, 29th year of King Seong), the combined forces of Baekje, Silla, and Gaya attacked Goguryeo; Baekje gained six counties in the lower Han River region, and Silla gained ten counties in the upper Han River region. However, in 553 (14th year of King Jinheung), Silla turned its troops around and occupied even the lower Han River region that Baekje had recovered, establishing Sinju (新州) there. Through this, the Baekje–Silla Alliance collapsed, and the following year the Battle of Gwansanseong took place. The dates in this article are according to the lunar calendar.

Background

• Goguryeo's turmoil: In the mid-6th century, Goguryeo's politics were unsettled by the assassination of King Anjang (531), internal strife among maternal relatives before and after the accession of King Yangwon, and in the northwest it faced pressure from the Northern Qi and the Turks.
• Baekje's revival: After the move of the capital to Sabi (538), King Seong of Baekje, having reorganized the state, sought to recover the Han River basin, a long-held goal since the loss of Hansung in 475.
• Silla's northward advance: From the late 540s to 551, Silla advanced toward Danyang and Chungju in the upper reaches of the South Han River. The Silla Jeokseong Stele in Danyang records that Silla occupied the Jeokseong (Danyang) region around this time and rewarded local collaborators. In 550, Isabu also seized Dosalseong of Goguryeo and Geumhyeonseong of Baekje.

Course of Events

Joint Attack of 551

In 551, King Jinheung changed the era name to "Gaeguk (開國)" and began ruling in his own right. In the third month of that year, King Jinheung visited Nangseong (around present-day Cheongju or Chungju), which he had newly acquired. In the same year, Silla and Gaya, centered on Baekje, jointly attacked Goguryeo. The Baekje forces first defeated Pyongyang (the area of present-day Seoul, called "South Pyongyang" at the time) and recovered six counties in the lower Han River region. According to the Biography of Geochilbu in the Samguk Sagi, Silla's eight generals, led by Geochilbu, attacked Goguryeo together with Baekje and, riding the momentum, took ten counties inside Godyeon (高峴, presumed to be present-day Cheollyeong) beyond Jukryeong.

Rapprochement between Goguryeo and Silla in 552

Facing the threat in the northwest, Goguryeo sought to join hands with Silla. The Korean Encyclopedia of Culture entry on "King Jinheung" explains that in 552 the so-called Yeo-Na (麗羅) secret agreement was concluded at Goguryeo's proposal, and the entry on the "Battle of Gwansanseong" also holds that Silla was able to attack Baekje with Goguryeo's tacit consent.