Commanders of the Battle of Gwansanseong, 554.
From left to right : Prince Buyeo chang(扶餘昌), King Seong of Baekje(聖王), Mononobe no Maganomuraji(物部 莫奇武連)

Baekje had maintained a century-long alliance with its neighbor Silla, to balance the threat of the northern kingdom Goguryeo. With the aid of Silla and the Gaya confederacy, Seong led a long campaign to regain the Han River valley, the former heartland of Baekje which had been lost to Goguryeo in 475. Baekje regained its original capital in 551. The campaign culminated in 553 with victories in a series of costly assaults on Goguryeo fortifications.

However, under a secret agreement with Goguryeo, Silla troops, arriving on the pretense of offering assistance, attacked the exhausted Baekje army and took possession of the entire Han River valley. Incensed by this betrayal, the following year Seong launched a retaliatory strike against Silla’s western border. This attack was led by the crown prince (subsequent king Wideok) and joined by the Gaya confederacy and Yamato(Japan). But Seong and 30,000 Baekje troops were killed in the disastrous Battle of Gwansanseong. This defeat led to significant erosion of royal power.