In 551, a Baekje–Silla alliance attacked Goguryeo in order to capture the important Han River region from Goguryeo, planning to split it between them. In 553, Baekje gained the critical region after expending itself with a series of costly assaults on Goguryeo fortresses, but Silla troops, arriving on the pretense of offering assistance, attacked the exhausted Baekje troops and took possession of the entire Han River region, leading to a war between the two former allies in which the Baekje monarch was killed.