After successfully concluding his campaign in Baekje, Jangsu then turned his attention toward the second peninsular kingdom of Silla. Silla had been a vassal state of Goguryeo since Gwanggaeto defeated the Baekje and Wa troops invading Silla in 400. To secure the allegiance of his de facto protectorate, Jangsu demanded the younger brother of King Nulji of Silla to become a political hostage. King Nulji broke off relations with Goguryeo in 454. Jangsu invaded Silla in 468, expanding his domain into parts of Gangwon Province, and again in 489, capturing 7 walled cities and expanding his domain into parts of North Gyeongsang Province. With his victory over Silla, Jangsu erected a stone monument in present-day Chungju, praising the accomplishments of his father and himself. This monument remains at the same site, holding historical importance as the only surviving Goguryeo stele in the Korean peninsula.