In the mid to late 3rd century, during King Goi’s reign, the Yeongseo region of present-day Gangwon Special Self-Governing Province was incorporated into Baekje’s sphere of influence. Between the late 3rd and early 4th centuries, Silla advanced beyond the Jukryeong into the Yeongseo region, leading to conflicts between the two states. In the late 4th century, under King Geunchogo’s reign, Baekje came to control the political powers of the Wonju area, a fact that can be confirmed through tombs and artifacts excavated from the Beopcheon-ri site. In 396, Goguryeo launched a southern campaign and seized 58 fortresses and 700 villages from Baekje. Among them, Gomoruseong (古牟婁城), identified with present-day Wonju, was later renamed Pyeongwonseong (平原城). When Silla secured ten guns in the upper Han River region in 551, Wonju was incorporated into its territory, and Pyeongwongun (平原郡) was established there. After unification of the Three Kingdoms, Silla installed Bukwonsogyeong (北原小京) at Pyeongwongun in 678, both to effectively control the newly annexed former Goguryeo territories and the Yeongseo and Yeongdong regions of Gangwon, and to strengthen defenses against Tang forces advancing through the Jukryeong route following the Battle of Seokmun. With the completion of the Nineju (九州) system in 685, the security of Bukwonsogyeong was further consolidated. As tensions with Tang gradually eased, Bukwon, like other Sogyeong, acquired the character of a auxiliary capital(陪都), serving to reinforce nationwide governance. Its administrative seat was located in present-day downtown Wonju, and in the 5th year of King Sinmun’s reign (685), Bukwongyeongseong (北原京城), identified with today’s Geumdaeseong was constructed. The jurisdiction of Bukwonsogyeong encompassed present-day Wonju, Seowonmyeon in Hoengseonggun, and parts of Gangcheonmyeon and Buknaemyeon in Yeojusi, Gyeonggi Province. In 757, Bukwonsogyeong (北原小京) was renamed Bukwongyeong (北原京). In the late 9th century, Bukwongyeong was changed to Bukwonbu (北原府), and later, during Later Goguryeo, it was renamed Wonju (原州).