"Samguk Sagi" Book 04. Silla's Records. In 512, Usan-guk（于山國） was Ulleungdo（鬱陵島）
Usan-guk, or the State of Usan, occupied Ulleung-do and the adjacent islands during the Korean Three Kingdoms period. According to the Samguk Sagi, it was conquered by the Silla general Kim Isabu in 512. He is said to have used wooden lions or tigers to intimidate the residents into surrendering. It has been written that the alias of Usan-guk is Ulleung-do. Usan-guk rarely entered into historical records, but appears to have continued a largely autonomous existence until its annexation to Goryeo in 930.
According to Sejong Sillok, Goryeosa, and Mangi Yoram, Usan-guk consisted of Ulleung-do and Usan-do. Koreans believe Usan-do is the Liancourt Rocks (Dokdo in Korea, Takeshima in Japan) because old Korean geographical records indicate that Ulleungdo and Usando can only see each other on a clear day, although some Japanese dispute the interpretation of some of the records.