Battle of Baekgang
Under the Ritsuryō system, Tsushima became a province of Japan. This province was linked with Dazaifu, the political and economical center of Kyūshū, as well as the central government of Japan. Due to its strategic location, Tsushima played a major role in defending Japan against invasions from the Asian continent and developing trade lines with Baekje and Silla of Three Kingdoms of Korea. After Baekje, which had been aided by Yamato Japan, was defeated in 663 by Sillan and Tang Chinese forces at the Battle of Baekgang, Japanese border guards were sent to Tsushima and Kaneda Castle was constructed on the island.