Tadakuni, who returned to Satsuma, belonged the Southern Court and raised an army at his own castle, Ichiuji-jo Castle, just after Imperial Prince Kaneyoshi (the ninth son of Emperor Godaigo) was appointed as Seisei Shogun (literally, "great general who subdues the western barbarians") and Yasusue SANJO, a court noble of the Southern Court, arrived at Satsuma as advance agent in 1337.
薩摩に戻った忠国だったが、延元2/建武(日本)4年（1337年）、懐良親王（後醍醐天皇の九男）が征西将軍に任命され、先遣として南朝方公家の三条泰季が薩摩へ到着するや、南朝方に属して居城の一宇治城で挙兵する。