Yasumune accompanied Sadatsuna when Sadatsuna was ordered by Tokimune HOJO (the regent of the Kamakura bakufu) to go down to Kyushu as a supreme commander of the Menggu Punitive Force when Mongol attempted to invade Japan (Genko), and Yasumune made his base in Oki-jo Castle located in Setaka, Yamato-gun, Chikugo Province.
貞綱が元寇の際、執権北条時宗の命を受け、蒙古討伐軍総大将として九州に下った時、同行し、筑後国山門郡瀬高の大木城を拠点とした。