In 1578, during the generation of Koremasa ASO, however, when the Otomo clan was badly defeated by the Shimazu clan in the Battle of Mimi-kawa, local samurai lords in Higo allied with the Shimazu clan or Ryuzoji clan, which was an emerging clan at that time, to invade the territories of the Aso clan.
しかし阿蘇惟将の代となった1578年、大友氏が耳川の戦いで島津氏に大敗を喫すると肥後の国人衆は島津氏や新興勢力の龍造寺氏と誼を通じ、阿蘇氏の領域を脅かすようになる。