However, since they had gained too much power, Yoshitsura Isshiki, who was the family head of the Isshiki clan, was killed by Yoshinori ASHIKAGA, the sixth-generation shogun of the Muromachi, and an attack by the Takeda clan resulted in the weakening of family power.
しかし、力をつけすぎたために室町幕府の第六代将軍・足利義教に時の一色氏の当主一色義貫が殺害されてしまったうえ、武田氏の攻撃を受け勢力を縮小させてしまう。