It is said that the defection of the Mori clan was caused mainly by the involvement of Hidetsuna KAMEI, a vassal of the Amago clan, in the succession race of the Mori clan, but it seems that Tsunehisa's strong ulterior motive also played a part.
毛利氏の離反は、毛利氏の後継争いに尼子家臣・亀井秀綱が介入したことが大きな原因とされているが、実際には経久の強い意向が働いていたと思われる。