Peace Negotiation of Izumo and Aki
The Peace Negotiation of Izumo and Aki (Unkei wagi) was an unsuccessful effort to negotiate between the Amago clan of Izumo Province and the Mōri clan of Aki Province that occurred over a period from Eiroku 4 (1561) to Eiroku 5 (1562).
In the era of Amago Haruhisa, the Amago clan based in Izumo Province witnessed the peak years of its prosperity, serving as the military governors of eight provinces in the Sanin-Sanyō Region of western Japan. After defeating the Ōuchi clan, the Mōri of Aki Province viewed the Amago as their primary rival in the region. Mōri Motonari launched invasions of Iwami Province in a bid to capture the Iwami-Ginzan silver mine, but was repelled on two occasions by Haruhisa, including the Collapse at Oshibara (Oshibara kuzure) and the Battle of Gōrozaka. Consequently, the Amago maintained exclusive rights to the mine.
In the twelfth month of 1561, Haruhisa suddenly died and was succeeded by his eldest son, Amago Yoshihisa. Yoshihisa’s succession as the new leader caused turmoil among the retainers of the clan. In an effort to resolve their concerns, Yoshihisa made a request to Ashikaga Yoshiteru, the shōgun of the Muromachi bakufu, to mediate a reconciliation with the Mōri, but the mediation effort failed after a six-month period.
The Iwami-Ginzan silver mine was located in the eastern portion of Iwami which leaned toward support for the Amago. Retainers of the Amago including the Onsen and Tako clans had important roles. Yoshihisa dispatched Honjō Tsunemitsu, Ushio Hisakiyo, and Tako Tokitaka from Izumo to support Fukuya Takakane after Takakane launched a rebellion against the Mōri in western Iwami to shore-up an unfavorable position.
However, in an effort to save face for Ashikaga Yoshiteru, and to address the dissatisfaction of his retainers that had been simmering since the era of his great-grandfather, Tsunehisa, as well as to mediate an end to the internal conflicts, Yoshihisa desired negotiations with the Mōri to occur soon. Mōri Motonari offered to engage in settlement negotiations on the condition of non-intervention in Iwami. Yoshimura immediately agreed, but, from the perspective of the Fukuya family, this meant an elimination of their position along with that of the commanders and kokujin affiliated with the Amago clan who supported the Fukuya. As a result, the front line of the Amago in Iwami promptly collapsed and Honjō Tsunemitsu separated from the clan, while Tako Tokitaka took his own life in the Iwayama Castle of Iwami.
Owing to the change in circumstances in Iwami, the Mōri rejected an effort to reconcile and, in 1562, invaded Izumo. After the Mizawa, Mitoya, Akana, and Yonebara clans of western Izumo switched sides and provided an advantage to the Mōri, Yoshihisa responded by entering into an alliance with Ōtomo Yoshishige, the powerful sengoku daimyō from northern Kyūshū. Meanwhile, supporters of the Amago from eastern Izumo, Hōki, Bitchū, and Mimasaka provinces combined in opposition to the Mōri. As a result, this expedition by the Mōri lasted for four years.
Thereafter, Ashikaga Yoshiteru sought a three-way settlement that included Ōtomo Yoshishige from Buzen Province, but with a battle for Gassantoda Castle near at hand, Motonari was not supportive. With the impending approach of a battle, Yoshiteru determined that re-opening of the negotiations was not realistic. Meanwhile, based on the expectation of Yoshishige that a pincer attack from the Amago was not likely, the negotiations headed in the direction of a settlement between the Mōri and Ōtomo. Owing to the sudden death of Mōri Takamoto, the negotiations were extended, but in the seventh month of 1564, a settlement was reached between the Mōri and Ōtomo so the Ōtomo did not become involved in conflict between the Mōri and the Amago.
Hōki and Bitchū fell to the Mōri forces, leaving Yoshihisa isolated in Gassantoda Castle, and he surrendered in the eleventh month of 1556.