Collapse at Oshibara
Date: Kōji 2 (1556) or Eiroku 1 (1558)
Location: Oshibara in the vicinity of the Iwami-Ginzan silver mine in Izumo Province
Outcome: The Shishido army (fighting on behalf of the Mōri) fell into disarray after being attacked by the Amago with falling boulders from both sides of a narrow ravine leading to the subsequent loss of Yamabuki Castle to the Amago.
The Collapse at Oshibara occurred either in Kōji 2 (1556) or Eiroku 1 (1558) at Oshibara in which the Mōri clan suffered a major defeat by the Amago clan. This conflict is also referred to as the Ushibara kuzure or Collapse at Ushibara.
Contest for the Silver Mine
From 1526, operations at the Iwami-Ginzan silver mine began in earnest under the governance of Ōuchi Yoshioki. In 1530, occupation of the mine by Ogasawara Nagataka, a local gōzoku, or family of means and influence, initiated a struggle for control of the operations. Although, three years later, the Ōuchi recovered the mine, in 1537, Amago Tsunehisa invaded Iwami Province and began to intervene in the contest for the silver mine.
Around the fourth month of 1553, the Ōuchi (who controlled the mine at the time) appointed Sasuga Naganobu to the stronghold of Yamabuki Castle on the mountainside of Ginzan. In 1555, however, after Sue Harukata (who was in charge of house affairs for the Ōuchi clan) lost and was killed in fighting against Mōri Motonari at the Battle of Itsukushima, those forces withdrew from Iwami so that, henceforth, a struggle between the Mōri and the Amago for control of the mine ensued.
Iwami-Ginzan and Oshibara
Until the Edo period, when the Edo bakufu placed Iwami-Ginzan under its direct jurisdiction, rights to the mining operations were exclusively held by merchants. Assorted daimyō such as the Mōri and the Amago clans levied transit fees for the transport of silver ore (and, later, processed silver) from the mine. To secure these rights, the Ōuchi constructed Yamabuki and Yataki castles nearby Ginzan. The mountain for the mining of silver reached a higher altitude than the mountains on which the castles were situated but it would be against their interests to harm the miners and their families so the daimyō did not interfere much with them.
With respect to securing Iwami-Ginzan, Yamabuki Castle became a key focal point. The stronghold was constructed on a steep mountain so could not be taken by force and there were only two options for attack: (i) offer favorable conditions to the castle lord in exchange for surrender, or (ii) block the supply of provisions until the defenders capitulate. In cases when the first option were unsuccessful, then the second option would be chosen. These two options were fundamental with respect to the struggle for control of the mine. Oshibata was located en route to a road that formed a T-intersection with the main road used by the Amago army to march to Iwami-Ginzan. The battle occurred at a vital transit location centered around Kametani Castle with residences of blacksmiths and military staff in the surrounding area.
A loss by the Amago of their base at Kamedani Castle in Oshibara would enable the Mōri to sever supply lines to the Amago army attacking Yamabuki Castle. Transporting military supplies by sea route would require defeating the Mōri forces that had gathered in the environs of the castle. In other words, securing Oshibara and Kamedani Castle would enable the Amago to threaten the supply lines of the Mōri army. Oshibara had significant strategic value to the aim of each side to secure Iwami-Ginzan.
Theories concerning the timing of the battle
Theory of Kōji 2 (1556)
In the third month of 1556, at the height of the Subjugation of Bōchō (during the assault of Susumanuma Castle), Kikkawa Motoharu (the second son of Mōri Motonari) joined with Shishido Takaie and Kuchiba Michiyoshi on a deployment to Iwami, compelling the surrender of Sasuga Naganobu (the lord of Yamabuki Castle) in the fifth month. The Mōri took control of Iwami-Ginzan and used Biwakō Castle as the base for the expeditionary forces. Biwakō was constructed by Michiyoshi around 1531. As of this time, the invasions of Suō and Nagato provinces were not yet complete so Motoharu was likely sent to Iwami to counter the Amago in Iwami and to lay the groundwork for the broader campaign in Suō and Nagato.
Meanwhile, Amago Haruhisa (the lineal grandson of Tsunehisa) quickly withdrew from Bizen Province (an expedition in support of Uragami Masamune) and assembled an army for the purpose of recapturing Iwami-Ginzan. In the opening stages of the conflict, the Mōri repelled the Amago army. Allied forces of the Amago and Ogasawara clans then returned to re-engage the Mōri in battle at Oshibara.
Theory of Eiroku 1 (1558)
After taking control of Yamabuki Castle and Iwami-Ginzan in the fifth month of 1556, and after having pacified Suō and Nagato provinces, toward the end of the fifth month of 1558, the Mōri attacked and toppled several castles held by the Ogasawara. In the sixth month, the Mōri approached Nukuyu Castle. Amago Haruhisa, along with Honjō Tsunemitsu deployed to Yunotsu, but, owing to heavy rain that caused swelling of the Gō River, could not go to Nukuyu Castle on the other side of the river. The two armies stared at each other across the river. During this time, Kobayakawa Takakage (the third son of Mōri Motonari) persuaded Ogasawara Nagakatsu to vacate the castle in the eighth month.
Meanwhile, having failed in providing support to Nukuyu Castle, Haruhisa turned his forces around and, to attack Yamabuki Castle, engaged in battle in Oshibara. There is also a theory that Nukuyu Castle did not fall.
Course of events
After 25,000 mounted soldiers from the Amago army attacked Yamabuki Castle, the Mōri sent 7,000 mounted soldiers under the command of Shishido Takaie. At the outset, Honjō Tsunemitsu of the Amago army blocked the route for the provisioning of Yamabuki. He also prevented merchants from making deliveries to Yamabuki. Thereafter, Tsunemitsu converged with the main division under Haruhisa and surrounded the castle. In an effort to protect the castle, Motonari sent Takaie with reinforcements to attack. Haruhisa responded by deploying to Oshibara.
The location of the battlefield between the Shishido and Amago armies at Oshibara was a narrow ravine with several mountain steams enabling the Shishido to counter the numerical advantage of the Amago. The site also served to separate the Amago army from Kamedani Castle. It was an ideal location with one exception. After taking-up positions atop the steep mountains, the Amago army dropped boulders in a pincer attack against the Shishido. The Shishido did not anticipate an attack from the flanks along with further attacks against the soldiers at Kamedani Castle. Without anywhere to flee, the Shishido army fell into disarray and self-destructed, losing several hundred soldiers in the course of the rout. Having repelled the Shishido army, Haruhisa confronted the armies of Motonari and Motoharu that arrived later, continuing with the siege of Yamabuki. The Amago then made attacks in GInzan as well to disrupt the Mōri army and, after Sasuga Naganobu, the chamberlain of Yamabuki, killed himself, on 9/3, the castle fell.
Owing to this battle, Haruhisa recaptured Iwami-Ginzan and Yamabuki Castle. Honjō Tsunemitsu (the lord of Takayagura Castle) was appointed as chamberlain of Yamabuki owing to his significant contributions in battle. To secure their hold on Iwami-Ginzan, the Amago clan established a contact network with Onsen Hidenaga, a local gōzoku, or family of means and influence, and direct retainers of the Amago (Tako Tokitaka and Ushio Hisakiyo). Thereafter, despite numerous attempts, the Mōri were unable to recapture Iwami-Ginzan and lost in events such as the Battle of Gōrozaka. The Mōri could not succeed during Haruhisa’s lifetime. In 1562, the Mōri finally recovered Iwami-Ginzan through further manipulation and after the Peace Treaty between Aki and Izumo.