Battle of Kagamiyama Castle

鏡山城の戦い

Amago Clan

Aki Province

Ōuchi Clan

Date:  Sixth month of Daiei 3 (1523)

Location:  Kagamiyama Castle in Saijō in Aki Province

Outcome:  The Amago prevailed after Mōri Motonari enabled the Amago forces to take over the castle by persuading Kurata Naonobu to betray the defenders.

Commanders:  Amago Tsunehisa, Mōri Motonari

Forces:  4,000 from the Mōri including a band of kokujin from Aki plus an unknown number of Amago forces

Casualties:  Unknown

Commanders:  Kurata Fusanobu, Kurata Naonobu

Forces:  Unknown

Casualties:  Unknown but Fusanobu took his own life

The Battle of Kagamiyama Castle occurred in the sixth month of 1523 at Kagamiyama Castle in Aki Province.  The battle was waged between the Amago clan of Izumo Province and the Ōuchi clan based in Suō Province.

Prelude

Kagamiyama Castle was a mountain fortress located in Saijō in Aki Province.  Having designs to take control of Aki, the Ōuchi who governed Suō and Nagato built Kagamiyama Castle during the period from 1457 to 1466, and used it as a base for operations in Aki.

In 1522, Ōuchi Yoshioki dispatched Sue Takafusa as the commander-in-chief to Aki.  Takafusa resided in Aki from third month to the eighth month.  During this time, he launched attacks against Shinjō-Obata and Ōtsuka, but, owing to the defensive efforts of the Aki-Takeda army, did not yield much progress.

Meanwhile, Amago Tsunehisa, the sengoku daimyō of Izumo Province (who was expanding his influence into Aki and Bingo provinces) took advantage of an opportunity created by Yoshioki’s deployment of forces to northern Kyūshū by marching to Saijō in a bid to topple Kagamiyama Castle.  Together with a band of kokujin from Aki who served the Amago, he launched an assault on Kagamiyama Castle.  The Kikkawa came under the command of the Amago in 1518, the year after the Battle of Arita-Nakaide in which the combined Mōri and Kikkawa forces prevailed against the Aki-Takeda clan.  Similarly, the Mōri also came under the governance of the Amago.

Course of events

On 6/13 of 1523, Mōri Motonari joined Kikkawa Kunitsune to lead 4,000 soldiers in an assault on the castle.  During this period, Motonari served as the guardian of Mōri Kōmatsumaru who, at the age of two, became the head of the Mōri clan in 1516 following the death by illness of his father, Mōri Okimoto. Kōmatsumaru participated in this battle.  On behalf of the Ōuchi, Kurata Fusanobu (serving as the general of the castle) and, Kurata Naonobu (Fusanobu’s uncle serving as the lieutenant general) were based in Kagamiyama Castle.  Together, Fusanobu and Naonobu led forces to intercept the Amago army.  Owing to their valiant efforts, the Amago army could not easily approach near the castle and the two sides entered into a stalemate.  Motonari then devised a plan to persuade Naonobu to betray the Ōuchi on the condition that Motonari allow the Kurata family to become heirs to the clan.  Assigned to the outer citadel, Naonobu guided the Amago into the castle.  The entry of Amago forces into the castle grounds caused mass confusion, during which Fusanobu sheltered in the main citadel.  The defensive effort lasted one day and night, but then, on 6/28, the castle fell to the Amago.  Fusanobu took his own life in exchange for a promise to spare the lives of his wife and children.

After the fall of the castle, Tsunehisa acknowledged Fusanobu’s request, but criticized Naonobu’s betrayal and ordered his execution.  Motonari’s trickery was not only derided but, even though he made the most significant contribution to the battle, did not generate admiration for the Mōri clan.  In this battle for the castle, Tsunehisa became wary of the superior intellect and courage of Motonari, while Motonari also came to distrust Tsunehisa.

Aftermath

After the battle, Kōmatsumaru fell ill after returning to Aki and died on 7/15 of 1523.   Wary of Motonari, Tsunehisa used one of his retainers, Kamei Hidetsuna, to intervene in the ensuing succession struggle within the Mōri family.  Tsunehisa plotted to back Aiō Mototsuna, the younger brother of Motonari, but Motonari forestalled this action by purging Mototsuna and his supporters.  Tsunehisa’s intervention failed and Motonari became the next head of the clan.

In 1525, in a bid to regain power, the Ōuchi clan went on the offensive in Aki.  Until that time, Motonari had sided with the Amago, but, in the third month of 1525, he separated from the Amago and came under the command of the Ōuchi.  Motonari participated when Sue Okifusa led the Ōuchi army in an attack against Yoneyama Castle.  Motonari negotiated the surrender of Amano Okisada, the lord of Yoneyama Castle.  The poor behavior of Tsunehisa at the Battle of Kagamiyama is considered one of the reasons why Motonari separated from the Amago.  The Ōuchi army succeeded in recapturing Kagamiyama Castle, reducing the power of the Amago in Aki Province.

Kagamiyama Castle did not sit at a high elevation, so it was not impregnable.  Therefore, the Ōuchi newly constructed Sobaga Castle, and, thereafter, Tsuchiyama Castle, to serve as their main base, abandoning Kagamiyama Castle.