Battle of Imahashi
Date: Ninth and tenth months of Eishō 3 (1506)
Location: Imahashi Castle in the Atsumi District on a southern peninsula of Mikawa Province
Outcome: The Imagawa clan from Suruga and Tōtōmi provinces prevailed and Makino Kohaku, lord of Imahashi Castle, was killed in action, setting the stage for further incursions by the Imagawa in western Mikawa.
The Battle of Imahashi occurred in the ninth and tenth months of Eishō 3 (1506) in regard to Imahashi Castle in the Atsumi District of Mikawa Province during the Sengoku period.
In 1506, it became known that forces led by the Imagawa clan (the sengoku daimyō of Suruga and Tōtōmi provinces) attacked Makino Kohaku, a kokujin (or provincial family of influence) at Imahashi Castle in eastern Mikawa. In this battle, the Imagawa prevailed while Kohaku was killed in action. Subsequently, the Imagawa clan conquered Matsudaira Nagachika in western Mikawa and captured Imahashi Castle to use as a base of operations.
Under an alternate theory, Matsudaira Nagachika attacked the Makino clan allied with the Imagawa and toppled Imahashi Castle.
Prelude to the battle
At the time, the powerful Matsudaira clan of western Mikawa was opposed to the Imagawa clan so the Imagawa had reason to subjugate the Matsudaira.
Moreover, around this time, in eastern Mikawa, despite being under the command of the Imagawa clan, a struggle over territory arose between Makino Kohaku (the lord of Ishihashi Castle who was aiming to expand influence from the Hoi District into the Atsumi District) and Toda Norimitsu (the lord of Tahara Castle who was trying to increase his authority in the Atsumi District). The Toda clan then falsely claimed to the Imagawa that the Makino clan was colluding with the Matsudaira of western Mikawa. The Imagawa immediately responded by joining forces with the Toda to attack Imahashi Castle. Moreover, although this secret betrayal of the Toda is affirmed, the portion about the Imagawa adding forces to the Toda is uncertain.
Course of events
Imagawa Ujichika planned to conquer Mikawa with his uncle, Ise Shinkurō (later known as Hōjō Sōun) as the commanding general of the army. In the seventh month of 1506, Ujichika dispatched over 10,000 forces from Suruga, Tōtōmi, and eastern Mikawa under the Imagawa command to eastern Mikawa, and closely surrounded Imahashi Castle. During this time, there were movements from the side of Makino Kohaku to provide explanations or offer to settle, but the Imagawa refused and, during the ninth and tenth months, a genuine battle unfolded. By the beginning of the eleventh month, the castle fell and Kohaku took his own life.
Aftermath of the battle
The Imagawa army pressed on with other forces in eastern Mikawa who had surrendered and forced their way into western Mikawa to subdue the Matsudaira clan. The Matsudaira, however, fought valiantly, while the Imagawa forces on their distant expedition were threatened by the Toda who were relatives of the Matsudaira. Consequently, Ise Shinkurō was compelled to retreat to Imagawa territory without attaining his objective.
Under one theory, the Matsudaira forces chases the Imagawa to eastern Mikawa, so Imahashi Castle, which was the original objective of the attack, fell. As a result, the Makino family of Ishihashi weakened whereas the Toda clan of Tahara Castle increased their influence in the Atsumi District including with respect to Imahashi Castle. Moreover, remnants from the Makino family of Ishihashi fled to the Chita Peninsula in Owari Province and the territory of Tahara to await an opportunity to recover Imahashi Castle. In western Mikawa, Matsudaira Nagachika gained notoriety and the unifying power of the Anjō-Matsudaira family grew.
Reinforcements to the defenders of Imahashi Castle
At this time, Makino family members from Ushikubo led by Makino Shinjirō deployed as a rear guard at the Battle of Ishibashi, but after the defeat, the surviving troops re-grouped and returned to Ushikubo. Meanwhile, after receiving an urgent message regarding an invasion by the Imagawa, Matsudaira Nagachika from western Mikawa sent reinforcements to eastern Mikawa. Nagachika dispatched reinforcements to Yawata in the Hoi District. He positioned a rear guard in 大平河 in western Mikawa, but after receiving notice of a rout of the forces in eastern Mikawa, the Matsudaira army in 大平河 was shaken.
Invasion of western Mikawa in 1508
The attack by the Imagawa on Imahashi Castle in Mikawa in 1506 may be viewed as a separate event from the invasion of western Mikawa in 1508. A conflict in recorded accounts between references to a small number of forces as compared to more than 10,000 forces led by Ise Sōzui indicates the possibility of two separate attacks by the Imagawa in western Mikawa, one prior to the attack against Imahashi Castle, followed by another attack two years later. As a result of the Battle of Imahashi, the power and influence of the Imagawa may have expanded to the area near Okazaki in western Mikawa.
The Mikawa Conflict in the Eishō Era
One theory links, and refers to collectively as the Mikawa Conflict in the Eishō Era, the series of events including the Battle of Imahashi launched by the Imagawa, the Battle of Idano during which the Imagawa aimed to subjugate the Matsudaira in western Mikawa, the attack on Ishinomaki Castle, and battles between the Okudaira and Hosokawa in western Mikawa. In the background of these clashes, and related to maneuvers by the Muromachi bakufu during the Meiō era (1492 to 1501), stood a conflict between the Imagawa affiliated with the faction of Ashikaga Yoshiki (the tenth shōgun) and the Matsudaira clan who were close to Hosokawa Nariyuki, the military governor of Mikawa affiliated with the faction of Ashikaga Yoshizumi.