The Battle of Ameyama occurred on 8/4 of Kōji 2 (1556) in the Nukata District of Mikawa Province during the Sengoku period.  The battle was waged between the Imagawa and Okudaira clans.

This battle was one event during a rebellion by the kokujin, or provincial landowners, of Mikawa against the Imagawa clan (the shugo daimyō of the province) known as the Furious Drama of Mikawa.  

As conflict deepened between the Oda and Imagawa clans in Mikawa, tensions mounted among the kokujin (or kunishū) comprising the class of small-scale landowners in Mikawa.  From 1555 to 1558, the kokujin launched a large-scale rebellion against the Imagawa – the Furious Drama of Mikawa.

Within the Okudaira clan based in the Tsukude area, Okudaira Sadakatsu supported the Imagawa while his son, Okudaira Sadayoshi, demonstrated opposition to the Imagawa triggering, among other clashes, the Battle of Hijika.  From Achiha Castle in Ameyama, Achiha Sadanao (later the Ameyama-Okudaira family), Achiha Sadasuke, and Okudaira Sadayoshi acted in concert with Sadakatsu.  On 8/4 of Kōji 2 (1556), the Imagawa forces responded by attacking the kokujin at Ameyama.

Suganuma Sadamura, the lord of Noda Castle and second head of the Noda-Suganuma clan (aligned with the Imagawa), led forces up the valleys along the Ameyama River to attack.  Achiha Sadanao and Okudaira Sadayoshi installed wicket fences at the narrowest point between the mountains on either side of the valley and intercepted the approaching forces.  It is surmised that Achiha Castle was referred to as the Ameyama fortress owing to the defenses set-up for the Battle of Ameyama.

Based on a written commendation from Imagawa Yoshimoto to a retainer from the Mikawa-Makino clan named Nose Jinza, the attacking forces attempted to invade not only from downstream the Ameyama River but also by traversing the mountains from Chigiri in the Hoi District behind Ameyama on the other side of the ridgeline of the mountains in Mikawa.  Clashes with the Okudaira forces also occurred in this area.

After arriving at the wicket fences, Suganuma Sadamura noted the fortress was on steep ground, but that there were few defenders.  Confident that he could overcome them, he commenced an attack.  Okudaira Gorōemon (the younger brother of Okudaira Sadayoshi) leveraged the valley terrain to approach and launch an arrow that struck Sadamura in the throat, killing him.  Siblings and subordinates trailing from behind were also killed resulting in a bitter defeat for the Imagawa.

Thereafter, the Okudaira forces who waged their first battle at the Ameyama fortress could not halt the large army of the Imagawa so, in the end, surrendered.  Okudaira Sadakatsu ousted Okudaira Sadayoshi who orchestrated the rebellion from Mount Kōya so was pardoned and returned to the service of the Imagawa.

Anecdote

On 6/15 of Heisei 4 (1992), the ancient battleground was designated a historical site in the city of Okazaki known as the Ameyama Fortress Ruins and Ameyama Battlefield.