Battle of Mifuneyama
Year: 8/23 of Eiroku 10 (1567)
Location: Mifunedai in the foothills of Mount Mifune in the Kimitsu District of Kazusa Province
Outcome: Having designs to encroach on the Satomi territory in Kazusa, Hōjō Ujimasa built a fortress near Sanuki Castle and was then attacked and defeated by the Satomi army.
The Battle of Mifuneyama occurred on 8/23 of Eiroku 10 (1567) during the Sengoku period at Mifunedai (the Mifune plateau) in the Kimitsu District of Kazusa Province. The conflict was waged between Satomi Yoshihiro (a daimyō and sixth head of the Awa-Satomi clan of Awa Province) and Hōjō Ujimasa (a sengoku daimyō and fourth head of the Gohōjō clan of Sagami Province).
In the Second Battle of Kōnodai, the Satomi clan suffered a major defeat by the Hōjō clan. As a result, the Hōjō soon occupied former territory of the Satomi in the northern and western portions of Kazusa Province. In the eastern portion, the Hōjō had senior retainers of the Satomi, Masaki Tokitada and Toki Tameyori serve on their behalf. Further, in an effort to capture the base of Satomi Yoshihiro at Sanuki Castle, the Hōjō built a fortress at Mifunedai located at the foot of Mifuneyama (Mount Mifune). The work was personally supervised by Hōjō Ujimasa, the head of the Hōjō family. After construction of the fortress approximately just four kilometers north of Sanuki Castle, Yoshihiro responded to the threat by attacking the Hōjō soldiers stationed at Mifunedai. Upon learning of these developments, Ujimasa joined Ōta Ujisuke to travel by boat across Edo Bay to attack Sanuki Castle. Meanwhile, Ujimasa had his younger brother, Hōjō Ujiteru, and Hara Tanesada lead a detached unit from the Ichihara District upstream along the Obitsu River to attack Kururi Castle which was the base of Yoshihiro’s father, Satomi Yoshitaka.
Yoshihiro, along with Masaki Noritoki, burst out of Sanuki Castle to attack the Hōjō forces that were concentrated at Mifunedai. A fierce battle ensued in which the Satomi army defeated the Hōjō. At this time, Ōta Ujisuke was killed in action while serving as the rear guard. A violent battle also unfolded between the Hōjō and Satomi navies. Riding the momentum of their victory, the Satomi forces chased after the Hōjō. Fearing a pincer attack from land and sea, Ujimasa had all of the surviving Hōjō forces retreat to their home base in Sagami Province.
Consequences of the battle
After this victory, Yoshihiro succeeded in cutting-through the kokujin who had sided with the Hōjō, and he then launched a successful invasion of Awa and Kazusa provinces. This resulted in the re-association of the Mariyatsu territory with the Satomi clan. Without an expectation of support from the Hōjō, by the third month of 1569, Masaki Tokitada served the Satomi clan, while the Toke and Togane branches of the Sakai clan offered to serve the Satomi. Meanwhile, the Satake clan of Hitachi Province (which at one time had settled with the Hōjō) allied again with the Uesugi clan. The defeat in this battle had a significant impact on the Hōjō. In 1568, after a breaking of their alliance with the Takeda, the Hōjō were forced to change their strategy for the Kantō, extending offers to the Uesugi and the Satomi for a new alliance. The Satomi declined, while the Hōjō proceeded to ally with the Uesugi. The Satomi then entered into a three-way alliance with the Satake and Takeda clans.
The victory gave the Satomi a superior position vis-à-vis the Hōjō in Awa and Kazusa provinces until around 1575 when the Hōjō clan re-invaded their territory after spurning an alliance with the Uesugi in favor of one with the Takeda clan.