Battle of Ōsaki
Date: Tenbun 16 (1588)
Location: The Kami District in the northwest of Mutsu Province and other locations
Synopsis: The Date took the initiative by invading the Ōsaki territory and attacking Nakaniida Castle, but after bogging down in lowlands amid snowfall, were routed by Ōsaki forces bursting out of the castle. After Date forces gathered in Niinuma Castle, the Ōsaki army laid a siege which resulted in the tender of hostages under a negotiated settlement.
The Battle of Ōsaki occurred in Tenbun 16 (1588) in the Kami District of Mutsu Province. This conflict was waged between the army of Date Masamune (a sengoku daimyō and seventeenth head of the Date clan) and the allied forces of Ōsaki Yoshitaka (a sengoku daimyō and twelfth head of the Ōsaki clan) and Mogami Yoshiaki (a daimyō and the eleventh head of the Mogami clan).
Prelude to the conflict
Owing to a series of clashes continuing from Tenbun 11 to Tenbun 17 (1542 to 1548) between supporters of Date Tanemune (the fourteenth head of the Date clan) and those of his eldest son, Date Harumune known as the Tenbun Conflict, the Date clan lost a significant amount of its power. In the era of Date Terumune, the clan recovered most of its sphere of influence and took control again of the southern Ōu region (meaning the southern region of Mutsu and Dewa provinces). However, the diplomatic order in the southern Ouu region driven by the Date clan was shaken by the appointment of Toyotomi Hideyoshi to the position of kanpaku, or Chief Advisor to the Emperor in addition to a series of military actions triggered after Masamune’s succession as the seventeenth head of the Date clan. Upon the death of Masamune’s father, Date Terumune, most of the alliances and subordinate powers to the Date broke from the clan.
Located to the north of the territory controlled by the Date, the Ōsaki clan formerly served as the Ōshū tandai, a representative of the Muromachi bakufu to govern Mutsu Province in lieu of a military governor. The Ōsaki also showed signs of separating from the Date and becoming independent.
Soon after the Kyūshū Pacification led by Hideyoshi, a dispute arose within the Ōsaki clan between Niida Osabe Yoshikage and Iba Sōhachirō – two child companions of Ōsaki Yoshitaka (the twelfth lord of the Ōsaki clan). This escalated into a family conflict. Ujiie Yoshitsugu was a senior retainer of the Ōsaki clan serving as the shitsuji, or minister of the clan, and as the lord of Iwatesawa Castle. Together with Yoshikage, Yoshitsugu devised a plot to kill Sōhachirō and order their lord, Yoshitaka, to commit seppuku, based on a promise to serve Masamune in exchange for reinforcements. However, supporters of this plot decided to align with Yoshitaka in an effort to eliminate Yoshitsugu and the rest of the opposition.
In the first month of 1588, under the pretext of suppressing an internal conflict involving the Ōsaki clan, Masamune dispatched Hamada Kagetaka on his behalf and ordered Rusu Masakage, Izumida Shigemitsu, and Oyamada Yorisada to deploy. Meanwhile, Ōsaki Yoshitaka came to intercept them while placing Nanjō Takanobu in charge of Nakaniida Castle to prepare for a defensive battle from the castle.
Course of events
Masamune sent 10,000 (or, by other accounts, 5,000) soldiers to the Ōsaki territory.
On 2/2, Izumida Shigemitsu led the vanguard of the Date army in an attack on Nakaniida Castle, but, lowlands surrounding the castle and a heavy snowfall limited their movement, so the forces had to withdraw. Acting on the opportunity, the Ōsaki army burst out of the castle and crushed the Date forces. Moreover, Kurokawa Haruuji (the lord of Tsurudate Castle and the father-in-law of Rusu Masakage) switched sides from the Date to the Ōsaki and attacked the Date forces assaulting Nakaniida Castle. Caught in a pincer attack, the Date forces fell into disarray and evacuated to Niinuma Castle, only to be surrounded by Ōsaki forces in pursuit. On 2/23, Rusu Masakage (who was holed-up in the castle) reached a settlement with the Ōsaki through the mediation of Kurokawa Haruuji to tender Izumida Shigemitsu and Nagae Katsukage (older brother-in-laws of Kasai Harunobu and Sōma Yoshitane) as hostages in exchange for lifting of the siege.
On 2/29, Masakage departed from Niinuma Castle and withdrew while gathering the defeated soldiers. Shigemitsu and Katsukage were incarcerated in Arigafukuro Castle. Katsukage was released while Shigemitsu was moved to the main base of Mogami Akimitsu at Yamagata Castle. Through the entreaty of Masamune’s mother, Yoshihime, a settlement was reached so that, on 7/22, Shigemitsu could finally return home.
Mogami Yoshiaki (from a cadet family of the Ōsaki clan and whose formal wife came from the Ōsaki) did not permit military intervention by Masamune, and led 5,000 soldiers as reinforcements to join with the Ōsaki army for attacks on the Kurokawa and Shida districts in the Date territory.
In the southern part of the Date territory, on 2/12, Ashina Yoshihiro sent Ōuchi Sadatsuna to attack Nawashiroda Castle while Ishikawa Mitsumasa, the lord of Odemori Castle, received support from Sōma Yoshitane to abandon the Date. These developments were precursors to the Battle of Kōriyama.
Skirmishes with the Mogami in Date territory persisted over a period of days. On 4/28, Date forces killed 101 members of the Yamagata group at Akiu and brought 21 heads to Masamune.
Meanwhile, Uesugi Kagekatsu took advantage of the focus of the Mogami on supporting the Ōsaki to disregard the orders prohibiting personal conflicts between daimyō by encroaching on Shōnai in territory controlled by the Mogami.
Consequences of the battle
In the fifth month, Masamune’s mother, Yoshihime (Yoshiaki’s younger sister) headed toward the battlefield to mediate a settlement. Yoshiaki sent a letter to Yoshihime requesting her to persuade Masamune to settle. In the seventh month, Izumida Shigemitsu who had been tendered to the Ōsaki as a hostage was taken to Yamagata Castle and a settlement was reached on the condition that he continue as a hostage (although, in fact, after Shigemitsu was taken to Yamagata, before long he was sent to Masamune). Thereafter, Yoshihime herself negotiated with Yoshiaki in Yamagata Castle in regard to pardons for Ujiie Yoshitsugu who was in conflict with Ōsaki Yoshitaka and Kurokawa Haruuji who was formerly subordinate to but later separated from Date Masamune. Negotiations continued to resolve the conflicts within the Ōsaki clan and, in the ninth month, the parties finally reached a settlement.
This battle was, in the end, an extension of an internal conflict in the Ōsaki clan. Ujiie Yoshitsugu (the secretary of the Ōsaki clan) requested Date Masamune to send reinforcements while Mogami Yoshiaki requested the same from his lord, Ōsaki Yoshitaka. This was done on the basis that upon resolution of the internal conflict besetting the Ōsaki clan, the forces would be withdrawn and Yoshiaki pressured Yoshitaka to forgive Ujiie and his supporters in view of Masamune’s wishes.
In the eighth month of 1588, Uesugi Kagekatsu ordered forces led by Honjō Shigenaga and Daihōji Yoshikatsu (father and son) to launch a full-scale invasion into the Shōnai region of the Mogami territory at the Battle of Jūgorigahara resulting in a victory for the Uesugi.
With the support of the Mogami, the Ōsaki repelled Masamune. Nevertheless, in 1589, after Masamune prevailed at the Battle of Suriagehara and destroyed the Ashina clan, the Ōsaki could not withstand the pressure and submitted to the Date again.