Dates: Fourth to seventh months of Tenshō 16 (1588) (or, under another view, the second to seventh months)
Location: In the area surrounding Kōriyama and Kubota Castles in the Asaka District of Mutsu Province
Outcome: After a prolonged confrontation between opposing armies in fortified positions near Kōriyama Castle, settlements were finally reached between the Date on one side and the Satake and Ashina on the other.
The Kōriyama Conflict covers a series of battles between the Date army of Date Masamune and the Sōma army of Sōma Yoshitane (supported by the forces of Ashina Yoshiro) for control of an area encompassing Kōriyama Castle and Kubota Castle in the Asaka District of Mutsu Province.
On 10/9 of 1586, Tamura Kiyoaki, a sengoku daimyō and twenty-fifth head of the Tamura clan in Mutsu, died. In an event known as the Tenshō Tamura Disturbance, the demise of Kiyoaki led to chaos within the Tamura clan, making them vulnerable to encroachment by neighboring powers. In the fourth month of 1588, an invasion of the Tamura territory by the Sōma clan triggered a counterattack by the Date.
There are two views with respect to the time frame for the Kōriyama Conflict. Under one view, the conflict occurred from the fourth to seventh months of 1588 marked by the foregoing response of the Date to the invasion by the Sōma of the Tamura territory. The Ashina provided reinforcements to the Sōma in battle against the Date. Under the alternate view, the conflict occurred from the second to seventh months of 1588 which includes the period from the second to fourth months during which the Ashina fought against the Date on their own accord.
In the third month of 1587, after Ashina Yoshihiro (the second son of Satake Yoshishige) assumed the role as head of the Ashina clan, he began to actively promote attacks against the Date clan. Before the end of the year, he launched several attacks against Nawashiroda Castle with the aim of dividing the Date and Tamura while attacking the Nihonmatsu.
Attacks by Ashina Yoshihiro (second to fourth months)
In the second month of 1588, Date Masamune intervened in an internal conflict in the Ōsaki clan and lost at the Battle of Ōsaki. Ashina Yoshihiro viewed this as an opportunity to expand so he dispatched Ōuchi Sadatsuna to lead a vanguard force of 4,000 soldiers into the Date territory. On 2/12, Sadatsuna attacked Nawashiroda Castle and then converged with a follow-on contingent to mount attacks against castles aligned with the Date, including Kōriyama, Kubota, Takakura, and Motomiya. To intercept the invaders, forces from the south led by Date Shigezane (the lord of Nihonmatsu Castle) along with reinforcements under Katakura Kagetsuna (the lord of Ōmori Castle) and Shiroishi Munezane (the lord of Miyamori Castle) all together totaled only 600 soldiers. Nevertheless, Shigezane managed to defend against the invasion for two months, overcoming the attacks by the Ashina forces. However, to the north, Mogami Yoshiaki participated by sending reinforcements for the Ōsaki, launching attacks in multiple locations across the Date territory. Moreover, Ishikawa Mitsumasa (the lord of Odemori Castle) relied upon Sōma Yoshitane to abandon the Date. This caused Masamune himself to turn his attention toward potential conflict with the Sōma, preventing the dispatch of reinforcements to the front lines in the south.
In an effort to overcome this situation, Sanejitsu convinced Masamune to sign a statement pledging to award territory in the Date District including Hobara and Kaketa. Sanejitsu then proposed this to Sadatsuna in return for serving the Date. At this moment, tensions within the Ashina family deepened between new members who originated from the Satake clan to serve Yoshihiro and multi-generational retainers of the Ashina from Ōshū. Consequently, Sadatsuna accepted the offer from Shigezane and switched sides to the Date. On 4/18, the Ashina forces attacked Motomiya Castle in a bid to eliminate Date forces and Sadatsune for having abandoned the Ashina, however, these forces were confronted along the shores of the Abukuma River by over 1,000 soldiers led by Sadatsuna and fled in defeat.
Confrontations at Kōriyama and Kubota castles (fifth to seventh months)
On 5/22, Masamune himself led soldiers on an attack against Odemori Castle but, owing to bad weather, the forces temporarily withdrew to Ōmori Castle. However, on 5/12, Sōma Yoshitane secured the territory of the Tamura clan which was in disarray following a division between Date and Sōma factions after the demise of Tamura Kiyoaki (Masamune’s father-in-law whose wife was the aunt of Yoshitane). With the aim of serving as the rear guard for the Ashina forces and Odemori Castle, he headed toward Miharu Castle but was blocked by Hashimoto Kentoku and others from the Date faction in the Tamura family so he could not enter the castle and retreated. Upon learning of the withdrawal of the Sōma forces, Masamune gathered troops at Miyamori Castle, and once again commenced attacks against Odemori Castle. On 5/16, Odemori fell and Ishikawa Mitsumasa fled to the Sōma territory. On 5/17, Ōkura Castle was captured, and, on 5/18, Gassan, Dōmeki, and Ishizawa castles fell. On 5/19, Yoshitane withdrew from Funehiki Castle and the Sōma forces on the battlefront in the direction of Higashi-Adachi collapsed. Faced with precarious circumstances, Yoshitane requested support from Satake Yoshishige, Ashina Yoshihiro, and Iwaki Tsunetaka. The Satake and Ashina immediately responded but, after having invaded the territory of the Tamura while Tamura Kiyoaki was alive, Tsunetaka took issue with Yoshitane entering Miharu Castle and refused to send reinforcements. The fact that the territory of the Tamura was the subject of a three-cornered battle among the Date, the Sōma, and the Iwaki added to the complexity of this battle.
In the sixth month, the allied armies of the Satake and Ashina advanced troops toward Kōriyama. This defied the expectations of Masamune who anticipated an invasion in the direction of Motomiya near Miyamori Castle. Masamune also departed Miyamori Castle and traveled from Motomiya toward Kōriyama to provide support. Tamura Akiyori and Tamura Akimori of the Tamura clan came running as reinforcements while, on 6/14, Rusu Masakage (a member of the Date clan) also arrived in support. On 6/12, the allied armies of the Satake and Ashina approaching Kōriyama and Kubota castles confronted from afar the Date forces, whereupon each side built fortresses and, over the next forty days, engaged in a series of small skirmishes. According to an account of Masamune, the allied armies had approximately 8,000 mounted soldiers compared to only 600 on the side of the Date. Based on another account, the allied armies had 4,000 mounted soldiers and the Date had 600, so, in either case, the Date bore a significant disadvantage.
Upon orders of Masamune, Shigezane set-up a camp for Date forces on Mount Sannō. By digging trenches and building mounds, the forces reinforced the position so the allied armies could not capture it. Moreover, Sasagawa Castle alongside the Abukuma River was aligned with the Date, so the allied armies had to keep in check the Date forces behind them. Through the day and night, the opposing forces fired four to five thousand rounds from arquebuses in a fierce fight. On 7/4, a unit led by Niikuni Sadamichi (aligned with the Ashina) passed in front of Katakura Kagetsuna and Date Shigezane who were defending Kubota Castle. After Kagetsuna had his younger brother, Katakura Saemon, chase after them, Saemon became surrounded by Ashina forces. In an effort to save him, Kagetsuna and Shigezane engaged in battle against the Ashina, but pulled back after a bitter confrontation. Although Itō Shigenobu was killed in action, the Date forces killed over fifty enemy soldiers in the counterattack before retreating.
For various reasons, neither side could launch a full-scale attack. The Date had a small force, and despite the Ōsaki and Mogami ceasing their advance and the start of settlement negotiations, the northern portions of the Date territory remained vulnerable. Moreover, the Date had not fully recovered from their loss at the Battle of Ōsaki so were not in a position to go on the offensive. Meanwhile, on the side of the Ashina, Satake Yoshishige had been repeatedly warned by Toyotomi Hideyoshi to abide by the order issued in the twelfth month of the prior year that prohibited territorial conflicts between daimyō, and told to promptly settle differences with his son, Yoshihiro, and his nephew, Masamune. Therefore, he could not openly lead forces in an effort to eliminate Masamune and was compelled to continue to wait-and-see. He may also not have possesses sufficient forces for a decisive battle. While the order from Hideyoshi prohibiting conflicts between daimyō may have had an inhibiting effect, ultimately it was other factors that likely prevented a final showdown.
After each side established positions confronting one another over Kōriyama Castle, the conflict wore on for a long period. On 7/2, Iwaki Tsunetaka solicited Ishikawa Akimitsu to propose that Akimitsu serve as a mediator for a settlement with Masamune. The clashes with bow and arrow and with arquebuses ceased on 7/5. Although the negotiations with the Ashina bogged down over differences concerning the lines of demarcation for disputed territory, on 7/16, a settlement was reached between the Date and Satake, and, on 7/18, between the Date and the Ashina, as confirmed by the Satake. On 7/21, the opposing armies withdrew.
Aftermath of the conflict
On 8/5, Masamune entered Miharu Castle, placed Tamura Muneaki (Megohime’s younger cousin) as head of the Tamura clan, and succeeded in securing the territory of the clan as a series of battles ended in victory for the Date. However, Iwaki Tsunetaka intervened for the purpose of preventing the territory of the Tamura from falling into the hands of Sōma Yoshitane or Date Masamune, but his calculation failed when Yoshitane abandoned Miharu Castle only to be replaced by Masamune. In the fourth month of 1589, Tsunetaka deployed in a bid to garner control of the territory of the Tamura and fought against Masamune.
The battle marked the end of a string of defeats extending for a period of three years from the Battle of Tedoribashi. Having been increasingly cornered since the death of Date Terumune (Masamune’s father), the Date leveraged this as an opportunity to reverse their fortunes and expand their territory. Nevertheless, this battle violated an order issued the preceding year by Toyotomi Hideyoshi forbidding provincial daimyō from engaging in personal conflicts among themselves. Thereafter, during the Oushū Retribution, eight districts acquired by Masamune (Asaka, Iwase, Shirakawa, Ishikawa, Yama, Kawanuma, Ōnuma, and Aizu) were subject to seizure.