The Yoshinobu Incident occurred in the tenth month of Eiroku 8 (1565) as an assassination plot against Takeda Shingen, the nineteenth head of the Kai-Takeda and sengoku daimyō of Kai Province. As an outcome of this incident, Takeda Yoshinobu, the eldest son and designated heir of Shingen, was removed from the line of succession and Yoshinobu’s band of retainers was purged, causing internal ruptures with significant consequences for the family. This is noted as a contributing factor to the subsequent decline and eventual elimination of the Takeda by Oda Nobunaga in 1582.
Takeda Harunobu is widely known under the name of Takeda Shingen which he adopted after entering into the priesthood in the second month of 1559. For historical accuracy, he is referred to below as Harunobu until 1559 and as Shingen thereafter.
Harunobu did not have a good relationship with his eldest son and designated heir, Yoshinobu. At the age of fifteen, Yoshinobu married the daughter, Reishōin, of Imagawa Yoshimoto, the sengoku daimyō of Suruga Province, an ally of the Takeda clan. Owing to this relationship, the alliance with the Imagawa clan was given particular importance. The relations between Harunobu and Yoshinobu appear to have begun to deteriorate even before the Battle of Okehazama in which Yoshimoto was killed in the fifth month of 1560 by the Oda army while attempting to march through Owari Province.
Harunobu noted in a letter dated 7/16 of Tenbun 24 (1555) as follows:
“I send this secret letter in my own writing. To begin, in regard to Yoshinobu, owing to the Imagawa, he has forgotten his relationship with me as his father. I am like a grandfather to lord Gorō (Ujizane), and, since Nagakubo, provided military support to the Imagawa. Despite my cordial response to their requests, it is not acceptable to be neglected in this manner. He appears to have become estranged and is said to be faithless, so when he returns to the province, I intend to engage in direct talks with him. In that case, I heard from Hōjō Ujiyasu that he was told that the provincial landowners in Etchū desired a mediated settlement for a territorial accord. Every effort must be made for a resolution, but please note that I do not deliberately intend for a mediation. Accordingly, I inform you of these events by this sealed letter.”
According to this letter, from early on, Yoshinobu appealed to his father, Harunobu, regarding the lack of support given to Imagawa Yoshimoto and this issue caused fissures in the relationship between them. Moreover, even while Yoshimoto was still alive, Harunobu was deepening relations with Oda Nobunaga – an archenemy of the Imagawa clan. In 1554, after Harunobu took control of southern Shinano, the Tōyama clan, kunishū, or provincial landowners, in eastern Mino, offered to yield their allegiance to Harunobu. The Tōyama were also relatives of the Oda clan enabling close relations with Oda Nobunaga. As a result, the Tōyama were obedient to both the Oda and Takeda clans.
At the time, Harunobu was in conflict with Nagao Kagetora of Echigo Province so he sought to avoid a war in eastern Mino by choosing to cultivate friendly relations Nobunaga. In a letter from Nobunaga dated 11/23 of Eiroku 1 (1558), he offers thanks for a visit during deployment by Akiyama Torashige, the district governor of Ina and a retainer of the Takeda and said that he wanted to be sent a large falcon. It can be surmised that friendly relations had already been established between Nobunaga and Harunobu. Therefore, it is possible that Yoshimoto and others in the Imagawa clan harbored distrust toward the Takeda.
On 5/19 of Eiroku 3 (1560), at the Battle of Okehazama, Imagawa Yoshimoto was killed by the Oda forces led by Oda Nobunaga. Yoshimoto was succeeded by his eldest son and designated heir, Imagawa Ujizane, but following the demise of Yoshimoto, Matsudaira Motoyasu (later known as Tokugawa Ieyasu), the lord of Okazaki Castle in Mikawa Province who had formerly been obedient to Yoshmoto, became independent. Meanwhile, retainers of the Imagawa based in Tōtōmi Province rioted in an event known as the Enshū Disturbance. The Imagawa territory fell into a state of chaos. Observing the turmoil, Shingen initially agreed to support the Imagawa, but then changed his mind and instead began devising plans to seize territory from the Imagawa. Aware that Ujizane was struggling to contend with his opponents, Shingen sent a secret messenger to retainers of Anayama Nobutada who was serving as an intermediary for the Imagawa and ordered him to inquire about the situation in Tōtōmi. If it appeared that Ujizane would lose, he planned to launch an invasion of Suruga.
In the ninth month of 1565, negotiations began with Oda Nobunaga in regard to an alliance. This was to address the recurrence of conflicts between the Oda and Takeda in regard to the treatment of the Tōyama clan of eastern Mino who had affiliations with both clans. These negotiations proceeded swiftly and, in the eleventh month, a political marriage was arranged whereby Shingen’s fourth son, Takeda Katsuyori, wed Nobunaga’s niece (adopted daughter from Tōyama Naokado) named Ryūshōin.
Having earlier wed Yoshimoto’s daugther, Yoshinobu placed an emphasis on the alliance with the Imagawa and, in the wake of the Battle of Okehazama, could not tolerate in any manner an alliance with the Oda. To maintain relations with Yoshinobu, Shingen negotiated with Ashikaga Yoshiteru, the thirteenth shōgun of the Muromachi bakufu, and received a special privilege to award Yoshinobu the title of vice third deputy shōgun. Nevertheless, in 1562, Yoshinobu strongly objected to the appointment of Katsuyori (Yoshinobu’s younger brother of a different mother) as the lord of Takatō Castle in Shinano. At this time, the relationship between Shingen and Yoshinobu continued to be strained.
By coincidence, the eldest sons of the allied clans (Takeda Yoshinobu, Imagawa Ujizane, and Hōjō Ujimasa) were all born in 1538. In the case of Ujizane and Ujimasa, their fathers retired and each of them inherited the headship of their respective clans in 1559. Meanwhile, Yoshinobu likely harbored resentment for not inheriting the headship of the Takeda clan similar to the succession of his peers at this time, in addition to diplomatic policies that were not in his view commensurate with their interests in the Imagawa clan.
Course of events
Prior to the formation of an alliance with the Oda clan, Yoshinobu and his retainers including Obu Toramasa, Nagasaka Katsushige, and Sone Suō-no-kami, plotted a rebellion. He also had friendly relations with Anayama Nobutada who served as an intermediary for the Imagawa. After numerous consultations with Katsushige and Suō-no-kami, on 7/15 of Eiroku 7 (1564), Yoshinobu paid a secret visit to the residence of Toramasa under the pretext of viewing garden lanterns. Inspectors under the command of Shingen learned of the visit and notified Shingen. In addition, Obu Masakage (Toramasa’s younger brother), served as an informant regarding a secret meeting between Yoshinobu’s retainers and Toramasa. As evidence, he produced a letter from Yoshinobu. Shingen shed tears, and praised Masakage for his loyalty in reporting the planning rebellion by his older brother, Toramasa.
The plan for the coup d’état by Yoshinobu and his faction was thus exposed and, on 10/15 of Eiroku 8 (1565), Toramasa was apprehended and forced to commit seppuku. In a letter from Shingen dated 10/23 addressed to Obata Gengorō, a member of the Obata clan of Kōzuke, he stated: “Owing to the deeds of Obu Toramasa, a plot to hinder relations between Shingen and Yoshinobu became known, so I sentenced him. The relationship with Yoshinobu did not, from the beginning, exist so please do not be concerned.”
Nagasaka Katsushige and Sone Suō-no-kami were executed on grounds of treason. Over eighty members of Yoshinobu’s band of retainers were executed and the remainder expelled. Yoshinobu was incarcerated at the Tōkō Temple in Kōfu. On 12/5 of Eiroku 9 (1566), Anayama Nobuyoshi, the younger brother of Anayama Nobutada, was forced to take his own life for being found of conspiring with Yoshinobu.
Initially, Shingen considered restoring relations with Yoshinobu. In the spring of 1566, he engaged high priests from the Rinzai sect including Kaisen Jōki of the Eirin Temple, Junkoku Kōshin of the Chōzen Temple, and Randen Ejō of the Tōkō Temple to serve as mediators in an effort to reach a settlement with Yoshinobu. The situation, however, did not improve.
As a result, Yoshinobu’s retainers who had been ousted were not permitted to return to the service of the Takeda. On 10/19 of Eiroku 10 (1567), Yoshinobu died at the age of thirty. One military chronicle notes that he died either by taking his own life or from illness.
Owing to this incident, Shingen lost his lineal heir while the family incurred major disruption. In an effort to bring stability, in the eighth month of 1566 and, again in 1567, Shingen ordered retainers to submit written oaths of personal allegiance. Shingen then designated his fourth son, Takeda Katsuyori, to become his successor.
The Yoshinobu Incident led to a break in the alliance of the Takeda and Imagawa clans. In the spring of 1568, Imagawa Ujizane demanded the return of his younger sister named Reishōin who had been wed to Yoshinobu. Shingen, however, was not supportive of the request and delayed a response. Ujizane turned to another ally, Hōjō Ujiyasu, to mediate the return of his sister. Shingen responded with the condition that unless Ujizane signed an oath to preserve their alliance, he would not honor the request. Ujizane was thereby forced to submit a written oath whereupon Reishōin was permitted to return to the Imagawa.
This decisively broke the relationship between the Takeda and Imagawa. To oppose Shingen, Ujizane sent a letter to Uesugi Terutora of Echigo Province asserting that Shingen knew no limits to his betrayal and requested an alliance and military support. On 12/6 of the same year, Shingen launched the Invasion of Suruga against Ujizane that led to the end of the Imagawa as a sengoku daimyō family. Thereafter, the former territory of the Imagawa was the subject of ongoing clashes between the Takeda, the Tokugawa, and the Gohōjō clans.