The Mikawa Discord, or Mikawa sōgeki, occurred from Kōji 1 (1555) to Eiroku 1 (1558) as a large-scale conflict when provincial families rebelled against the Imagawa clan in Mikawa, particularly in the eastern portions of the province.

Course of events

Advance of the Imagawa clan into Mikawa

From the beginning of the sixteenth century, the Imagawa clan gradually encroached from their home in Suruga Province into Mikawa Province.  From 1546, this advance accelerated as the Imagawa vied against the Oda of Owari Province for territory and influence in Mikawa.  In 1547, Imagawa Yoshimoto (the shugo daimyō and sengoku daimyō of Suruga and Tōtōmi provinces) eliminated Toda Yasumitsu based in Tahara Castle.  This occurred after Yasumitsu was entrusted to send Takechiyo, a child and designated heir to the Anjō-Matsudaira clan, as a hostage to Sunpu, but failed in this task and turned him over to the Oda instead.  After attacking Yasumitsu at Tahara Castle, the Imagawa supplanted the influence of the Toda family in the Atsumi Peninsula and Mikawa Bay.

Meanwhile, Oda Nobuhide (the sengoku daimyō of Owari Province) compelled the surrender of Matsudaira Hirotada, father of Takechiyo and lord of Okazaki Castle in the Nukata District.  In 1548, the Imagawa prevailed in the Battle of Azukizaka and the Oda retreated, while Hirotada reverted to service of the Imagawa.  In 1549, after the sudden demise of Hirotada, the Imagawa seized Okazaki Castle, while bringing under their protection the youthful Takechiyo (later known as Matsudaira Motonobu → Matsudaira Motoyasu → Tokugawa Ieyasu), to Sunpu in their home base of Suruga Province.  Thereafter, the Oda recaptured Anjō Castle, while the Imagawa suppressed a rebellion by Kira Yoshiyasu and took him away to Sunpu.  Meanwhile, following the death of Oda Nobuhide, the Oda were beset by internal conflicts, allowing the Imagawa to gain an advantage.

In the second month of 1555, Yamaguchi Noritsugu, lord of Narumi Castle in Owari abandoned the Oda in favor of the Imagawa.  In the third month, at the coming-of-age ceremony for Matsudaira Takechiyo, Imagawa Yoshimoto served the honor of crowning him, after which Takechiyo wed the daughter of Sekiguchi Ujizumi known as Tsukiyama-dono.  From the time of Matsudaira Nobutada, the head of the Anjō-Matsudaira family had received one of the characters from the name of the head of the Kira clan, but, on this occasion, Takechiyo received the name Motonobu with one character from the name of Yoshimoto, demonstrating both internally and externally that the Imagawa had replaced the Kira as the provincial lords of Mikawa.

Rebellion in Mikawa

In the ninth month of 1555, the Imagawa confronted challenges to their rule in Mikawa from a broad range of actors and families.  First, Suzuki Nobushige of Asuke Castle, together with the Tōyama clan of Mino Province, raised arms against the Imagawa.  Then, Kira Yoshiyasu, the lord of Nishio Castle who had been allowed to return to Mikawa after having been taken as a hostage to Sunpu, joined with Mizuno Nobumoto, the lord of Ogawa Castle, to rebel.

The opposition grew to include, among others, the influential Okudaira clan.  Namely, Okudaira Sadayoshi, the eldest son of Okudaira Sadakatsu, rebelled and was ousted to Mount Kōya whereby it took over a half year to suppress the resistance, while rebellions within the family continued again the next year.

As the predominant clan in Mikawa, over the generations, the Matsudaira proliferated into an array of illegitimate branches, known as the Eighteen Matsudaira.  In fact, there were fourteen branches, comprised of the Ogyū, the Tōjō, the Takiwaki, the Ōkusa, the Fujii, the Fukama, the Takenoya, the Katanohara, the Goi, the Fukōzu, the Mitsugi, the Nagasawa, the Nomi, and the Sakurai.  The outbreak of the rebellion reverberated across these families, triggering conflicts between and within individual families.

In 1556, Sakai Tadanao of Ueno Castle (a quasi-independent senior retainer of the Anjō-Matsudaira clan) raised arms.  Meanwhile, an internal struggle arose in the Tōjō-Matsudaira family (also known as the Aono-Matsudaira) in which Matsudaira Tadashige, the head of the family and ally of the Imagawa, was killed in battle.  Matsudaira Chikanori of the Ogyū-Matsudaira family departed from the Imagawa and attacked the Takiwaki-Matsudaira family.

Other provincial families experienced divided loyalties.  Suganuma Sadatsugu abandoned the Imagawa and came into conflict with his younger brothers, Suganuma Sadanao and Suganuma Sadauji, who remained loyal to the Imagawa.  In the eighth month of 1556, Suganuma Sadamura led Imagawa forces against kokujin from the Okudaira and Achiha clans but was defeated in the Battle of Ameyama.  The Makino clan endured troubles between factions who were for and against the Imagawa.

By 1557, it appeared the Imagawa army had suppressed dissent in each region of Mikawa, but, in 1558, rebellions arose among other provincial families such as the Kawai and the Itō.  Suzuki Shigetatsu (Suzuki Hyūga-no-kami) of Terabe Castle acted in concert with the rebels by killing Matsudaira Omoe of the Nomi-Matsudaira family.  Imagawa Yoshimoto sent Matsudaira Motoyasu of the Anjō-Matsudaira family on his first deployment in the Siege of Terabe Castle.  The Imagawa backed Kira Yoshiaki to serve as the head of two branches of the Kira clan that had been separated since the Nanboku era, the Saijō-Kira clan and the Tōjō-Kira clan.  Yoshiaki was moved to Tōjō Castle and, by the tenth month, Nishio Castle came under the direct control of the Imagawa.  The rebellion subsided for a period after the Imagawa army prevailed in a clash against the Oda (which supported the rebels) at the Battle of Nagura-Funadobashi.

A reason why the rebellion flared across Mikawa may have owed to the political alliances between families forged through marital relationships, such as among the Okudaira, the Suganuma and the Makino clans of eastern Mikawa.  This was reinforced by repeated protests arising from territorial unions among the Matsudaira, the Suzuki, and the Kira clans.  This turned into an anti-Imagawa movement opposing an expansion of the rule of the Imagawa in Mikawa (or a movement connected to the Oda of Owari).  As a result, from the perspective of the Imagawa, the protests had to be dismantled to secure their governance, and not only through military means, but also by intervening in the internal affairs of families.  Prior to attaining this objective, Yoshimoto encountered his fate at the Battle of Okehazama and the influence of the Imagawa in Mikawa rapidly declined.

The rise and fall of the Imagawa governance of Mikawa

Owing to the suppression of a series of rebellions, almost all of Mikawa came under the control of the Imagawa.  Around 1558, Imagawa Yoshimoto retired and transferred his role to his son, Imagawa Ujizane. Thereafter, Yoshimoto delegated management of the internal affairs of the clan to Ujizane, while Yoshimoto himself continued to focus on operations in Mikawa.  In 1560, Yoshimoto led his forces into Owari Province whereupon he was ambushed and killed in a surprise attack by Oda Nobunaga at the Battle of Okehazama.  One of the purposes of this venture was to pacify the border area with Mikawa and to ensure the provincial families did not revolt again.

As a result, however, the death of Yoshimoto as well as errors committed in the wake of his demise led to the autonomy of Matsudaira Motoyasu (an event known as the Sanshū Disturbance) and the reemergence of riots that the Imagawa had previously suppressed.  The inability of the Imagawa to resolve these problems militarily as well as the failure of intervention policies with respect to family matters invited members of the faction opposed to the Imagawa to revert to Matsudaira Motoyasu.  Having observed these miscues, influential families in Tōtōmi Province subsequently rebelled (known as the Enshū sōgeki, or Tōtōmi Discord), hastening the collapse of the Imagawa’s rule of Mikawa within a short period of time.