Battle of Anjō Castle
Years: Tenbun 9 (1540); Tenbun 14 (1545); Tenbun 18 (1549)
Location: The environs of Anjō Castle in the Hekikai District of southwest Mikawa Province and, in general, in and around the border area between Owari and Mikawa provinces
Outcome: In the First Battle of Anjō Castle in 1540, allied forces of the Oda and Mizuno prevailed against the Matsudaira, but whether this resulted in the capture of Anjō is subject to multiple theories; in the Second Battle of Anjō Castle in 1545, Oda forces operating from the castle along with reinforcements defeated the Matsudaira who had misjudged the force posture of the Oda and retreated to Okazaki Castle; in the Third Battle of Anjō Castle in 1549, the Imagawa and their Matsudaira proxies prevailed, ousting Oda Nobuhiro and recapturing the castle.
Commanders: Oda Nobuhide, Mizuno Tadamasa
Forces: 3,000 (2,000 mounted soldiers from the Oda, 1,000 foot soldiers from the Mizuno)
Casualties: Unknown but over 1,000 from both sides in total
Commanders: Oda Nobuhide
Forces: 600 + reinforcements
Casualties: Unknown[Third Battle]
Commanders: Oda Nobuhiro, Hirate Masahide
Commanders: Imagawa Yoshimoto
Casualties: Unknown but over 1,000 from both sides in total[Second Battle]
Commanders: Matsudaira Hirotada, Honda Tadatoyo
Casualties: Unknown[Third Battle]
Commanders: Taigen Sessai
Forces: 10,000 (combined Imagawa and Matsudaira)
The Battle of Anjō Castle encompasses a series of battles over a period of nine years from Tenbun 9 to 18 (1540 to 1549) between the Oda clan and the Imagawa, together with the Matsudaira serving as a local proxy in Mikawa on behalf of the Imagawa. In addition to battles in 1540, 1545, and 1549 for control of Anjō Castle and its environs in southwestern Mikawa, the conflict was marked by numerous ancillary clashes as the opposing sides competed for influence in the border area between Owari and Mikawa provinces. Despite tangible progress by the Imagawa vis-à-vis the Oda over the decade of the 1540’s, the situation changed dramatically after the killing of Imagawa Yoshimoto, a sengoku daimyō and the influential leader of the Imagawa clan, at the Battle of Okehazama in 1560.
The initial battle was triggered by a plan of the Oda to advance into Mikawa, while the final battle occurred when the Imagawa aimed to expel the Oda from western Mikawa and to expand their domain in place of the Oda. In the end, the Oda lost and their influence in Mikawa dramatically declined. The Oda held on to only one corner of an area around Kariya on the border between Owari and Mikawa. Meanwhile, the Imagawa solidified their governance by seizing the territory of the main branch of the Matsudaira clan in Mikawa, and, with the exception of one district, the Imagawa controlled almost all of the southwest portion of Mikawa.
Matsudaira Kiyoyasu, the seventh head of the Matsudaira clan, achieved notoriety among neighboring provinces in the course of unifying Mikawa. In 1535, this momentum carried him to invade Owari and attack Moriyama Castle, however, during the siege, he was struck down by one of his retainers in an event known as the Collapse at Moriyama. His unexpected death caused turmoil within the clan, leading to a series of defections by influential families in Mikawa. Matsudaira Nobusada (head of the Sakurai-Matsudaira clan and great-uncle of Matsudaira Hirotada) ousted Hirotada (the eldest son of Kiyoyasu) from the clan’s base at Okazaki Castle and seized control of the clan. Hirotada then reached out to Kira Mochihiro for support. Mochihiro served as an intermediary between Hirotada and Imagawa Yoshimoto, the sengoku daimyō from Suruga.
The next year, Nobusada vacated Okazaki Castle in response to pressure from Yoshimoto, enabling Hirotada to return. Nobusada was later pardoned for his actions. At this time, Matsudaira Nobutaka from the Mitsugi-Matsudaira family supported the return of Hirotada to Okazaki and became his guardian. Nobutaka and his younger brother, Matsudaira Yasutaka, served in the Battle of Idano following an attack by the Oda on Okazaki in the wake of the sudden death of Kiyoyasu in the Collapse at Moriyama, contributing to the defense of Okazaki Castle. However, the Matsudaira clan rapidly deteriorated owing to conflict among followers after Kiyoyasu’s death. Many influential families in eastern Mikawa began to pledge their allegiance to the Imagawa. The Oda began to apply pressure from the west in an effort to contain an expansion of the Imagawa’s influence in Mikawa.
Anjō Castle was located on the end of the Zetsujō Plateau in the eastern portion of the Hekikai Plateau on the western bank of the Yanagi River flowing through western Mikawa. The southern portions of the Hekikai Plateau were called the Anjō-ga-hara, a wilderness area home to a mixed grove with swamps and wetlands. The environs of Anjō Castle were no exception and, owing to the mixed groves and swampland, the base was also referred to as Mori Castle (meaning the forest castle). Despite its small size, the topography made Anjō Castle easy to defend and difficult to attack. Beginning in 1471, after Matsudaira Nobumitsu, lord of Iwazu Castle in Mikawa, toppled Anjō through artifice, Anjō served as the headquarters of the clan for four generations over fifty years until Kiyoyasu moved to Okazaki Castle in 1524. As a result, Anjō served as a base from which to govern other castles under Matsudaira control throughout the Hekikai District (with the exception of the environs of Kariya under the control of the Mizuno clan and Ōhama in the Hazu District).
Anjō further served as a front-line base for the defense of the main base in Okazaki. For the Oda, Anjō offered a foothold to prevent the Imagawa from reaching Owari by impeding their ability to seize territory in Mikawa, and the Matsudaira needed Anjō to defend Okazaki Castle. In fact, the Battle of Azukizaka occurred after the loss of Anjō by the Matsudaira. For the Imagawa to claim possession of Mikawa, they could not permit continued occupation by the Oda. Therefore, after the death of Hirotada, the Imagawa fully intervened in this battle.
The First Battle of Anjō Castle
In the second month of 1540, Matsudaira Hirotada took the initiative to attack Narumi Castle in Owari, but was defeated. Thereafter, to reinforce the defense of Anjō Castle, Hirotada appointed Matsudaira Nagaie (the great-uncle of Kiyoyasu) to serve as the chamberlain, in addition to five members of the family and almost 5,000 soldiers. In the sixth month, Oda Nobuhide, lord of Furuwatari Castle in Owari, accompanied Mizuno Tadamasa of Kariya Castle to lead 3,000 soldiers (comprised of 2,000 mounted soldiers from the Oda army and 1,000 foot soldiers from the Mizuno) to attack Anjō Castle. The Oda forces assembled in the Takadai woods to the north of the castle, while a detached force from the Mizuno took positions to the south. The Matsudaira forces were comprised primarily of foot soldiers but were confronted with an opposing force of three times as many soldiers, placing them at an inevitable disadvantage. On 6/6, the Matsudaira forces split-up between positions to the north and south, then launched into attacks against the Oda and Mizuno soldiers. The Matsudaira fought well, at once driving back the allied Oda and Mizuno forces, but ultimately were unable to overcome their inferior position. Over fifty bushi including Nagaie died in battle, and between both sides, over 1,000 men lost their lives, illustrating the ferocity of the conflict.
Timing for the fall of Anjō Castle
There are several theories in regard to the timing of the fall of Anjō Castle. These include: (i) the castle fell at the First Battle of Anjō Castle in 1540, (ii) the castle did not fall at the First Battle of Anjō Castle after it ended in a draw, and (iii) after the Oda learned of the death of Matsudaira Nagachika on 8/22 of 1544, Oda Toshimune led 3,000 soldiers on another attack of the castle that failed, but, in the next month, Oda Nobuhide finally succeeded on his third attempt to capture the castle. The theory described in (iii) is consistent with various events. Namely, in 1541 (the year after the First Battle of Anjō), the Mizuno and Matsudaira clans reconciled, whereupon Mizuno Tadamasa offered his daughter, Odai-no-kata, to wed Matsudaira Hirotada. After the death of Tadamasa in 1543, his successor, Mizuno Nobumoto, expressed a willingness to cooperate with the Oda, and, in 1544, rejected the alliance with the Matsudaira. However, this theory is inconsistent with the outbreak of the First Battle of Azukizaka in 1542.
Alternatively, there is a theory that the castle did not fall in 1544, but later fell in 1547. There are questions whether this theory is consistent with the First Battle of Azukizaka as well as the Battle of Anjō Castle in 1545.
In any case, after the fall of Anjō Castle, family members including Matsudaira Kiyosada (the son of Matsudaira Nobusada), Matsudaira Tadamichi of the Sasaki-Matsudaira family, and Sakai Tadanao (a senior retainer) surrendered to the Oda clan, while retainers of the Matsudaira clan who held castles on the west bank of the Yanagi River in the Hekikai District gradually transitioned to Oda control. Finally, the Oda’s influence extended to the east bank of the Yanagi River, closing in on Okazaki Castle.
The Second Battle of Anjō Castle
In the first month of 1543, Matsudaira Nobutaka (who was serving as the guardian of Hirotada) and his younger brother, Matsudaira Yasutaka, seized the territory left behind by Matsudaira Chikanaga of the Iwazu-Matsudaira family. Nobutaka acquired enough authority to surpass the main branch of the family. Hereditary retainers such as the Abe and Honda clans were not receptive to Nobutaka’s rise, and urged Hirotada to expel him. Hirotada himself was not eager to expel Nobutaka owing to the support he received from Nobutaka enabling his return to Okazaki. However, unable to resist the advice of his senior retainers, he sent Nobutaka to Sunpu to serve as a messenger of New Year’s greetings to the Imagawa, and then ousted him and seized his domain. Nobutaka appealed to Hirotada for a pardon, exclaiming that he did not seek to rebel, and requested mediation by Imagawa Yoshimoto. Nevertheless, those opposed to Nobutaka rejected a reconciliation and, in light of their position, Yoshimoto declined to intervene. In response to the mistreatment, Nobutaka, along with Matsudaira Tadamichi and Sakai Tadanao, joined the Oda and constructed Yamazaki Castle. This castle was located on a small hill on the end of the Hekikai Plateau to the northwest of Anjō Castle. Lowlands extended as far as could be seen to Okazaki Castle. The unobstructed view allowed them to grasp the advance of enemy forces as well as their size. It served an important function as a branch of Anjō Castle; moreover, Yamazaki Castle addressed a vulnerability of Anjō where the approach from the northwest was not protected by swamp land.
In the ninth month of 1545, after Hirotada received news of the defeat of the Oda invasion of Mino, he mobilized forces with a plan to capture Anjō Castle. The Matsudaira did not expect Nobuhide had arrived at Anjō Castle, and further expected that after their loss, the Oda would suffer from low morale. In addition, the Matsudaira had an advantage based on their knowledge of the local topography, so they let down their guard in regard to the risk of a confrontation. The Oda had over 600 men in the castle along with reinforcements, and wielded arquebuses representing the newest class of weapons on the battlefield. Nobuhide strategically placed his reinforcements outside of the castle in careful preparation for a pending assault on Anjō.
The armies violently collided at Kiyonawate near Anjō Castle. The Matsudaira first learned of the presence of reinforcements after being subject to attack from behind by the Oda, and attempted to crush these forces, but the soldiers positioned in the castle used this as an opportunity to strike out and attack the Matsudaira from the opposite side. Retainers who feared their escape route would be cut-off urged Hirotada to retreat before incurring a fatal blow, but he ignored them and insisted on continuing the attack. Matsudaira forces who heard that Nobuhide had deployed with his army became unsettled while the contingent was split in two, without any path of retreat. Hirotada lost hope in securing his own safety and committed to die in battle, but a senior retainer named Honda Tadatoyo (the uncle of Honda Tadakatsu) dismissed this view and, in lieu of Hirotada, charged the enemy formation and succeeded in drawing the attention of the Oda forces so that Hirotada and the remaining Matsudaira forces could retreat to Okazaki Castle. Tadatoyo himself died in the charge. After this loss, Hirotada mounted a failed attack against Ueno Castle protected by Matsudaira Ietsugu (the son of Matsudaira Kiyosada) and Sakai Tadanao. Ueno Castle was in a strategic location to the northwest of Okazaki Castle. At this time, the main branch of the Matsudaira clan completely withered while the dependency upon the Imagawa steadily increased.
The Third Battle of Anjō Castle
In the ninth month of 1546, Hirotada once again attacked Ueno Castle, and finally succeeded in its capture. Thereafter, through the mediation of Yoshimoto, he succeeded in reaching a settlement with Sakai Tadanao while Matsudaira Kiyosada was confined in Sakurai. Despite these signs of progress, the main branch of the Matsudaira remained in a weakened state. In 1547, Hirotada protected Okazaki Castle and requested support from the Imagawa clan to push back the Oda forces toward the west. At this time, Matsudaira Nobutaka built fortresses in the Nukata District on the northern and southern banks of the Otsu River. Okazaki Castle obstructed the east, so it became difficult to maintain communication with allies in the direction of the Hoi District controlled by the Imagawa. Moreover, Matsudaira Tadamichi built fortresses in Uewada and in Tsutsubari on the west bank of the Yanagi River in the Hekikai District, threatening Okazaki Castle from the south and the west, posing a dire situation for the main branch of the Matsudaira clan. Unable to continue on his own accord as a local daimyō in western Mikawa, Hirotada sought help from Imagawa Yoshimoto, and, in return, Yoshimoto demanded that Hirotada tender his eldest son, Takechiyo (later known as Tokugawa Ieyasu), as a hostage. To Hirotada, Yoshimoto had been a benefactor who aided his return to Okazaki Castle and Hirotada did not, without the support of the Imagawa, attempt to compensate for the weaknesses of the clan, so he consented to the demand and sent Takechiyo to the home base of the Imagawa in Sunpu.
En route to Sunpu, Takechiyo was captured by Toda Yasumitsu, the lord of Tahara Castle, and sold to the Oda clan, thereby becoming a hostage of the Oda. Yoshimoto responded by ordering Amano Kagetsura, the chamberlain of Yoshida Castle, to attack Tahara Castle, upon which Yasumitsu and the main branch of the Toda clan were decimated. Nobuhide used Takechiyo to demand that Hirotada defect from the Imagawa and come under the command of the Oda. Instead, Hirotada clearly expressed his allegiance to the Imagawa, and the Matsudaira clan came fully under the governance of the Imagawa. The Imagawa then established a base in the village of Yamanaka for attacks in western Mikawa, commencing construction in the seventh month of 1547.
In the ninth month of 1547, Hirotada fought and lost in battle against Matsudaira Nobutaka and Matsudaira Tadamichi on the Yanagi River at the Battle of Tokawahara. Serving in the rear guard, Matsudaira Tadatsugu and Torii Tadamune (the older brother of Torii Mototada) died in the battle. When Nobutaka’s forces assaulted Tadamune’s home base at Watari Castle, he was shot and killed head-on by arquebus-wielding enemies. At this same time, the Oda attacked Umetsubo Castle in the western portion of the Kamo District and compelled the resident Miyake clan to pledge their obedience. Owing to a decline in the military capabilities of the main branch of the Matsudaira, it became difficult to fight militarily against Tadamichi from the fortress in Uewada, whereupon Hirotada ordered his retainers to kill him in the tenth month of 1547. Next, in the eleventh month, Hirotada attacked Matsudaira Shigehiro at Motojuku Castle for colluding with the Oda.
In the third month of 1548, Nobuhide responded to Hirotada’s animosity by mobilizing for an attack on Okazaki Castle. Since the abduction of Takechiyo, Hirotada continued to express his allegiance to the Imagawa. In recognition of Hirotada’s resolute stance toward the Oda, Yoshimoto sent reinforcements to the Matsudaira, advancing by 3/6 to the Fuji River where the forces established a base in the Nukata District. After scouting these movements, on 3/9, the Oda army crossed the Yanagi River and set-up their position at the Uewada fortress. The armies violently clashed on 3/19 at the Second Battle of Azukizaka. The Oda suffered an overwhelming loss, fleeing to Anjō Castle while Oda Nobumitsu (the younger brother of Nobuhide) remained in Uewada to serve as the rear guard. Oda Nobuhiro (Nobuhide’s son) stayed to guard Anjō while other forces returned to their base in Furuwatari Caste in Owari Province. The Imagawa then returned to their base near the Fuji River.
Theories of an attack and capture of Okazaki Castle by Oda Nobuhide
Regarding the circumstances in 1547, there is a theory that, Okazaki Castle was not only surrounded by enemy-held fortresses including Kamiwada, Ōhira, Oka, and Tsutsuhari, it was in fact toppled by Nobuhide in the ninth month. Moreover, with respect to the details of Takechiyo becoming a hostage of the Oda, rather than an abduction, Hirotada himself may have turned Takechiyo over to the Oda clan at the time of surrender of Okazaki Castle.
As for Hirotada’s position, owing to the Battle of Tokawahara on 9/28, in addition to the assassination of Matsudaira Tadamichi around this time, Nobuhide delayed action in Mikawa. Soon thereafter, Hirotada may be considered to have confirmed his intent to oppose Nobunaga again. Although the Okazaki group may have joined the Battle of Azukizaka in the third month of 1548, there are no traces of action taken from Okazaki Castle toward the Oda army that retreated to Anjō Castle via Kamiwada. After the victory by the Imagawa in the same battle, Hirotada may have reverted to their side. In this case, members of the Okazaki group who joined the Imagawa turned to them because it was against their conscience to surrender to the Oda.
In the fourth month of 1547, Nobuhide permitted Matsudaira Nobutaka of Yamazaki Castle to deploy for the purpose of toppling Okazaki Castle with his own forces. This led to a clash known as the Battle of Mimidori-Nawate on the grounds near the Myōdai Temple in the Nukata District, and Nobutaka died in battle after being shot by an arrow.
From the inception, Hirotada did not actively seek to oust Nobutaka (who was his uncle) but was guided by a faction of senior retainers opposed to Nobutaka. Hirotada felt personally responsible that Nobutaka sought support from the Oda and became his adversary. Hirotada desired that Nobutaka be captured alive, but instead Nobutaka died in battle. Upon viewing the remains of Nobutaka, he lamented to his retainers for their ruthlessness in ousting his own kin who then died in battle, causing him to wail out loud. Owing to the expulsion of Nobutaka and Tadamichi, Hirotada began to regain power over the course of the year enabling his forces to attack Umetsubo Castle under the control of the Oda and to reconsolidate the fractured family. However, in the third month of 1549, he died in Okazaki Castle. There are theories that he died of illness or was assassinated.
In response to the news of Hirotada’s death, Yoshimoto took swift action. To complete the takeover of the Matsudaira domain, that same month he deployed an army of 10,000 soldiers (including Matsudaira forces) led by Taigen Sessai to Mikawa. Sessai first sent some of the forces to Okazaki Castle; and, to sever the route from Owari, dispatched forces toward Narumi and Ōdaka. He then attacked surrounding fortresses including Yamazaki Castle. Matsudaira Tadamichi died and was succeeded by his younger brother, Matsudaira Tadanari, of the Sasaki-Matsudaira branch of the family. In view of the developments, Tadanari relinquished his role as the head of Umemori and other areas in the Aichi District and joined the Imagawa. After isolating Anjō Castle, Sessai launched an attack on the castle from the north led by Matsudaira forces. Owing to their valiant efforts, the attackers breached the third and second perimeters of the castle and pressed toward the inner citadel. However, after becoming too anxious to capture Oda Nobuhiro, Honda Tadataka (father of Honda Tadakatsu), the lead commander of the Matsudaira, penetrated too quickly and died in battle. This rattled the forces so that Sessai determined it was no longer possible to continue the attack and withdrew the entire force to Okazaki.
In an event known as the Fourth Battle of Anjō Castle, in the ninth month of 1549, Sessai deployed again, establishing a position on Mount Arakawa in the Hazu District which had served as a base for Arakawa Yoshihiro. At this time, there was a continuous land route from Mount Arakawa to Anjō Castle. Then, as an initial step, the Imagawa attacked Nishio Castle ruled by the Tōjō-Kira clan in the Hazu District who had cooperated with the Oda and combined with their army. Kira Yoshiyasu, the lord of Nishio Castle, maintained a political alliance through marriage with the Shiba clan who had been enemies of the Imagawa for generations. This action drew a reproach from Yoshimoto, but, after the fall of the castle, the defenders capitulated and retainers of the new Oda faction were sentenced with the permission of Yoshimoto. In response to the advance by the Imagawa army from the south toward Anjō Castle, the Oda dispatched reinforcements led by Hirate Masahide who stubbornly resisted the opposing forces. Nevertheless, after a vicious attack by the Matsudaira, Nobuhiro was captured and Anjō Castle fell toward the end of the year. On this occasion, the Imagawa army made effective use of the arquebus, and, after the fall of Anjō Castle, the Imagawa toppled Kariya Castle the following year. The Oda forces in the Hekikai District and the surrounding areas collapsed, barely maintaining control of Shigehara Castle near the border with Owari as their only base in the district.
Events after the battles of Anjō Castle
After his capture, Nobuhiro became a hostage of the Imagawa so the Imagawa exchanged Nobuhiro for Takechiyo who was a hostage of the Oda. By virtue of bringing Takechiyo back to Mikawa, the Matsudaira avoided extinction of the clan, rewarding Hirotada for his loyalty to the Imagawa. However, Yoshimoto did not agree that Takechiyo should reside in Okazaki Castle, and promptly had him sent as a hostage to Sunpu. This enabled Yoshimoto to pursue his plan to control Mikawa, folding western Mikawa into his domain.
First, Yoshimoto ordered Amano Kageyasu and Ii Naomori to guard Anjō Castle, and dispatched Yamada Kagetaka to serve as the chamberlain of Okazaki Castle. He initally reconciled with the Mizuno clan and had them return to Kariya Castle. After yielding allegience to the Imagawa, Mizuno Nobuchika, the lord of Kariya Castle (and younger brother of Mizuno Nobumoto, the lord of Ogawa Castle) later rebelled against the Imagawa and betrayed them in favor of the Oda. Ultimately, Yoshimoto failed to secure the support of the Mizuno. Nobuchika was attacked and eliminated by Okabe Motonobu, a retainer of the Imagawa.
Niwa Ujikiyo, lord of Iwasaki Castle, controlled land in the border area between Owari and Mikawa, and had been subservient to the Imagawa until 1550. In 1551, Niwa Ujihide, lord of Fujishima Castle, and his son, started an internal conflict against Ujikiyo and his son, Ujisato. Ujihide requested reinforcements from Oda Nobunaga. Ujikiyo learned this news in advance, so as the Oda army approached Yokoyama in the Aichi District, the Iwasaki-Niwa army launched a surprise attack and defeated the Oda. Ujihide and his son then fled to Mikawa. Also in 1551, after a sudden revolt by Matsudaira Jinjirō of the Tōjō-Matsudaira family (who had served the Imagawa), Jinjirō was driven out of his territory and his younger brother, Matsudaira Tadashige, succeeded him as head of the Tōjō-Matsudaira family. Meanwhile, Oda Nobuhide died, triggering a succession struggle between his eldest son (Oda Nobunaga) and Nobunaga’s younger brother (Oda Nobukatsu, also referred to as Nobuyuki). The same year, Nobunaga intervened in a conflict in the Niwa clan, attacking UJikiyo and his son, Ujisato. A battle ensued at Yokoyama in the Aichi District and he lost. Finally, in the same year, Yamaguchi Noritsugu, lord of Narumi Castle in Owari, betrayed Nobunaga in favor of the Imagawa.
In 1552, Nobunaga attacked Yamaguchi Noriyoshi (Noritsugu’s son), giving rise to the Battle of Akatsuka. Nobunaga could not defeat Noriyoshi and the battle ended in a draw. Thereafter, Noritsugu attacked Ōdaka Castle. He then deceived Kondō Kageharu, lord of Kutsukake Castle, and persuaded him to support the Imagawa. Next, the Hanai clan, lord of Teramoto Castle located in the western portion of the Chita District submitted to the Imagawa. In 1554, Shigehara Castle in the Hekikai District also fell to the Imagawa. To subjugate the Mizuno, the Imagawa built Muraki fortress and threatened Ogawa Castle. Facing dire circumstances, Mizuno Nobumoto requested support from the Oda, and, based on Nobunaga’s victory at the Siege of Muraki Fortress, those at Ogawa avoided a crisis. Although the Oda rescued the Mizuno from their crisis in the Chita District, critical locations in the Hekikai District and the border area between Owari and Mikawa had already fallen under control of the Imagawa. As a result, influential families in the western portion of the Kamo District aligned with the Oda were exposed to pressure from the Imagawa forces and began to surrender one after another. In 1554, Nishi-Hirose Castle, in a strategic location in the northern part of western Mikawa held by the Oda, was attacked by several commanders from the Miyake clan including Miyake Takasada, lord of Higashi-Hirose Castle under Imagawa control.
In 1555, Yoshimoto sent Mikawa forces on a long march to the Kaitō District in Owari to threaten Oda forces from the west. The Mikawa forces joined Hattori Tomosada from Ninoue in the Kaisai District of Owari to attack and topple Kanie Castle. Meritorious contributions by soldiers including Ōkubo Tadatoshi led to them being named the Seven Spears of Kanie. Owing to this course of events after the battles of Anjō Castle, the situation reversed so that the Imagawa forces encroached on the Oda domain in Owari. In the ninth month of 1555, Kira Yoshiyasu, suddenly revolted after earlier surrendering to and having been pardoned by the Imagawa in the wake of a battle in the tenth month of 1549. He then colluded with the Oda. After surrendering in 1549, Yoshiyasu served as a commander for the Imagawa with the support of Yoshimoto. In 1554, through the influence of Yoshimoto, became head of the Tōjō-Kira family and the Seijō-Kira family, unifying the two branches of the Kira clan. In 1557, Yoshiyasu served as the hair stylist for Takechiyo at his coming-of-age ceremony. Despite this honor, Yoshiyasu’s retainers urged him to defect, whereupon he suddenly enticed the Mizuno forces from Ogawa Castle into Nishio Castle. Soldiers in other castles including Arakawa, Hazu, Kasuzuka, and Katahara did not consent and remained obedient to the Imagawa.
Meanwhile, in the Kamo District, in the first month of 1556, Matsudaira Masanori, the eldest son of Matsudaira Noritō, of the Takiwaki-Matsudaira clan (supporters of the Imagawa) died in battle against Matsudaira Chikanori. Around this time, Sakai Tadanao also rebelled, but, during the second month of 1556, returned to the service of the Imagawa. At the end of the third month, Takiwaki Castle fell after an attack by Matsudaira Chikanori, resulting in the death of Matsudaira Noritō, and his father, Matsudaira Norikiyo.
In the third month of 1556, Nobunaga attacked Arakawa in the Hazu District. Matsui Tadatsugu, a commander in the Tōjō-Matsudaira clan, intercepted the Oda forces at Noderahara in the Hekikai District. Tadatsugu received a letter of commendation from Yoshimoto for his achievements for his resistance to the Oda. The same year, after the Oda invaded the western portion of the Kamo District and attacked the Miyake at Umetsubo Castle, Yoshimoto appointed a chamberlain. Shibata Katsuie, a senior retainer of Nobunaga, then attacked Ukigai Castle, but Sakai Tadatsugu and Harada Ujishige successfully defended the castle, and after the Imagawa sent reinforcements including Ōkubo Tadakatsu, Katsuie was repelled and forced to flee in defeat.
In 1556, Okudaira Sadakatsu, lord of Kameyama Castle near the village of Tsukede in the Shitara District was the subject of a scheme by Nobunaga. Sadakatsu, together with Sudanuma Sadatsugu (the lord of Damine Castle that served as the headquarters for the Suganuma clan) and supporting members of the Okudaira clan, rebelled against the Imagawa and attacked Hadanashi Castle. Yoshimoto responded by sending an army to attack Hijika Castle defended by Okudaira Sadanao, the younger brother of Sadakatsu. After holding out in the castle, the Okudaira repelled the attacking forces, killing Matsudaira Tadashige on the side of the Imagawa. Although Sadakatsu pushed back the Imagawa, the territory held by the Okudaira was separated from Owari so Sadakatsu could not count on reinforcements other than relatives. Moreover, he confronted an unstable situation including an internal conflict in the Oda clan that led to the Battle of Inō.
Yoshimoto then ordered Suganuma Sadamura (of the Noda-Suganuma family), along with commanders from the eastern Mikawa forces such Honda Tadatoshi and Toda Nobumitsu to subjugate the Okudaira clan. As a result, the environment became increasingly dire for Sadakatsu. The Achiwa clan, senior retainers of the Okudaira based at Ameyama Castle in the Nukata District, were attacked by the Imagawa. Although the defenders killed Suganuma Sadamura, the castle fell to forces under Honda Tadatoshi. Sadakatsu retreated to Kameyama Castle, but the Okudaira had no prospect of prevailing against the Imagawa, so one-half year after rebelling, Sadakatsu returned to the allegiance of the Imagawa and pleaded for a pardon from Yoshimoto. Suganuma Sadatsugu took a similar approach, but was killed by his younger brother, Suganuma Sadanao (allied with the Imagawa) and the rebellion suppressed.
In 1555, Kira Yoshiyasu led an insurrection against the Imagawa, but, by 1557, he surrendered and was pardoned again by Yoshimoto. In the spring of the same year, a settlement conference was held in Uenohara in Mikawa. Yoshimoto appointed Yoshiyasu to serve as the representative of Mikawa, and he participated in the meetings with Shiba Yoshikane, the military governor of Owari representing the Oda. The two representatives could not agree on the order of seats, causing a loss of decorum at the conference. Failed negotiations ensued, causing Yoshiyasu to lose face, whereupon he requested protection from Nobunaga and absconded to Owari. However, Yoshiyasu then plotted with Yoshikane (who harbored ill-will toward Nobunaga) and the Ishibashi clan based at Toda Castle in Owari, collaborating with the Hattori of Ninoue to enable Imagawa forces to enter Owari from a sea route. While in progress, this scheme became known to the Oda so all of them were banished from Owari. Yoshiyasu sought support from Yoshimoto, but Yoshimoto did not countenance a second betrayal after even saving Yoshiyasu’s life, so he had Yoshiyasu taken to Sunpu and incarcerated. As a result, in the autumn of 1557, the Imagawa seized Nishio Castle and assigned a chamberlain. Yoshimoto placed Kira Yoshiaki, the younger brother of Yoshiyasu, in Tōjō Castle, enabling the Kira clan to survive. Further, the Imagawa seized and placed under their direct control the Seijō territory.
In 1558, Miyake Takasada and the Miyake family, together with Suzuki Shigenori, lord of Terabe Castle, abandoned the Imagawa for the Oda. Yoshimoto responded by dispatching Mikawa forces led by Matsudaira Motoyasu to eliminate the traitors. Leading his first assignment, Motoyasu used fire as the means to attack Terabe Castle, causing the Suzuki to re-join the Imagawa. Motoyasu then toppled the base of the Chūjō clan in the Kamo District, and recaptured Umetsubo Castle which had been attacked by the Oda. He subjugated Ibo Castle protected by Miyake Masasada, and then attacked Miyake Takasada at Higashi-Hirose Castle. Masasada and Takasada were from different branches of the Miyake clan, and while Masasada submitted to the Imagawa, Takasada resisted, and later, Takasada recaptured Higashi-Hirose Castle. Meanwhile, Motoyasu attacked Nishi-Hirose Castle. Owing to Motoyasu’s results, a majority of the western portion of the Kamo District returned to control of the Imagawa, and, in recognition of these achievements, Yoshimoto presented Motoyasu with a long sword from the village of Yamanaka in Mikawa. Finally, in the same year, the Oda attacked Shinano Castle in Owari but the defenders mounted a successful counterattack led by Matsudaira Ietsugu to achieve an overwhelming victory.
After the battles of Anjō Castle, the influence of the Oda in Mikawa Province clearly waned, while their control of the eastern portions of Owairi and the Chita District unraveled. Attempts to expand their authority over the southern and northern portions of Mikawa ended in a series of failures. The situation leaned completely in favor of the Imagawa while the Oda were in a state of distress. In 1559, Yoshimoto summoned Yamaguchi Noritsugu and his son, Yamaguchi Noriyoshi, to Sunpu and had them commit seppuku. He then seized the territory of the Yamaguchi clan while Narumi and Ōdaka castles came under direct control of the Imagawa.
Nobunaga resisted the movements of the Imagawa by building an array of fortresses including Tange, Zenshōji, and Nakashima in the environs of Narumi Castle, along with Marune and Washizu near Ōdaka Castle. These permit the Oda to sever communications in the direction of Mikawa. This caused a crisis at Narumi and Ōdaka, entrapping the Imagawa forces stationed in the northern portion of the Chita District. In 1560, an effort by the Imagawa to support these forces led to the Battle of Okehazama and the unexpected death in battle of Yoshimoto as he attempted to traverse Oda territory in Owari Province.