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Suruga campaign against Takeda clan

In October 1571, Takeda Shingen broke the alliance with the Oda-Tokugawa forces and allied with the Odawara Hōjō clan. He decided to make a drive for Kyoto at the urgings of the shōgun Ashikaga Yoshiaki, starting with invading Tokugawa lands in Tōtōmi. Takeda Shingen's first objective in his campaign against Ieyasu was Nishikawa Castle, Yoshida Castle and Futamata Castle. In 1572, after besieging Futamata, Shingen would press on past Futamata towards the major Tokugawa home castle at Hamamatsu. Later, Ieyasu asked for help from Nobunaga, who sent him some 3,000 troops. Early in 1573 the two armies met at the Battle of Mikatagahara, north Hamamatsu. The considerably larger Takeda army, under the expert direction of Shingen, overwhelmed Ieyasu's troops and caused heavy casualties. Despite his initial reluctance, Ieyasu was convinced by his generals to retreat. The battle was a major defeat, but in the interests of maintaining the appearance of dignified withdrawal, Ieyasu brazenly ordered the men at his castle to light torches, sound drums, and leave the gates open, to properly receive the returning warriors. To the surprise and relief of the Tokugawa army, this spectacle made the Takeda generals suspicious of being led into a trap, so they did not besiege the castle and instead made camp for the night. This error would allow a band of Tokugawa soldiers to raid the camp in the ensuing hours, further upsetting the already disoriented Takeda army, and ultimately resulting in Shingen's decision to call off the offensive altogether. Takeda Shingen would not get another chance to advance on Hamamatsu, much less Kyoto, since he died from unknown cause shortly after the siege of Noda Castle later that same year.

In 1574, as Shingen was succeeded by his son Takeda Katsuyori, the conflict continued as the Tokugawa forces under Honda Tadakatsu and Sakakibara Yasumasa seized many of the Takeda clan's castles, including Komyo castle. At some point, Ieyasu tried to capture Inui Castle in Tōtōmi Province, however strong resistance from its garrison commander, Amano Kagehira, forced Ieyasu to abort the siege. During their retreat, Kagehira launched a counterattack to pursue Ieyasu, which was repelled by Mizuno Tadashige and Torii Mototada who led the rearguard.

In 1575, on April, Ōga Yashirō a deputy governor of over 20 villages in Oku district of Mikawa under Matsudaira Nobuyasu, was arrested by Ōkubo Tadayo Yashirō and paraded him around Hamamatsu Castle, before Tadayo executed Yashirō brutally by mutilating him alive with a saw, while Tadayo crucified the wife and children of Yashirō. According to the investigation, Yashirō was implicated of betrayal allegation for colluding with Takeda Katsuyori to invade the Tokugawa clan's territory. According to a letter, Yashiro had teamed up with Takeda Katsuyori of Kai to seize Okazaki Castle. However, one of Yashiro's colleague, Yamada Hachizō, betrayed Yashiro and passed on information about the rebellion to Nobuyasu. Meanwhile, Ieyasu himself also learned further crime of Yashirō of corruptions in governance through one of his vassal's report. On June, during Takeda Katsuyori's raid through Mikawa Province, he attacked Yoshida Castle and besieged Nagashino Castle. Ieyasu appealed to Nobunaga for help and Nobunaga came personally with 30,000 strong men. The Oda-Tokugawa forces of 38,000 won a great victory and successfully defended Nagashino Castle. Though the Takeda forces had been destroyed, Katsuyori survived the battle and retreated back to Kai Province. For the next seven years, Ieyasu and Katsuyori fought a series of small battles, as the result of which Ieyasu's troops managed to wrestle control of Suruga Province away from the Takeda clan.