text: In 1584, Tadakatsu participated in the Komaki Campaign of Tokugawa against Toyotomi Hideyoshi. During this conflict, Tadakatsu entered negotiation with the Akai clan, a samurai clan from Tanba Province, which led by Ashida Tokinao, younger brother of Akai Naomasa, as Tokinao aspired for the revival of the Akai clan by contributing with Tokugawa clan. Tokinao then raised an army in Tanba on Ieyasu's side. As Tadakatsu was stationed Komaki while Ieyasu departed to engage Toyotomi troops at Nagakute, Tadakatsu observed a huge host under Hideyoshi himself move out in pursuit. With a handful of men, Tadakatsu rode out and challenged the Toyotomi army from the opposite bank of the Shōnai River. Toyotomi Hideyoshi was said to have been impressed by Tadakatsu and commanded his army to not attack the small units of Tadakatsu. After the Battle of Komaki and Nagakute, the front line in northern Owari reached stalemate. Ieyasu and Oda Nobukatsu led 20,000 soldiers and besieged three castles: Kanie Castle, Maeda Castle, and Shimoichiba Castle. The Kanie castle were defended by Maeda Nagatane and Takigawa Kazumasu. Tadatsugu, Okanabe Mori, and Yamaguchi Shigemasa spearheading the attack towards Shimoichiba castle. On June 22, Nobukatsu and Ieyasu launch an all-out attack on Kanie Castle. The soldiers led by Tadatsugu, While Tadakatsu, Ishikawa Kazumasa, Ii Naomasa, Sakakibara Yasumasa, and Matsudaira Ietada deployed in reserve before entering the battle with Ieyasu himself. On June 23, Ieyasu entered the castle with Sakakibara Yasumasa, thus the castle were subdued.
  Famed Four Tokugawa's Guardians under Toyotomi Regency
  on November 13, 1585, Ishikawa Kazumasa defected from Ieyasu to Hideyoshi. This accident caused Ieyasu to undergone massive reforms of the structures of Tokugawa clan military government. At first, Ieyasu ordered Torii Mototada, who served as the county magistrate of Kai, to collect military laws, weapons, and military equipment from the time of Takeda Shingen and bring them to Hamamatsu Castle (Hamamatsu City, Shizuoka Prefecture). Later, he also appointed two former Takeda vassals, Naruse Masakazu and Okabe Masatsuna, as magistrates under authority of Ii Naomasa and Honda Tadakatsu, while he also ordered all of former Takeda vassals who now serve him to impart any military doctrines and structures they knew during their service under Takeda clan., and lastly, he ordered the three of his prime generals, the so-called "Tokugawa Four Heavenly Kings," Ii Naomasa, Honda Tadakatsu, and Sakakibara Yasumasa, to serve as supreme commander of this new military regiments.