Kazumasa, also accompanied Ieyasu in the
Siege of Terabe 1558, and later at
Siege of Marune 1560. After 1560, when Ieyasu abandoned the Imagawa, Kazumasa then became a valued retainer and administrator under him. In 1562, he took part in the
Siege of Kaminogo Castle, when Ieyasu managed to convince
Imagawa Ujizane to release his family, Kazumasa acted as guardian of the
Imagawa family, which at the time was a very dangerous task. By 1567, the majority of daimyō forces in the Tokugawa armies were organized in two divisions, each with a separate commander. Kazumasa was placed over the forces of 13 Tokugawa daimyō-vassals and his counterpart,
Sakai Tadatsugu, was given command over the forces of 18 daimyō-vassals. In 1573, He participated in the
Battle of Mikatagahara. and 1575 in the
Battle of Nagashino. After
Toyotomi Hideyoshi's victory over
Shibata Katsuie in 1583, Ieyasu expressed his congratulations to Hideyoshi through Kazumasa. Later, Kazumasa and
Sakakibara Yasumasa accordingly issued statements attacking Hideyoshi, due to the Tokugawa's decision. Kazumasa served at the
Komaki headquarters during the
Komaki-Nagakute Campaign in 1584. In 1585, Kazumasa, very dismayed by what he saw as Tokugawa's foolhardy path of resistance against
Toyotomi Hideyoshi, later he switched sides to Hideyoshi. This inconvenienced Ieyasu, who had to reconstruct his military organization and defensive policies, since Kazumasa had significant knowledge about Tokugawa organization. ==Death==