In 1573, while still lord of the Hata district of Tosa,
Ichijō Kanesada was unpopular and had already suffered the defection of a number of important retainers. Seizing the opportunity, Motochika with his brother
Kōsokabe Chikayasu, wasted no time in marching to attack on the Ichijō's headquarters at
Nakamura, and Kanesada fled to
Bungo, defeated. In 1575, at the
Battle of Shimantogawa (Battle of Watarigawa), he defeated the
Ichijo family. Following his conquest of Tosa, Motochika turned north and prepared for an invasion of
Iyo province. The lord of that province was
Kōno Michinao, a daimyo who had once been driven from his domain by the Utsunomiya clan, returning only with the assistance of the powerful
Mōri clan. However, it was unlikely that Kōno could count on that sort of help again as the Mōri were embroiled in a war with Oda Nobunaga. Nonetheless, Chōsokabe's campaign in Iyo did not go off without a hitch. In 1579, a 7,000 men Chōsokabe army, commanded by
Kumu Yorinobu, met the forces of Kōno Michinao, led by
Doi Kiyonaga at the
Battle of Mimaomote. In the ensuing battle, Kumu Yorinobu was killed and his army defeated, though the loss proved little more than an unfortunate delay. The next year, Motochika led some 30,000 men into
Iyo Province, and forced Kōno Michinao to flee to Bungo province. In 1580,
Oda Nobunaga disapproved of Motochika's conquest of Shikoku, despite being allied with them. This commotion started from Motochika's effort to respond the
Miyoshi clan's threat, by strengthened ties not only with Nobunaga but also with
Mōri Terumoto to isolate the Miyoshi clan diolomatically. However, Motochika did not change his policy even after Nobunaga began a full-scale invasion of
Chugoku Region against the
Mōri clan, which caused Nobunaga to distrust grow suspicious about Motochika's allegiance. However, historian Yoshihiro Kawashima stated the relationship between the Mōri and Chosokabe clans had already broken down by the time the Mōri and Miyoshi clans made peace in 1575, and that the alliance between the Chosokabe and the Sanuki Kagawa clan (the adoption of Kagawa Chikazu) itself was the result of the shared interests of the two clans, who were pro-Oda and under threat from both the Mōri and Miyoshi clans. With little interference from either the Mōri or the Ōtomo, Chōsokabe was free to press onwards, and in 1582, he stepped up ongoing raids into
Awa province and defeated
Sogō Masayasu and the
Miyoshi clan at the
Battle of Nakatomigawa. Later, Motochika advanced to
Sanuki province and defeated
Sengoku Hidehisa at
Battle of Hiketa. By 1583, Chōsokabe Motochika had controlled
Tosa Province,
Iyo and subdued both
Awa and
Sanuki. Over the ensuing decade, he extended his power to all of
Shikoku island, making Motochika's dream of ruling all of Shikoku a reality. After unifying Tosa, he formed an alliance with Oda Nobunaga, who was promoting the unification project in the central government, through kinship with his wife. ==Conflict with Hideyoshi==