Working together with his brothers Yoshihiro, Toshihisa, and Iehisa, he launched a campaign to unify Kyūshū. Starting in 1572 with a victory against Itō clan at the battle of Kizaki and the Siege of Takabaru in 1576, Yoshihisa continued to win battles. In 1578, he defeated the Ōtomo clan at the battle of Mimigawa, though he did not take their territory. Later, in 1581, Yoshihisa took Minamata castle with a force of 115,000 men. In early 1584, he was victorious in Battle of Okitanawate against Ryūzōji clan and defeated the Aso clan.

By the middle of 1584, the Shimazu clan controlled; Chikugo, Chikuzen, Hizen, Higo, Hyūga, Osumi, and Satsuma, most of Kyūshū with the exception of Ōtomo's domain and unification was a feasible goal.