The Battle of Ichirai was fought in the year of 1539 between two rival factions of the Shimazu clan

Shimazu clan

The were the daimyō of the Satsuma han, which spread over Satsuma, Ōsumi and Hyūga provinces in Japan.The Shimazu were identified as one of the tozama or outsider daimyō clans in contrast with the fudai or insider clans which were hereditary vassals or allies of the Tokugawa clan,The Shimazu were...

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Shimazu Katsuhisa

Shimazu Katsuhisa

was the fourteenth head of the Shimazu clan and the third son of Shimazu Tadamasa following the Sengoku or Warring States period of 16th century Japan....

, who presided over the Shimazu family, did not have a son and he was driven out by Shimazu Sanehisa, who was the head of another branch, the Sasshū (薩州家). Sanehisa then laid claim to be the head of the clan without being properly recognized by the rest of the families. Katsuhisa asked Shimazu Tadayoshi

Shimazu Tadayoshi

was a daimyo of Satsuma Province during Japan's Sengoku period.He was born to a branch family of the Shimazu clan, the Mimasaka Shimazu family but after his father Shimazu Yoshihisa died, his mother married Shimazu Unkyu of another branch family, the Soshū...

 for help to regain his position, and Tadayoshi sent his son Shimazu Takahisa

Shimazu Takahisa

, the son of Shimazu Tadayoshi, was a daimyo during Japan's Sengoku period. He was the fifteenth head of the Shimazu clan.On 1526, Takahisa was adopted as the successor to Shimazu Katsuhisa and became head of the clan. He launched a series of campaigns to reclaim three provinces: Satsuma, Osumi,...

to be adopted by Katsuhisa as a condition for his help. In 1526, Katsuhisa handed over the position of the head of the family to Takahisa but it wasn't until 1539 though, during the Battle of Ichirai, when Tadayoshi defeated Katsuhisa (who would regain power later) that Takahisa came to be recognized by all members of the Shimazu clan as the head.