[1] The actual date of Sannohe Castle’s foundation is unknown; however, it was one of a series of fortifications established by the Nanbu clan in the late Kamakura period or early Muromachi period when the clan relocated its seat from Kai Province to Mutsu Province and started to secure its control over the frontier Nukanobu District.

The clan residence remained at nearby Shōjujidate Castle in what is now the town of Nanbu until that fortified house was destroyed in an uprising in June 1539.

In 1590, the Sannohe faction led by Nanbu Nobunao organized a coalition of most of the Nambu clans and pledged allegiance to Toyotomi Hideyoshi at the Siege of Odawara.

In return, he was recognized as chieftain of the Nanbu clans, and confirmed as daimyō of his existing holdings in the northern districts of Mutsu Province.

However, Kunohe Masazane, leader of a rival faction, felt that he had a stronger claim to the title of clan chieftain, and immediately rose in rebellion.