Kōzuke Province
Kamiizumi was born as Kamiizumi Hidetsuna in his family castle in Kōzuke Province (modern day Maebashi, Gunma Prefecture). His family were minor landed lords in the service of the Yamanouchi branch of the Uesugi clan. At the time of his birth, Kōzuke Province was being contested by the Uesugi, the Hōjō, and the Takeda clans. His family was originally a branch of the Ōgo clan that moved to nearby Kaigayagō Kamiizumi and took its name for their own. When the main Ōgo clan moved to Musashi Province, the Kamiizumi family took over Ōgo Castle, at the southern foot of Mount Akagi.