Coming from a province conquered by the Takeda, Masamori belonged to the sakikata-shu (the group of vanquished enemies) within the Takeda establishment, but proved himself repeatedly by loyal service. He fought for the Takeda at Mimasetoge (1569) and Mikatagahara (1573), where he led the cavalry vanguard. At the Battle of Nagashino (1575), he supplied the largest cavalry contingent, commanding 500 mounted samurai and 1000 footmen. In the central company, commanded by Takeda Nobukado, he charged the Oda line beside the other Kozuke warlords. Eventually, as the Takeda army made no headway, the attack was called off but not until grave losses had been suffered.