The Battle of Numajiri was part of a campaign of eastern expansion by Hojo Ujiyasu, who brought the Hōjō into conflict against the Satake clan of Hitachi Province. The Satake was led by Satake Yoshishige, who was renowned for his ferocity in battle; and was also known by the nickname "Ogre Yoshishige" (鬼義重, Oni Yoshishige).

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Allied with the Satomi clan and Utsunomiya clan, Yoshishige resisted Hôjô expansion and fought against the Later Hōjō clan, who were extending their power into southern Hitachi.^[1] At Numajiri, 20,000 men under Yoshishige fought against 80,000 Hōjō troops.^[1] The Satake won, due in part to the use of over 8600 matchlock guns by their troops.^[1]