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Isono Kazumasa (磯野 員昌?, 1534–1583) was relatively the most senior of retainer beneath the clan of Azai throughout the latter Sengoku Period of Feudal Japan. Initially at the time in which Kazumasa began his service under the Azai, Sukemasa was the present head over the clan, and Kazumasa began his great line of feats by supporting the former in a desperate conflict against the Rokkaku that took place in 1561. Within this event, the Rokkaku attempted to ultimately take over the entire Sawayama castle through means of besiegement, but were met with failure in this attempt after Kazumasa was sent as relief, convincing Sukemasa to allow this respective retainer complete jurisdiction over Sumamata castle as a reward for his reliability. Following the succession of Azai Nagamasa, Kazumasa willingly supported the former within many variable conflicts throughout the years that had passed, the most notable being Anegawa of 1570, where Kazumasa was said to have fought with such conviction and courage on the battlefield that his favorite horse was shot out beneath him and he unhesitatingly mounted a new horse immediately following the first, charging even deeper into the Oda ranks that surrounded him. In furthered justification to this, Kazumasa penetrated so far past the soldiers of Shibata Katsuie and Toyotomi Hideyoshi that he would have initially breached into the main camp of Oda Nobunaga if the additional units of Mori Yoshinari and Sakuma Nobumori did not immediately assist their respective allies.
Even with such a showing of great heroism, Kazumasa was tasked with an overwhelming obligation by the year of 1573: Oda Nobunaga had already torn apart the Asakura and was charging upon the Sawayamasa castle with over 35,000 soldiers to back his aspirations. And as Nagamasa was too afraid of the humiliation that he would suffer from if he were to surrender to Nobunaga a second time, he took a hold of Kazumasa's aged mother, swearing that she will be crucified if Kazumasa fails in the ultimate attempt of forcing off the Oda besiegers from Sawayama. As Kazumasa was thus determined to fight on to his very last breath less his mother were to suffer such an excruciating death, he managed to hold out for over 8 months before initially possessing no remaining provisions at which his army would be able to feed upon. Knowing that defense was now an impossible ideal, Kazumasa had little other choice then to surrender to Nobunaga, who in return admired the warrior for his loyalty and conviction, awarding him Takashima District of western Omi. Remaining within such a fief, Kazumasa surmiseably detested Nagamasa with an intense hatred, supporting the Oda with renewed resolve, even though abandoning his domain by the year of 1578, where it is said that he remained within the same respective province, employing a simple life of farming. Kazumasa then died in 1583. Also, while Nagamasa was married to Nobunaga Oda's younger sister Oichi, Kazumasa was very fond of Oichi, this was one of the reasons why he fought with such courage.
- Isono_Kazumasa - SamuraiWiki. (Samurai Archives) FWSeal & CEWest, 2005