Lifespan: Daiei 3 (1523) to 9/10 of Tenshō 18 (1590)
Title: Governor of Tanba
Lord: Azai Hisamasa → Azai Nagamasa → Oda Nobunaga
Father: Isono Kazumune
Children: Yukinobu, Masanaga, daughter (formal wife of Kobori Masatsugu)
Adopted Children: Kazutsugu (son of Anyōji Ujitane), Nobuzumi (eldest son of Oda Nobukatsu, nephew of Oda Nobunaga)
Isono Kazumasa served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Azai clan and served as the lord of Sawayama Castle in Ōmi Province.
Service as a retainer of the Azai clan
In 1523, Kazumasa was born as the son of Isono Kazumune. The Isono clan served as hereditary retainers of the Kyōgoku clan, but, after ceding to the rise to prominence of Azai Sukemasa, joined his command. Kazumasa’s father, Kazumune, was welcomed by Isono Kazuyoshi, who came to Sawayama Castle as an adopted son from a family connected to the Isono clan. The main branch of the Isono clan based at Isonoyama Castle went west to become retainers of the Harada clan, the lords of Takasu Castle in Chikuzen Province in northern Kyūshū. The family later moved to harbor town of Hakata and engaged in the craft of metal casting.
After the death of his father, Kazumasa’s uncle, Kazukiyo, inherited the family, followed by Kazumasa. Based at Sawayama Castle, Kazumasa leveraged his military prowess to achieve valorous results in battle against the rival Rokkaku clan of southern Ōmi. He served as the leader of the vanguard of the Azai army. He was extolled, along with Ōnoki Kunishige, Nomura Sadamoto, and Mitamura Hidetoshi, as a member of the Four Wings of the Azai.
On 6/28 of Genki 1 (1570), at the Battle of Anegawa, Kazumasa slashed his way deeply into the Oda army, momentarily approaching the main base of Oda Nobunaga. However, Inaba Yoshimichi (Ittetsu), Ujiie Bokuzen, and Andō Morinari from the Oda army rushed over to the defense. In addition, members of the Tokugawa army who had defeated the Asakura army joined the Oda forces, causing the Azai army to collapse and pull-back in defeat. Nevertheless, Kazumasa’s bold approach toward the main base of the Oda became a legendary story.
Service as a retainer of the Oda clan
After the Battle of Anegawa, the land route to Odani Castle held by the Azai clan was severed by the Oda clan based at Yokoyama Castle. This left Sawayama Castle isolated in the midst of enemy positions. This led, on 2/24 of Genki 2 (1571), to attacks against Kazumasa at Sawayama Castle whereupon he surrendered to Nobunaga.
After surrendering, in exchange for Sawayama Castle, Kazumasa was granted the Takashima District in Ōmi. Around this time, veteran generals of the Oda family were located in the environs of Lake Biwa, and the granting of the Takashima District represented extraordinary treatment on a par with Kinoshita Tōkirō (later known as Toyotomi Hideyoshi) at Yokoyama, Niwa Nagahide at Sawayama, Nakagawa Shigemasa at Azuchi, Shibata Katsuie at the Chōkō Temple, Sakuma Nobumori at Nagahara, and Akechi Mitsuhide at Usayama, evidencing a recognition within the Oda family of his military achievements. Kazumasa, however, was obliged to adopt Nobunaga’s nephew, Tsuda Nobuzumi, as his designated heir.
Thereafter, in the ninth month of 1573, Kazumasa apprehended Sugitani Zenjubō and, in the eighth month of 1575, participated in the suppression of the Echizen Ikkō-ikki.
In the first month of 1576, Tsuda Nobuzumi traveled from the Takashima District to Kyōto. In the twelfth month of 1576, Nobuzumi issued documents to Kyūboku Shōnin and, in the seventh month of 1577, to Yokoe Suizenji, to recognize their rights to territory. Around this time, Kazumasa experienced a loss of authority and transfer of the headship of the clan.
Flight and aftermath
On 2/3 of Tenshō 6 (1578), Kazumasa fled after being reprimanded for betraying the will of Nobunaga. His territory in the Takashima District of Ōmi was transferred to Nobunaga’s nephew, Tsuda Nobuzumi. The subject of the reprimand is not certain, but, according to one theory, Nobuzumi and Nobunaga pressed him to transfer the headship of the clan, but he refused. His whereabouts after fleeing are uncertain, but, after the deaths of Nobuzumi and Nobunaga at the Honnō Temple Incident in the sixth month of 1582, he returned to the Takashima District and engaged in farming. He died on 9/10 of Tenshō 18 (1590) at the age of sixty-eight.
His son, Isono Yukinobu, along with other members of the family, served Ishida Mitsunari and, later, Tōdō Takatora (during the Tenshō era, Takatora served Kazumasa), continuing the family name. His grandson, Yukihisa, joined the Tōdō army at the Siege of Ōsaka, and at the Battle of Yao-Wakae, killed Mashita Moritsugu. His daughter wed Kobori Masatsugu, and was the mother of Kobori Masakazu, a tea master.