Lifespan: 14xx to 8/1 of Eishō 4 (1507)
Bakufu: Muromachi – military governor of the lower districts of Yamashiro Province
Lord: Hosokawa Masamoto → Hosokawa Sumiyuki
Father: Kōzai Motonao
Adopted Children: Motomori (son of Hatano Hidenaga) (?)
Kōzai Motonaga served as a bushō during the Sengoku period. He was a retainer of the Hosokawa clan and served as the deputy military governor of the southern half of Yamashiro Province. Motonaga was the lord of Arashiyama Castle in Kyōto.
The Kōzai clan originated as bushi from Sanuki Province in Shikoku, but, around the time of Kōzai Motonao, moved to the capital of Kyōto, and Motonao’s younger brother inherited his land in Sanuki. In 1497, Motonao’s son, Motonaga, was appointed to serve as the deputy military governor of the lower districts of Yamashiro Province (while Akazawa Tomotsune was the deputy military governor for the upper districts of Yamashiro). In 1504, Motonaga joined with another retainer of Hosokawa Masamoto (the deputy shōgun of the Muromachi bakufu), Yakushiji Motoichi, to abandon their lord in favor of one of his adopted sons, Hosokawa Sumiyuki, and launch a rebellion. Motoichi’s younger brother, Yakushiji Nagatada, continued to support Masamoto, leading an attack against the rebels at Motoichi’s base in the First Siege of Yodoko Castle. The Hosokawa army prevailed, and, after the fall of the castle, Motoichi took his own life after being captured in Kyōto while Tomotsune fled to Yamato Province.
In 1507, Motonaga burned down the Kamo Shrine in Kyōto for refusing to contribute funds for a deployment to Tanba Province by his lord, Masamoto.
Masamoto had three adopted sons, Hosokawa Sumiyuki, Hosokawa Sumimoto, and Hosokawa Takakuni. Owing to the absence of a natural heir, disputes arose among the family as to who should become Masamoto’s designated successor, with rival factions forming in support of each of the three adoptees. Motonaga supported Sumiyuki, but when it became known that he was unlikely to become the successor, Motonaga made a plan with Yakushiji Nagatada and Takeda Magoshichi by which, on 6/23, Masamoto was assassinated and they backed Sumiyuki as his successor. The killing of Masamoto is known as the Lord Hosokawa Incident which occurred in the context of the broader succession struggle known as the Eishō Disturbance. This faction initially expelled one of the other sons, Hosokawa Sumimoto, from Kyōto, whereupon he fled to Iga Province, but following counterattacks by the factions supporting Sumimoto and the other adopted brother, Hosokawa Takakuni, Motonaga was killed after being struck by a stray arrow in fighting near the residence of Sumiyuki in Kyōto. Meanwhile, Sumiyuki took his own life, bringing to an end after only forty days his term as the ostensible head of the Hosokawa-Keichō family.
There are numerous theories with respect to Motonaga’s motivations for the assassination of Masamoto. With the support of Miyoshi Yukinaga (the kasai, or head of house affairs), Sumimoto became the designated heir of the Hosokawa family, enabling Yukinaga to expand the influence of this faction. This included intervening in affairs in Motonaga’s home province of Sanuki. In addition, Motonaga was fearful of Masamoto’s peculiar personality, and, faced with dimming prospects for Sumiyuki to become the successor, he resented Yukinaga’s growing influence within the Hosokawa family. By eliminating Masamoto, he sought to gain the authority needed for Sumiyuki to become Masamoto’s successor.