Miyoshi Sōi served as a bushō during the Sengoku period.  He was a member of the Miyoshi and served as a retainer of the main branch of the family.  Sōi is known (along with Miyoshi Nagayasu and Iwanari Tomomichi) as one of the members of the Miyoshi Group of Three.  His first name was Uemon Taifu Masakatsu, followed by Shimotsuke-no-kami Masanari, then, after entering the priesthood, Chōkansai Sōi.  Consistent with usage in his life, this profile uses the name Masakatsu until 1565 when he changed his name to Sōi.

There are various theories concerning his father.  Masakatsu was either the son of Miyoshi Masanaga or of the second son of Miyoshi Yukinaga, Magosaburō Yorizumi.  If he was the son of Yorizumi, then Miyoshi Masanari, the lord of Sanga Castle, was his older brother.  Miyoshi Isan was likely his younger brother.  A letter from Miyoshi Nagayoshi refers to Miyoshi Sōsan (Masanaga) and Uemon Taifu Masakatsu as father and son.  Whether Masakatsu refers to Isan or Sōi, if these two were brothers, then Sōi’s father was Masanaga.

Conflict with Miyoshi Nagayoshi

Along with his father (Masanaga) and younger brother (Isan), Masakatsu served Hosokawa Harumoto.  In 1544, he was transferred headship of the clan by Masanaga, but Masanaga retained his grip as the holder of power in the clan.  In 1547, Masakatsu joined his father at the Battle of Shari Temple.  As a close associate of Hosokawa Harumoto, Masanaga wielded authority which led to conflict with Miyoshi Nagayoshi, the head of the main branch of the family.  In the midst of tensions between Harumoto and Nagayoshi, in the tenth month of 1548, Nagayoshi rebelled against Harumoto and Masanaga.  As Masanaga’s son, this also made Masakatsu a target, and after holing-up at his base in Enami Castle in Settsu, he was surrounded.  He held on until the sixth month of 1549, but, after his father proved unable to help and was killed by Nagayoshi at the Battle of Eguchi, Masakatsu fled Enami Castle.

Subsequently, after Harumoto and Ashikaga Yoshiteru (the thirteenth shōgun of the Muromachi bakufu)  fled from Kyōto to Ōmi Province, Masakatsu collaborated with Kōzai Motonari of Sanuki Province and Hatano Harumichi of Tanba Province to engage in all-out war against Nagayoshi.  In the third month of 1551, after Nagayoshi was the target of a failed assassination by Yoshiteru, Masakatsu and Motonari took advantage of the chaos to invade Kyōto and then withdrew.  In the seventh month, Masakatsu entered the capital again and holed-up in the Shōkoku Temple, but, in an event known as the Battle of Shōkoku Temple (of 1551), one of Nagayoshi’s commanders, Matsunaga Hisahide, and his younger brother, Matsunaga Nagayori, burned down the temple, causing Masakatsu to flee.  The Shōkoku Temple was also the location of one of the preliminary battles of the Ōnin-Bunmei War in 1467 between the Eastern and Western armies, at which time it was damaged by fire but not completely destroyed.

In 1553, after Harumoto and Yoshiteru conspired against Nagayoshi, in the seventh month, Masakatsu and Motonari entered the capital.  In the eighth month, following the fall of Ryōzen Castle to the Miyoshi army, Yoshiteru and others fled again to Ōmi.  In the ninth month, after slipping into Tanba, Nasakatsu toppled Yagi Castle and killed its lord, Naitō Kunisada.  Yagi Castle was then retaken by Matsunaga Nagayori and he fled.

Return to service of Nagayoshi and the Miyoshi Group of Three

In the ninth month of 1558, Maskatsu became a retainer of his bitter enemy, Nagayoshi.  Thereafter, he contributed to the expansion of Nagayoshi’s influence, and, in 1562, contributed on the battlefield in attacks against Hatakeyama Takamasa at the Battle of Kumeda and at the Battle of Kyōkō Temple.

In 1564, after the death of Nagayoshi, Masakatsu gained prominence as one of the guardians of Nagayoshi’s nephew, Miyoshi Yoshitsugu.  Together with Miyoshi Nagayasu and Iwanari Tomomichi, Masakatsu and these individuals were referred to collectively as the Miyoshi Group of Three.  Along with Matsunaga Hisahide, the Group of Three held sway in the Miyoshi family.  In the third month of 1565, Masakatsu entered the priesthood and adopted the name of Chōkansai Sōi.  On 5/19 of Eiroku 8 (1565), Sōi joined with the other members of the Group of Three and Hisahide to attack the palace of Ashikaga Yoshiteru (the shōgun) during which Yoshiteru was killed in the fighting.  This is known as the Eiroku Incident.

Conflicts with Matsunaga Hisahide

Gradually, the relationship between the Group of Three and Hisahide deteriorated in a power-struggle for control of the Kinai.  On 11/16 of Eiroku 8 (1565), the Group of Three assaulted Iimori Castle in Kawachi Province, and brought Yoshitsugu to Takaya Castle.  The Group of Three then had Yoshitsugu promise to break ties with Hisahide and back Yoshiteru’s cousin, Ashikaga Yoshihide (the Awa kubō) as the fourteenth shōgun of the Muromachi bakufu.  The Group of Three also succeeded in garnering the support of Shinohara Nagafusa at the home base of the Miyoshi clan in Awa Province in Shikoku.  Toward the end of the twelfth month, they had Yoshihide issue an edict to have the army occupy Yamato (the home province of Hisahide) and, after allying with Tsutsui Junkei, threatened Hisahide.

Meanwhile, Hisahide stirred-up Hatakeyama Takamasa and Yasumi Munefusa (who had fled to Kii) to attack Takaya Castle.  In the second month of 1566, in Kawachi, the Hatakeyama army entered into a final showdown against the Group of Three.  On 2/17, however, the Group of Three roundly defeated the Hatakeyama army and Hisahide withdrew to Yamato.  In the fourth month, the Group of Three invaded Yamato a second time, but Hisahide fled his castle and assembled forces in Sakai to threaten Takaya Castle again.  As a result, the Group of Three obtained reinforcements led by Ikeda Katsumasa of Settsu and Atagi Nobuyasu of Awaji and approached Sakai with an army three times the size of Hisahide’s contingent.  On 5/30, Hisahide fled again without fighting.  This time, Shinohara Nagafusa backed Ashikaga Yoshihide and led a huge army from Awa to land in Hyōgo.  The Group of Three finally announced the demise of Nagayoshi and held a memorial service.  With reinforcements from Nagafusa, the Group of Three was able to almost completely eliminate elements of the opposition in the Kinai.

On 8/23 of Eiroku 9 (1566), Sōi received Yoshihide at Koshimizu Castle and treated him as his lord, but this drew scorn from Yoshitsugu.  In the second month of 1567, Yoshitsugu absconded and turned to Hisahide to oppose the Group of Three.  Miyoshi Yasunaga also abandoned the Group of Three so Hisahide regained power and the Miyoshi family ruptured amid conflict again.  In Yamato Province, a series of battles related to Hisahide’s base at Tamonyama Castle ran for approximately one-year from 4/18 to 10/11 of Eiroku 10 (1567).  In the course of this fighting, a national treasure was burned down in the Battle at the Giant Buddha of Tōdai Temple, with the statue dating to the eighth century.  The situation on the ground, however, favored the Group of Three.  Despite suffering a loss at the Tōdai Temple, Sōi and his allies re-grouped, toppled Shigisan Castle (aligned with Hisahide), and surrounded Hisahide’s base at Tomonyama Castle.

In the ninth month of 1568, after Oda Nobunaga marched upon Kyōto to install Ashikaga Yoshiaki as the fifteenth shōgun of the Muromachi bakufu, Yoshitsugu and Hisahide approached the Oda clan while the Group of Three chose the path of resistance.  On 9/29, following the successive losses of Shōryūji and Yodoko castles, the resistance forces witnessed a near total collapse.  Sōi, as the lord of Kizu Castle in Yamashiro, also withdrew from his base.  A falling out between Yoshiaki and Nobunaga only served to further intensify Sōi’s conflict with the Oda and the Ishiyama-Hongan Temple.

After a military campaign in the Kinai resulted in a definitive advantage for Nobunaga, Sōi’s movements cannot be confirmed.  The year of his demise is not verified, but, according to one account dated 5/26 of Eiroku 12 (1569), Sōi died on 5/3 of the same year in Awa Province.  The headship of the clan was inherited by his younger brother, Isan.

Other notes

In the diaries of Luís Fróis, a Portuguese missionary residing in Japan during this period, Miyoshi Sōi is referred to as Shimotsuke-no-kami.  Upon the strong encouragement of Shinohara Nagafusa, he supported Christian evangelical activities in the capital.  After a judgment against a Christian relative (Kawashima Joan), Sōi made efforts to obtain guarantees with respect to Joan’s life, identity, and assets.

The younger sister of Miyoshi Yoshitsugu became the wife of Tarao Tsunatomo, one of the Wakae Group of Three who betrayed Yoshitsugu.  Her son, Magokurō, was raised by Sōi.  Magokurō adopted the name of Miyoshi Ikegachi and served Oda Nobunaga.  He inherited the family name of Yoshitsugu and was granted a fief of 5,490 koku in Kawachi Province, later participating in battles against followers of the Ishiyama-Hongan Temple.  Ikegachi served Toyotomi Hideyoshi, Kuroda Nagamasa, and Asano Nagaakira.  In the Edo period, his descendants served as retainers of the Hiroshima domain until the Meiji Restoration.

Sōi’s personal seal was in the form of a Mandarin duck.