Separation of Suō and Aki
The Separation of Suō and Aki occurred on 5/12 of Tenbun 23 (1554) when Mōri Motonari of Aki Province severed relations with Sue Harukata and Ōuchi Yoshinaga of Suō Province. This break in relations led to attacks on numerous castles under Ōuchi control in the Saeki District of Aki.
In 1551, Ōuchi Yoshitaka (the thirty-first head of the Ōuchi clan) was cornered by a senior retainer named Sue Takafusa and compelled to take his own life in an event known as the Tainei Temple Incident. The Mōri clan had submitted for many years to the Ōuchi, but, after Yoshitaka’s death, attacked numerous outlying castles aligned with the Ōuchi in Aki Province and supported a rebellion against Takafusa. Thereafter, Ōuchi Yoshinaga succeeded Yoshitaka as head of the Ōuchi clan while Sue Harukata (formerly known as Takafusa) held the real power in the clan and the Mōri continued to maintain in a subservient relationship with the Ōuchi.
Mōri Takamoto, the eldest son of Motonari, had previously advocated for killing the enemies of Yoshitaka who was his father-in-law (Takamoto’s wife was the daughter of Naitō Okimori and adopted daughter of Yoshitaka), but Motonari desisted from taking action based on a realistic assessment of the differences in power of the Mōri and Ōuchi. Instead, via this opportunity, he made efforts to garner control of the kokujin, or provincial landowners, in Aki and succeeded in expanding the influence of the Mōri family.
Course of events
Reasons for the fall out
After Amago Haruhisa (who engaged in repeated battles against the Ōuchi clan) deployed to Bingo Province which was controlled by the Ōuchi, he opposed Motonari and the kokujin from Aki in lieu of Yoshinaga and Harukata who were preoccupied in the aftermath of the killing of Yoshitaka. Conflicts raged from the seventh month of 1552 until the tenth month of 1553 when the Amago were forced to retreat.
Meanwhile, the end of the conflict with the Amago forces in Aki and Bingo gave rise to discord between Motonari and Harukata. After the fall of Hatagaeshiyama Castle in Bingo on the evening of 10/19, Motonari sought to have the Mōri protect the castle, while Harukata assigned a retainer of the Sue named Era Fusahide to serve as the chamberlain. Harukata’s rejection of Motonari’s wishes showed that he did take lightly the continued expansion of the Mōri influence so Fusahide also likely carried the responsibility to keep watch on their movements.
Around this time, Yoshimi Masayori, the lord of Sanbonmatsu Castle in Iwami Province (who was loyal to Yoshitaka) rebelled with the aim of overthrowing Sue Harukata. In the fifth month, Masayori had already dispatched a messenger to Yoshida-Kōriyama Castle to request support from the Mōri. While enduring bitter losses at the hands of Yoshimi’s forces fighting to avenge the killing of Yoshitaka, in the spring of 1554, Harukata planned a major expedition to Iwami and, around the first month of the year, pressed Motonari for the deployment of Mōri troops along with kokujin from Aki.
Severance of relations
Motonari received requests for support from Yoshimi Masayori as well as Ōuchi Yoshinaga and Sue Harukata. Initially, it was too early to assert his autonomy so he participated on the side of Harukata. However, Takamoto asserted that if the Mōri were compelled to join the Sue, then, after the elimination of Masayori, Motonari would be boxed-in. To ensure the survival of the Mōri family, Takamoto proposed that he and Motoharu deploy on behalf of Motonari to show the family was fulfilling its duties. He strongly opposed having Motonari himself deploy under Harukata who he argued would at some time incur the wrath of heaven as a traitor. Moreover, after the fall of Yoshida-Kōriyama Castle, Motonari was concerned about the possibility of movements by the Amago clan on his flank. In a letter from after the start of the new year, he noted that the Mōri and Sue would eventually part ways so that they should sever the relationship when advantageous to do so.
On 3/1 of Tenbun 23 (1554), the Ōuchi army under the command of Ōuchi Yoshinaga deployed intent on attacking Sanbonmatsu Castle, leading to the Siege of Sanbonmatsu Castle. Motonari, however, did not show signs of participating in the deployment. Despite Motonari being the leader of the kokujin groups in Aki, beginning from the end of the second month, Harukata bypassed Motonari by sending letters directly to them to demand their deployment in a ploy to separate the Mōri from the other kokujin in Aki.
In the third month, however, a secret messenger was caught by Hiraga Hiroyasu and Hiraga Hirosuke. Hiroyasu and the others were loyal to the Mōri and indebted to Motonari for enabling them to revitalize the Hiraga clan. Horiyasu put the secret messenger monk and previous letters in front of Motonari. This violated a promise to grant Motonari authority over the kokujin, or provincial landowners, in Aki and Bingo provinces. Motonari decided on a final show down with Sue Takafusa. On 5/11, Motonari soon communicated to the Amano, the Hiraga, and the Asonuma clans to sever relations with the Ōuchi and Sue.
There is also a theory that the rebellion occurred after Motonari and Masayori entered into a secret pact, but, in the beginning, Motonari had decided to participate on the side of the Sue so this is viewed as an embellishment from later eras. Meanwhile, having anticipated a parting with Harukata, Motonari may have been waiting for a consensus opinion among the Mōri family members (driven by the likes of Takamoto) to sever relations with the Sue, but there are no historical accounts to substantiate the view that Motonari had, from early on, been waiting for an opportunity to become independent, so this may also be an embellishment from later eras.
Pacification of Aki Province
On 5/12, Motonari raised arms with a deployment of 3,000 troops comprised of forces from the Mōri, the Kikkawa, and the Kobayakawa, in addition to kokujin from Aki such as Kumagai Nobunao. The army first attacked Satōkanayama Castle where Kurita Higo Nyūdō served as the chamberlain, but, a retainer of the Mōri named Kodama Narikata persuaded Nyūdō to vacate the castle without a fight whereupon Nyūdō was sent back to Suō Province. Many members of the garrison had been sent on an expedition to Iwami so few had remained behind to defend the castle. Next, when the Mōri attacked Koi Castle, Koi Naoyuki (the lord of the castle) lacked the will to fight and surrendered.
Thereafter, the Mōri army marched to Sakurao Castle defended by Era Katanobu. The forces toppled Kusatsu Castle. Based on earlier solicitations of the defenders by Motoharu and Nobunao, the castle was soon occupied. Owing to the fall of Sakurao Castle on the opposite shore from Itsukushima, the Mōri were further able to capture Itsukushima. This was facilitated by Hotate Naomasa through negotiations with local residents. Naomasa was a retainer of the Mōri who had been active as a merchant trader in the environs of Hiroshima Bay. In addition to Itsukushima, in a single day, the Mōri took control of four outlying castles including Satōkanayama, Koi, Kusatsu, and Sakurao.
By the middle of the same month, Motonari had taken control of Nihojima Castle, and assigned Kodama Narikata to Kusatsu Castle, Kagawa Mitsukage and Tōrinbō to Nihojima Castle, Katsura Motozumi to Sakurao Castle, and Koi Naoyuki to Itsukushima. To protect the seas, Narikata led the naval forces comprised of the Kawanouchi group – guards who were former retainers of the Aki-Takeda family. Mitsukage was an admiral of the former Takeda navy while Tōrinbō (a monk from the Ikkō sect) had close ties to the Kawanouchi group. Meanwhile, the forces destroyed Kadoyama Castle to prevent the Ōuchi from using it as a base to launch counterattacks, further strengthening their defenses.
After sweeping all Ōuchi elements from Aki, the Mōri clan quickly made preparations for a final showdown against the Ōuchi and Sue clans. Surprised at the movements made by Motonari, Harukata severely criticized Motonari for plotting acts of treason and for being wicked and atrocious. And, in a bid to recapture Itsukushima and its environs which constituted strategic locations for both sea and land routes, Harukata sent a retainer named Miyagawa Fusanaga, and, on 6/5, a conflict erupted at the Akashi Pass known as the Battle of Oshikibata.