Date: Tenshō 12 (1584) to Tenshō 13 (1585)
Location: Various locations across Higo Province
Synopsis: The defeat of the Ryūzoji clan at the Battle of Okitanawate in the third month of 1584 provided an opportunity for the Shimazu clan of Satsuma Province, operating with the allied Sagara clan, to launch a full-scale invasion of Higo Province, eliminating the Aso clan and their retainers who had formerly been aligned with the Ryūzōji.
The Aso Conflict occurred from Tenshō 12 (1584) to Tenshō 13 (1585) in Higo Province in Kyūshū. The conflict was waged by the Shimazu army led by Shimazu Yoshihisa against the Aso forces led by Aso Koremitsu.
Succession issues in the Aso clan
The Aso clan governed the Aso District in Higo while serving as ōmiya-zukasa, a high-ranking position among Shintō priests affiliated with the Ise Shrine, making them a preist-daimyō family. During the Sengoku period, despite interference by surrounding powers such as the Ōtomo and the Kikuchi, over a succession of alliances and ruptures, the Aso maintained their independence. In 1578, however, at the Battle of Mimikawa, the Ōtomo suffered a major defeat to the Shimazu, enabling the Shimazu to expand their influence into Higo Province. While subjugating local clans including the Nawa, the Jō, and the Amakusa Group of Five, the Shimazu commenced battle against the Aso clan who were sympathetic to the Ōtomo.
First, in 1580, the Shimazu clan attacked retainers of the Aso clan, namely, Nakamura Tadafuyu at Yazaki Castle and his younger brother, Nakamura Nidaibu at Amita Castle, both on the Uto Peninsula. Tadafuyu charged out of his castle and, along with the other members of his garrison, was killed in action. The next day, Nidaibu surrendered and vacated his castle. In 1581, over 1,000 members of the Sagara clan from Yatsushiro who submitted to the Shimazu advanced with the aim of attacking Kai Chikanao (Sōun), the lord of Mifune Castle in the Mashiki District. The forces set fires below Katashida Castle and repelled forces who burst out of the castle. Meanwhile, over 500 troops under Chikanao established a position at Hibikinohara in dense fog and launched a surprise attack against the Sagara forces.
Sagara Yoshihi (the commander-in-chief), along with Higashi Sakyō-no-jō (a magistrate of Yatsushiro) were killed. Yoshihi and Chikanao had entered into a mutual non-aggression pact but the defensive support to the Shimazu provided by the Sagara came to an end, while the Kōshi clan surrendered, enabling the Shimazu to expand their influence in Higo. This led Aso Koremasa, the head of the Aso clan, to oppose the Shimazu by abandoning the Ōtomo in lieu of an affiliation with the Ryūzōji who were a rising power.
In 1583, after the death of Koremasa, his younger brother, Aso Koretane, inherited the role of ōmiya-zukasa, but, just one month later, he died of illness. In the end, the role of ōmiya-zukasa was inherited by Koretane’s infant son, Aso Koremitsu, at the age of two. Kai Chikanao served as a guardian for Koremitsu, but, in the third month of 1584, at the Battle of Okitanawate, Ryūzōji Takanobu was killed and the Ryūzōji clan surrendered to the Shimazu clan. The Kumabe and other kokujin, or provincial landowners, in Higo changed their affiliation from the Ryūzōji to the Shimazu. This resulted in the isolation of the Aso clan in Higo. In the ninth month of 1584, Chikanao died. Under another theory, Chikanao revolted against Toyotomi Hideyoshi and was killed. In any event, in the tenth month, the Shimazu army commenced a full-scale invasion of the Aso territory in Higo.
Counterattacks by the Aso clan
The Shimazu army appointed Niiro Tadamoto and Inatomi Shinsuke as commanding generals to attack Mifune Castle. Mifune was not a stronghold, but, owing to a counterattack by Kai Chikahide (the son of Kai Chikanao), the Shimazu were unable to the capture the castle. After leaving some forces behind to pin-down the castle, the remaining forces turned to attack the base of the Aso clan at Iwao Castle. Having a three-year-old infant as the lord of the castle, there was no prospect of defending the castle so Koremitsu was taken by his retainers and fled. As a result, the Aso clan was extinguished although former retainers resisted the invasion by the Shimazu.
At Nangō Castle to the northeast of Iwao Castle, Nagano Korehisa resisted and faced an honorable defeat. Korehisa was on a par with the Kai clan as a mainstay of the Aso family while Nangō Castle was a former base of the Aso – at one time the Aso clan was called the ōmiya-zukasa or high-ranking Shintō preist of Nangō. These events shook the defenders of neighboring castles and Nagano and Shimoda castles fell to attacks by the Shimazu forces.
To the southeast of Nagano Castle lay Takamori Castle defended by Takamori Korenao, a senior retainer of the Aso family. Korenao rejected efforts by the Shimazu army to lure him, and, despite having a small army force, engaged in a final showdown outside of the castle during which he was killed. After learning that his father, Korenao, had been killed in battle, Takamori Koreoki surrendered to the Shimazu. This, however, was a plot by Koreoki who covertly requested reinforcements from the Ōtomo clan. While members of the Shimazu army rested in Takamori Castle, Koreoki suddenly attacked them. The unsuspecting Shimazu forces were decimated. Fearing a counterattack by the Ōtomo and Takamori armies, the remaining Shimazu forces abandoned efforts to capture Mifune Castle and retreated south to Hananoyama Castle.
On 8/10 of Tenshō 13 (1585), Chikahide summoned former retainers of the Aso clan to attack and topple Hananoyama Castle. At this time, commanders in the Shimazu army including Kinowaki Sukemasa and Kamata Masatora, along with commanders from the Sagara army who came to their assistance including Fukami Setsu-no-suke (the eldest son and legitimate heir of Fukami Nagatomo), Indō Keibu, and Makino Kageyu, were killed in action.
Counteroffensive by the Shimazu clan
Based in Yatsushiro, Shimazu Yoshihiro served as the deputy military governor of Higo Province. Following the withdrawal of the allied forces of the Shimazu and the Sagara, the main division led by Yoshihiro as the commander-in-chief invaded the Aso territory as a rear guard. The allied forces toppled Katashida, Hananoyama, and Mifune castles in succession. Kai Chikahide underwent the rites of tonsure and surrendered. Akai and Kiyama castles to the north of Mifune Castle also fell. Tsumori Castle to the east of Kiyama Castle fell, and, to the north, Ishihara Yoshitoshi defended Imaishi Castle but faced an honorable defeat, while Takaba Castle to the north of Imaishi Castle was also taken. As a result, the defenders at castles recovered the prior year by Takamori Koreoki including Iwao, Nagano, Nangō, and Shimoda feared an invasion by the Shimazu forces and vacated their premises one after another.
Takamori Koreoki of Takamori Castle headed toward Bungo Province to plan for a revival, but, while en route, he was betrayed by his retainers and killed after a pursuit by Shimazu forces. After the fall of Takamori Castle, the Shimazu completed their pacification of Higo Province.