Lifespan: Eishō 5 (1508) of Eishō 122 (1515) to 7/5 of Tenshō 11 (1583)
Lord: Aso Koretoyo → Aso Koremasa → Aso Koretane
Father: Kai Chikanobu
Children: Chikahide, Chikamasa, Nobushige, Naotake, daughter (wife of Kumanoshō Morimasa), daughter (wife of Hayakawa Kyūun), daughter (wife of the lord of Izu-Noyama Castle)
Kai Chikanao served as a bushō from the Muromachi to Azuchi-Momoyama periods. He was a retainer of the Aso clan. After entering the priesthood, he adopted the monk’s name of Sōun.
In 1523, Kikuchi Takekane raised arms at Tsutsugatake Castle so Koretoyo dispatched Chikanobu to eliminate him. This was the first experience for Chikanao in battle. Takekane fled to the Takaki District in Hizen Province. Owing to these results, Chikanao was awarded as his territory the town of Yao.
Service to the Aso clan
In 1541, Chikanao served as a deputy to Aso Koremasa (Senjumaru) as the commanding general to subdue Mifune Fusayuki, the lord of Mifune Castle for rebelling against the Aso clan after colluding with the Shimazu clan. At Kigurahara (Mifunehara), he garnered numerous enemy heads en route to a major victory, compelling Fusayuki to kill himself and capturing the castle. Owing to these achievements, Chikanao was awarded the territory of Senchō and Mifune Castle and appointed as its new lord. This is known as the Battle of Kigurahara.
Around this time, he underwent the rites of tonsure and adopted the name of Sōun.
In 1546, Sōun’s son-in-law, Kumanoshō Morimasa, pleaded for a short sword, but was refused so he had his wife (Sōun’s daughter) steal one. After discovery of the act, Morimasa feared the consequences so, via Hongō Hōki-no-kami of Uto, he colluded with the Shimazu and plotted to abandon the Aso clan. This, however, enraged Koremasa, who ordered Sōun to attack Kumanoshō Castle. At the Battle of Kumanoshō, Sōun defeated Morimasa but did not capture the castle. In 1549, once again, Koremasa ordered generals to deploy for an attack on the castle, resulting in the deaths of Morimasa and his entire family.
Sōun was ostensibly subservient to the Ōtomo clan, while, in Higo Province, he maintained relationships with the Sagara and Nawa clans as a means to preserve the Aso clan. In 1550, Ōtomo Yoshishige inherited the headship of the clan, and, in 1551, he deployed troops to subjugate Higo. The Aso clan submitted to the Ōtomo, dispatching Sōun to serve as a guide for the commanding general, Saeki Korenori. Korenori attacked Takaba Castle defended by Kōshi Takashige (aligned with Kikuchi Yoshitake’s faction) but was unable to topple the castle and withdrew on the condition the defenders tender a hostage.
Contending with the Shimazu and Ryūzōji clans
From around 1562, the Shimazu clan encroached on Higo Province, resulting in frequent clashes against the Sagara clan. Beginning in 1578, the Shimazu commenced an earnest invasion. Sagara Yoshihi, who controlled the Kuma, the Yatsushiro, and the Ashikita districts, joined forces with the Aso clan to resist. Meanwhile, the Ōtomo clan suffered a defeat to the Shimazu clan at the Battle of Mimikawa, reducing their influence in Higo. Thereafter, many of the kokujin in Higo aligned themselves with the Shimazu and Ryūzōji clans, but Sōun persisted in maintaining his alliance with the Ōtomo. The weakened state of the Aso clan led to internal divisions.
In the third month of 1580, allied forces comprised of individuals and clans affiliated with the Ryūzōji (Kumabe Chikanaga, Kōshi Chikatame, and the Kawajiri and Kanokogi clans) along with kokujin of Higo subordinate to the Shimazu led by Nawa Akitaka and Jō Chikamasa, raised an army to overthrow the Aso clan. Sōun and his eldest son, Kai Chikahide (Sōritsu), led 8,000 soldiers to intercept these forces, confronting them at Tanganose across the Shira River. According to reports from spies, Sōun learned that the Kumabe forces were enjoying drinking during the rainfall, so, at dawn the next morning, he crossed the river to mount a surprise attack. The Kumabe forces fell into disarray and were sent fleeing. As recognition for this victory, Ōtomo Yoshimune awarded Sōun the village of Ikenoura in the Akita District of Higo. This is known as the Battle of Tanganose.
Around this time, Kuronita Bungo-no-kami betrayed the Aso clan by colluding with the Itō clan. Although Bungo-no-kami was Chikahide’s father-in-law, Sōun did not allow the betrayal of his lord, so, with the exception of his wife who signed a pledge, he killed the members of the Kuronita and their followers and presented their heads to his lord. In the Aso family, there were over 70 yoriki, or security staff, known as the Iseri Group. The head of this group with the surname of Kaga plotted to join the Shimazu clan as a means to eliminate the Aso clan. Upon learning of this plan, Sōun killed all of them. This group included Sōun’s second son (Chikamasa), third son (Nobushige), and fourth son (Naotake). He killed Chikamasa, while Nobushige fled but was chased and killed on Mount 江俵, and Naotake fled to Hyūga. His eldest son, Sōritsu, had a plan to join the Ryūzōji clan so we pursued and captured. Kōshi Ise-no-kami pleaded for Sōritsu to be spared so Sōun had him write a pledge and spared his life.
Battle of Hibikigahara against Sagara Yoshihi
In the spring of 1581, Sōun cut ties with the Ōtomo clan and sent a hostage to the Ryūzōji, pledging to serve as their retainer. Meanwhile, in the ninth month, the Sagara surrendered to the Shimazu army, transferring the Ashikita District to the Shimazu. Sagara Yoshihi was then promptly ordered to attack Mifune Castle in the Mashiki District. Sōun, however, was the lord of Mifune Castle, and Yoshihi had earlier exchanged written oaths with Sōun to become allies whereby Yoshihi pledged not to attack the Aso Shrine and Sōun pledged not to attack the Shiraki-Myōken Shrine in Yatsushiro. A refusal to follow the orders would breach the settlement with the Shimazu which would not bode well for the future of the Sagara clan. Meanwhile, obeying the orders would mean betraying the trust of Sōun. Confronted with a difficult decision, Yoshihi opted to preserve the Sagara clan, committing to attack Mifune Castle. The Shimazu purposely aimed to have them fight against one another as a means of dividing the kokujin, or provincial landowners, in Higo.
At dawn on 12/1 of Tenshō 9 (1581), Yoshihi departed from his base at Furufumoto Castle, leading his army into the Aso territory. On the way, he made a prayer for victory at the Shiraki-Myōken Temple. At this time, he burned the written oath that he had previously exchanged with Sōun and prayed for his own demise. The wind caused battle flags to become entangled in the branches of camphor trees, portending an ill-fated deployment. From Yatsushiro, the Sagara forces traversed the Shabagami Pass to invade the Aso territory. Yoshihi divided the forces to have them attack Katashida and Kōsa castles. Yoshihi himself established a base at Hibikigahara. This site, however, was difficult to defend so his retainers recommended the encampment be moved to Shabagami, but Yoshihi refused. Meanwhile, after Sōun learned of their deployment, he did not immediately believe it was true. Once he learned it was, in fact, true, as a general in the Aso clan, Sōun was compelled to take measures to respond to the threat.
Following attacks by the Sagara forces, Katashida and Kōsa castles both fell over the next one to two days. At dawn on the second day, the Kai forces deployed, quietly traveling on the road toward Hibikigahara. On this day, a light rain created a dense fog. Sōun mobilized peasants from his territory to have them stand battle flags on Mount Iida, giving the appearance that forces had gathered in that location while he continued to head toward Hibikigahara. At this time, the Sagara forces noticed the movement of troops but mistook them for reinforcements from Uto Castle. From their plateau base at Hibikigahara, the Sagara forces celebrated their victories at Katashida and Kōsa castles, but enshrouded in fog, the Kai forces surrounded them and then launched a surprise attack. The Sagara camp turned into chaos. Refusing to retreat, Yoshihi was killed while seated on his camp stool by Ogata Kizō, a retainer of the Kai. In addition to the loss of their commanding general, over 300 of the Sagara forces were killed in action while the remainder fled in defeat. This is known as the Battle of Hibikigahara.
Having been trapped between the Shimazu and Aso clans (represented by Sōun), Yoshihi may have deployed with the intention of losing. After viewing the head of Yoshihi, Sōun was said to have shed tears and pressed his hands together in prayer, noting that Yoshihi’s death was inevitable because he broke covenants. Nevertheless, the strength of the Sagara clan enabled the Aso clan to survive for a long time, and he further noted that after the loss of Yoshihi, the Aso no longer had someone to depend upon so he predicted the Aso would confront their own demise within three years.
Although Sōun prevailed in battle against the Sagara clan, it became difficult for the Aso clan to contend with the Shimazu clan without the support of the Sagara clan. Thereafter, as a diplomatic strategy, Sōun made great efforts to preserve the Aso clan while standing in-between the two major powers of the Shimazu and the Ryūzōji. In the winter of 1582, he proposed a settlement to the Shimazu, but did not carry out even one of the conditions set forth by them, instead, seeking the return of former territory of the Aso clan, causing the negotiations to fail.
Sōun died of illness on 7/5 of Tenshō 11 (1583) at the age of seventy-five. Alternatively, as explained below, he may have been poisoned. Aware that the period of war was coming to an end, Sōun noted that he would not launch an offensive against the Shimazu clan, and intended to focus on defending Yabe (the main base of the Aso clan) and await the ascendancy of the individual who could unify the country. However, after scouts from the Shimazu learned of the death of Sōun, in 1585, Chikahide attacked Hananoyama Castle which was aligned with the Shimazu. This invited a counterattack from the Shimazu, causing Chikahide to quickly surrender. At the time, Aso Koremitsu, the lord of the Aso family, was only two years old. His mother took him and fled, ending the Aso clan serving in the role who had served as the high priests of the Aso Shrine in Higo.
Theory of poisoning
There is a theory that Sōun was poisoned by the daughter of Kai Chikahide (Sōun’s granddaughter).
Deeply loyal to the Aso clan, Sōun purged without mercy anyone who betrayed the family or violated their rules. For attempting to establish relations with Itō Yoshisuke of Hyūga Province, Chikanao murdered Chikamasa (the second son), Nobushige (the third son), and banished Naotake (the fourth son). Opposed to these actions, Kai Chikahide plotted to assassinate Sōun, but the plot was discovered. Typically, this would also have resulted in his killing, but Chikahide was his eldest son, and owing to pleas from his retainers, he refrained from that action. Even in the Sengoku period, the prospect of killing all four of one’s sons at once was an extreme degree of punishment.
Chikahide’s wife greatly feared Sōun, believing that his father-in-law would kill her husband as well. She then ordered her daughter (who later became the wife of Kiyama Jōtaku) to poison Sōun. She was the daughter of a retainer named Kuronita Chikasada, but Chikasada had once colluded with the Itō clan and was consequently killed by Sōun. With respect to the killing of Chikasada, Sōun had Chikahide’s wife vow in the name of God that she would not resent the killing of her father or plot revenge against him for the act. Therefore, Chikahide’s wife used the daughter because, first of all, it would be easier to catch him off-guard and, secondly, it would not violate the wording of the earlier vow.