Aso Koretoyo (阿蘇 惟豊, 1492–24 January 1539) was a daimyō of Sengoku period Japan and High Priest of Aso-Kuju Temple. A highly capable warrior, Koretoyo earned the nickname of "the Iron Monk" due to his heavy armor and impressive physique. Koretoyo engaged in numerous conflicts with the Ōtomo, Kamachi, and Sagara clans, driving the latter to ally themselves with the Shimazu. At first, Koretoyo was able to hold his own against the Shimazu, however, his lack of troops and natural resources forced him to retreat to his castle in the mountains, where he died fighting the forces of Shimazu Takahisa. He was succeeded by his son Koremasa, who became a Shimazu retainer.
Biography[]
Early life[]
The younger son of Aso Korenori, Koretoyo was brother to Aso Korenaga. As the firstborn, Korenaga was the named heir and was intended to marry the first daughter of Kikuchi Shigetomo in order to link the two clans. He renamed himself Aso Taketsune in 1502, whilst Koretoyo was made Daiguji (High-Priest) of the Aso-Kuju Temple. In 1513, Taketsune fell ill with a fever not long after the birth of Kikuchi Konomi and unfortunately for the Aso, he died before the marriage could take place. Korenori thus made Koretoyo his heir and promised him to Konomi instead. Not long after in 1516, Korenori fell from his horse whilst riding and died days later, presumably from an infection. Koretoyo set about securing his position as both head of the clan and as Daiguji. He integrated the adherents of Aso-Kuju Temple, many of whom were Sōhei (warrior monks) into the clan’s army. He also contacted a number of Yamabushi (hermit monks) whom he also, albeit with difficult, convinced to participate in his military campaigns. Koretoyo married Kikuchi Konomi sometime in the early 1520s.
Unification of Higo and the Battle of Yamae[]
With marriage securing his western flank, Koretoyo declared war on the Ōtomo to his east. In a series of skirmishes, he forced the Ōtomo out of the mountains and began construction on a number of forts stretching from Mount Taisen down almost to Mount Sobo. The Ōtomo themselves were suffering with an internal succession crisis after the early death of Ōtomo Yoshinaga, leaving the underage Yoshiaki as head of the clan. Accepting a hefty bribe from the Ōtomo, Koretoyo turned his attention elsewhere.
Whilst his marriage to Konomi left him as de facto ruler of northern Higo Province, following the death of her brother Masataka in 1509, nonetheless there were still pockets of Kikuchi loyalists throughout Higo Province. Through a swift and brutal campaign of suppression, Koretoyo subdued the samurai clans that opposed him, replacing them with those loyal to him. However, he was unable to rest for long as the Kamachi Clan to the north had attempted to use the disunity to expand their influence. Taking to the field in the summer of 1527, Koretoyo initially had difficulties due to unexpected flooding creating a rice shortage. Nonetheless, he managed to defeat Oki Masanaga, the regent for the young Kamachi Akimori, in three short engagements, first along the Kikuchi River, the second near Otsuyama and the final encounter forcing Masanaga over the Yabe River. Threatening Yanagawa Castle itself, Koretoyo was able to force Akimori and his advisors to negotiate peace, with the Kamachi surrendering authority over parts of southern Chikugo Province. On his way back to Higo, Koretoyo and his forces pillaged and burned the Renge-in Tanjō-ji temple, the site of a rival Sōhei sect.
In order to consolidate his claim on northern Higo, Koretoyo began construction of numerous forts along the Midori River, ostensibly in order to assert authority, but likely in preparation for an invasion of southern Higo. Sagara Haruhiro, head of the Sagara Clan and daimyō of southern Higo, sent a letter demanding the deconstruction of the forts, attempting to reason with Koretoyo by stating "a closed hand betrays the sword".[1] Instead, Koretoyo seized upon the opportunity, claiming that Haruhiro wished to invade northern Higo, thus he struck first. Aso forces crossed the Midorikawa in late summer, 1531. Koretoyo would swiftly besiege and capture the city of Yatsushiro before rushing south to take Hitoyoshi Castle, the seat of the Sagara, without securing the city first. However, Haruhiro had encamped near the town of Yamae with superior numbers, forcing Koretoyo to rest before continuing. When reinforcements from Yatsushiro arrived from the north and cut off his escape, Koretoyo was forced to encamp on the nearby hill, Yakushiyama.
Despite being heavily outnumbered and outmaneuvered, when offered the choice of escaping under cover of darkness, Koretoyo famously said "Escaping is admitting defeat, but not facing it. This is the greatest disgrace of all."[2] In the early hours of the morning, Koretoyo lead a surprise downhill charge against the reinforcements, the sheer ferocity of the attack catching the Sagara troops completely off guard. Their commander was killed and the remaining forces fled from the field. He then turned around and swiftly put to flight Haruhiro's regiment which had attempted to relieve his comrades. Although he was unable to capture or kill Haruhiro, the Sagara daimyō only remained in Hitoyoshi for two days before fleeing south into Satsuma Province. Koretoyo seized Hitoyoshi and named himself "Lord of Central Kyushu".
War with the Shimazu[]
Having fled south to the court of his ally, Shimazu Takahisa, Haruhiro warned the Satsuma daimyō of his dangerous northern neighbor. Takahisa had already been concerned with the aggression of Koretoyo, but he had been waiting for his war with the Ito to have been concluded before attempting any assaults. After the final defeat of the Ito at the Battle of Kunitomi, Takahisa was free to now move his forces against the Aso. He chose to first send an ultimatum to Koretoyo, in order to be perceived by neighboring daimyō as being justified, although it is unlikely to have been needed considering Koretoyo's reputation. Takahisa rounded up a large army and invaded Higo Province in summer, 1535.
Progress for the Shimazu was slow, mostly due to Takahisa awaiting the arrival of his retainer Ijuin Hidekuni along with his 3,000 men. This gave Koretoyo ample time to ready his defense. However, the Aso army was forced to garrison a number of castles and forts across Higo in order to prevent an uprising from pro-Sagara forces, leaving their total number at roughly 5,000 men. Meanwhile, even unbolstered by Hidekuni, Takahisa could field three times as many. Takahisa arrived at the Sendai River on July 1, only to discover that Koretoyo had already occupied the opposite bank and had destroyed a number of bridges further upstream. This left only a single wooden bridge further upstream or the treacherous Sogi Falls as a means of crossing. Takahisa thus set up camp and patiently awaited the arrival of Hidekuni, aware that time was on his side due to Koretoyo's dwindling supplies. Koretoyo swiftly became aware of his predicament and when scouts reported that Hidekuni was barely two days ride away, he knew he had to take action. In the early morning of 3 July 1535, Koretoyo and his men crossed the Sendai River in force and struck the unaware Shimazu army whilst it was still asleep. Having eschewed heavy armor and horses in order to maintain the element of surprise, the Aso forces were quickly able to gain a beachhead and began slaughtering Shimazu forces, but as Takahisa swiftly managed to take command and reorganize his routing troops, the battle began to turn against the Aso. Upon hearing of the arrival of Hidekuni and his men, Koretoyo gave the order for a retreat.
Despite still outnumbering the Aso, Takahisa had nonetheless sustained heavy casualties, far heavier than Koretoyo and thus was forced to retreat. The battle of the Sogi Falls is considered one of the most impressive victories from the period. However, Koretoyo was unable to maintain his momentum. Having to spend precious months subduing rebellions, he was unable to spend time recruiting, unlike Takahisa. By the spring of 1537, the Shimazu had returned, aware that Koretoyo was busy in northern Higo trying to rally up support for a southern campaign. Upon hearing of this, he sent a token force to hold Hitoyoshi Castle, led by his son Koremasa, but it was swiftly overwhelmed and destroyed, with Koremasa taken captive. Aware that he would not be able to face the Shimazu in the open field, he retreated to the mountains. Through a number of skirmishes, he managed to deal some minor victories over Takahisa's scouting forces, but was unable to push back the inexorable advance of the Shimazu. In 1538, Takahisa laid siege to Aso Castle. Despite its formidable defenses, Koretoyo had often neglected to tend to the maintenance of his farms and granaries, thus food stores were low. The remaining Aso forces held out valiantly but soon hunger and constant Shimazu assaults wore them down to just a few dozen defending the Tenshu. On 23 January 1539, Shimazu forces broke into the lower half of the Tenshu yet a heroic defense from just four Sōhei managed to hold off the attackers for nearly ten hours, including throughout the night. However, a renewed offensive on the 24th finally captured the top of the keep. Koretoyo was found dead beside his wife, having committed seppuku.
Koremasa would swear fealty to the Shimazu in return for maintaining his position as daimyō of Aso Province.