The Amago Clan in the Sengoku Period: A Private Historical Study ⑨
The Battle of Yoshida-Koriyama Castle
The large-scale expedition launched by Lord Amago Haruhisa immediately after he succeeded the head of the family and returned from the Harima expedition,
the "Battle of Yoshida-Koriyama Castle,"
which took place from 1540 to the following year,
staged in Yoshida, Takata District, Aki Province—present-day Akitakata City, Hiroshima Prefecture—
can be considered a historical watershed moment that drastically altered the political structure of the entire western region during this era.
The Amago clan, which had gained the power to carry out large-scale expeditions to other provinces from their base in Izumo,
the Ouchi clan, the giants of the western region based in Suo and Nagato,
and the Mori clan, which was seeking to transform from local Aki lords into Sengoku daimyo.
This clash, where these three different political powers intersected,
contained a profound conflict of strategies that cannot be fully explained
merely by the simple heroic historical schemas found in military chronicles like the "Intoku Taiheiki,"
such as "a reckless expedition due to Amago Haruhisa's youth"
or "a miraculous victory by the rare strategist Mori Motonari."
This time, I will introduce matters centered on this "Battle of Yoshida-Koriyama Castle."
Primary historical sources such as the "Mori Family Documents" and the "Mori Motonari Koriyama Siege Diary"
record the actual circumstances of this battle in detail.
In 1537,
Lord Haruhisa (then known as Haruhisa/Akiyoshi), who had inherited the family headship from his grandfather, the rare hero Lord Tsunehisa,
faced a typical "crisis of succession from a founding power."
In particular, the situation in Aki Province around 1540
presented a critical situation that the Amago clan could not overlook.
At the center of this
was the movement of the Aki-Takeda clan, the key to the "anti-Ouchi, anti-Mori" faction in Aki Province.
The Aki-Takeda clan shared the same roots as the famous Koshu-Takeda clan (such as Shingen),
was a family line of Aki provincial governors dating back to the Kamakura period,
and was a prestigious family that held authority as "Aki half-province governors"
based at Sato-Kanayama Castle (Kanayama Castle, present-day Hiroshima City).
The fact that they opposed the expansion of the Ouchi clan's power
and clearly declared themselves on the Amago side
meant for the Amago clan that they had obtained a "just cause" for intervention in Aki
and a "bridgehead" to reach southern Aki,
which became a factor in increasing the Amago clan's influence in the region at the time.
However, this system was greatly shaken.
On June 9, 1540, the head of the Takeda clan, Mitsukazu, died suddenly.
At the time, the Takeda clan was already under intense pressure from the main forces of the Ouchi side
and the emerging power Mori Motonari,
and was on the verge of destruction.
Mitsukazu's death paralyzed the command structure of Kanayama Castle
and was a situation that could lead to the collapse of the anti-Ouchi coalition in Aki.
Lord Haruhisa's subsequent expedition to Aki
reveals the aspect of a "decisive battle born of necessity"
to rescue the allied Takeda clan, which was on the verge of collapse.
Initially, Lord Haruhisa was conducting an expedition to Harima,
but faced with the crisis of the disappearance of his base in Aki,
he was forced to make the grave decision to withdraw from Harima and advance into Aki.
This can be interpreted as a choice of a "decisive battle"
based on extremely rational and urgent geopolitical requirements: to capture Yoshida-Koriyama Castle, the front line of the buffer zone with the Ouchi clan,
and to eliminate the pressure on the Takeda clan before annihilating the enemy's main force.
Upon invading, the Amago forces initially
attempted to enter from the Bingo road (Kōtachi direction), which was the shortest geographically.
However, this route was not realized due to the resistance of the Shishido clan,
who were based at Goryu Castle (present-day Kōda-chō, Akitakata City).
Although Shishido Motoyoshi and his son Takaie had previously been in fierce conflict with the Mori clan,
by this time, they had formed a strong marital relationship
by taking Mori Motonari's daughter (Goryu-no-kata) as a legal wife.
Because the Shishido clan physically blocked the Amago forces' entry via the Bingo road,
Lord Haruhisa was forced to detour to the "Iwami road," which passed through Iwami Province and crossed the Akana Pass.
This change in route led to the lengthening of the supply lines.
Furthermore, Lord Haruhisa incorporated the Yamauchi clan (Yamauchi Naomichi), a powerful local lord in northern Bingo, as the vanguard of the Amago forces.
While military chronicles claim the Amago forces numbered "30,000 cavalry," considering the stone yield and mobilization limits of Izumo, Iwami, and Bingo at the time, as well as the scale feasible for marching through mountainous terrain, it is estimated that the actual number was around 10,000 to 10,000-plus.
Although the mobilization of local lords like the Yamauchi clan contributed to increasing troop numbers, they did not necessarily possess high loyalty, making them a "double-edged sword" that carried the risks of increased supply consumption during long-term warfare and defection if the war situation worsened.
Upon entering Aki, the Amago forces advanced to Aoyama and Mitsuiyama, placing Yoshida-Koriyama Castle directly before them. The "Aoyama-Mitsuiyama Amago encampment," constructed at a position overlooking Yoshida-Koriyama Castle, has had its unique characteristics revealed by recent archaeological excavations.
The Aoyama Castle ruins (370m above sea level) and Mitsuiyama Castle ruins (367m above sea level) featured deep dry moats, earthen ramparts, and orderly arranged enclosures, possessing a permanent castle structure that went beyond a mere field encampment.
This suggests that the Amago clan, looking ahead to the governance of Aki after the capture of Koriyama Castle, intended for this site to function as a new base for their Aki strategy.
However, this encirclement strategy did not function perfectly. One of the factors was the movement of Kumagai Nobunao, a former retainer of the Aki-Takeda clan who was based at Takamatsu Castle (present-day Asakita-ku, Hiroshima City).
The Kumagai clan had previously turned to the Mori side due to conflicts with their former lord, the Takeda clan.
The Amago clan attempted to win back the Kumagai clan by utilizing the authority of their former lord, the Takeda clan, but Nobunao rejected this and remained firmly on the Mori side.
The failure to break down the Kumagai clan, following the failure with the Shishido clan, forced the Amago forces to conduct operations while leaving hostile forces at their rear (Goryu Castle) and flank (Takamatsu Castle), which became a decisive factor in hindering the completion of the encirclement network.
As the war situation reached a stalemate, the key to victory shifted to logistics.
The Amago forces had established a supply depot at "Miyazaki-Nagao," but Mori Motonari repeatedly launched guerrilla-style sorties, threatening this supply line.
According to records such as the "Mori Motonari Koriyama Siege Diary," fierce fighting took place around Miyazaki-Nagao from December of the ninth year of Tenbun to January of the following year. While military chronicles emphasize "self-destruction due to heavy snow," the reality that can be read from primary sources was a planned supply base destruction operation by the Mori army and the Ouchi clan reinforcements (said to be 10,000 led by Sue Takafusa) that arrived late.
The Amago army, having had their supplies cut off, rapidly lost their ability to continue fighting, compounded by the cold of the severe winter.
Then, on January 13th of the tenth year of Tenbun (1541), a general attack was carried out by the combined Ouchi and Mori forces.
In this battle, Lord Amago Hisayuki (younger brother of Lord Tsunehisa), an elder of the Amago family who held a central role, was killed in action.
Although he is depicted in military chronicles as a hero who scattered brilliantly while self-deprecatingly calling himself "Cowardly Yashu," the death of Lord Hisayuki is also recorded as a fact in primary historical sources, and this is considered to have been the direct trigger for the collapse of the entire Amago command structure and the disintegration of their battle lines.
Lord Haruhisa decided to withdraw the entire army. However, retreating with a force of over 10,000 that had penetrated deep into enemy territory, while under pursuit and through snow-covered mountainous terrain, was a military operation even more difficult than the attack itself.
What should be noted here is
the logistics management during the 'retreat battle' of the fleeing Amago forces.
The retreat route chosen by the Amago forces was
to head north from Yoshida, pass through Tajihi and Kuchiba, and
cross the 'Go-no-kawa River,' a major river in Iwami Province.
The most difficult point on this route, the river crossing site,
in what is now Misato Town, Ochi District, Shimane Prefecture (formerly Yamato Village),
has a massive ruin that still exists today
known as the 'Amago Encampment Site (Site Number: Yamato 60).'
Located on the ridge on the left bank of the Go-no-kawa River in Tuga-nishi, Misato Town, this site
stretches for approximately 900 meters and
possesses a unique structure consisting of numerous
flattened enclosures (kuruwa) and earthen steps along the ridge.
Excavations in the 1990s unearthed post holes and traces of daily life (gravel, old coins, iron fragments),
and based on its structural characteristics, it is estimated to have served the following functions.
First, it functioned as a 'base for accommodating and reorganizing a large army.'
The structure, which lacks the defense-oriented trenches seen in typical mountain castles
and instead features wide, flattened areas emphasizing habitability,
is thought to have served as a base to temporarily house and
reorganize the various units that had become disorganized during their retreat.
Second, it functioned as a 'bridgehead for river-crossing operations.'
The encampment is situated overlooking the river-crossing point of the Go-no-kawa.
During the retreat, the rearguard likely formed a defensive line here,
and while they kept the pursuing Mori and Ouchi forces in check,
the main force likely crossed the Go-no-kawa below using pontoon bridges or requisitioned boats,
serving a sophisticated river-crossing support function.
Supporting this large-scale retreat operation were
the Iwami Ogasawara clan (Ogasawara Nagataka, lord of Nukuyu Castle), who controlled the Go-no-kawa basin, and
the Akana clan (Akana Mitsukiyo, lord of Setoyama Castle), who guarded the Izumo border.
In particular, Ogasawara Nagataka, despite the sacrifice of losing his heir, Nagaharu, in this campaign,
provided the water transport capabilities of the Go-no-kawa to support the Amago forces' crossing.
Furthermore, with the Sengai-san Castle on the opposite bank serving as a surveillance base,
the Amago forces were able to ensure their safety during the most vulnerable moment of the crossing.
After crossing the river, the Amago forces crossed the Akana Pass, guarded by the Akana clan, and returned to Izumo Province.
Akana Setoyama Castle served as the final line of defense against the pursuing army
and supported the return of the Amago forces.
Although the retreat was successful, the political losses in Aki Province were devastating.
The Takeda clan of Sato-Ginzan Castle, left isolated by the Amago retreat,
was conquered by Mori Motonari in March of the same year,
marking the end of the Aki Takeda clan, a prestigious family dating back to the Kamakura period.
This meant that the Amago clan had lost their base for intervention in Aki Province,
and the local lords (kokujin) of Aki and Bingo provinces all shifted their allegiance to the Ouchi and Mori camp.
