Siege of Kōzuki Castle


Mōri Clan

Harima Province

Amago Clan

Date:  4/18 to 7/3 of Tenshō 6 (1578)

Location:  Kōzuki Castle in the Sayō District of Harima Province

Synopsis:  In this period, the Amago and Oda clans shared a mutual rival in the Mōri.  The Amago revival army led by Yamanaka Yukimori sought to restore themselves as an independent clan, while Oda Nobunaga had broader aims to conquer the nation.  This made conflict with the Mōri inevitable.  The Amago sought support from Hashiba Hideyoshi to hold Kōzuki Castle as a strategic base to oppose the Mōri, but Nobunaga placed a priority on subduing the Bessho clan at Miki Castle, leaving the Amago to contend with the Mōri who forced their eventual capitulation.

Lord:  Mōri Terumoto

Commanders:  Kikkawa Motoharu, Kobayakawa Takakage, Ukita Tadaie

Forces:  30,000

Losses:  Unknown

Lord:  Amago Katsuhisa

Commanders:  Amago Katsuhisa, Amago Ujihisa, Amago Michihisa, Yamanaka Yukimori, Jizai Motomichi

Forces:  2,300 to 3,000

Losses:  The commanders took their own lives to spare the lives of the garrison

The Siege of Kōzuki Castle occurred from 4/18 to 7/3 of Tenshō 6 (1578) at Kōzuki Castle in the Sayō District of Harima Province.  The conflict was waged between the armies of Mōri Terumoto and Amago Katsuhisa.


Although small in scale, Kōzuki Castle served as a durable fortress in a strategic location on the borders of Harima, Mimasaka, and Bizen provinces.  The castle was held by Akamatsu Masanori (affiliated with the Mōri clan) and Ukita Naoie as a military base for operations in the direction of Harima.  The castle was in fact on the front lines of the eastern edge of the Mōri’s sphere of influence.  In 1577, after the Oda clan entered Harima as a step in the Invasion of Chūgoku, the castle was toppled by Hashiba Hideyoshi.  This is known as the First Siege of Kōzuki Castle.  The Amago revival army, affiliated with the Oda family and led by Yamanaka Yukimori, the loyal commander of Amago Katsuhisa, was assigned to defend Kōzuki Castle.  Thereafter, the castle was temporarily taken back in a revolt by the Ukita clan but then fell again to the Oda army.

Even after the castle was reclaimed by the Amago, Hashiba Hideyoshi struggled to pacify elements in Harima such as Bessho Nagaharu and Kodera Masamoto who ostensibly submitted to the Oda clan but whose position was in fact uncertain.  In the second or third month of 1578, after the Bessho clan defected to the Mōri, a majority of the gōzoku, or wealthy families, in eastern Harima followed their lead, whereupon 7,500 soldiers amassed at the base of the Bessho clan in Miki Castle to prepare to hold-out against a siege.  This is known as the Siege of Miki.  Meanwhile, the advance of the Hashiba army shook the foundations of the Mōri’s influence in the eastern provinces, compelling the Mōri to quickly dispatch a large army to come to the aid of the Bessho clan to secure their defense.  At the same time, it is surmised there was an intention to act in concert with an ally, Uesugi Kenshin, who planned to resume operations for a march upon the capital of Kyōto.  In any event, as of this moment, the focal point on the front lines of the conflict between the Oda and Mōri clans moved to Miki Castle while the strategic importance of Kōzuki Castle to each of the armies was nearly lost.

The Mōri would have been expected to quickly enter Harima and directly come to the aid of the Bessho clan by threatening the main division of the Hashiba army from the rear but could not ignore the demands of Ukita Naoie who provided the base of operations for the Mōri army in the area (or to secure the road of retreat in Harima if the conflict bogged down in Harima).  At the behest of Naoie, leaders in the Mōri army, Kikkawa Motoharu and Kobayakawa Takakage, aimed to reclaim Kōzuki Castle and, by this means, indirectly contain the Hashiba army.

That same month, Mōri Terumoto (the commander-in-chief) deployed from Yoshida-Kōriyama Castle, Kobayakawa Takakage (the commander for the Sanyō region) deployed from Mihara Castle, and Kikkawa Motoharu (the commander for the Sanin region) deployed from Hinoyama Castle heading toward an assault on Kōzuki Castle.  Ukita Naoie of Bizen Province did not himself deploy but sent his younger brother, Ukita Tadaie, in his place.  The combined Mōri and Murakami navies stepped up operations and implemented a blockade in the waters off of Harima in the eastern portion of the Seto Inland Sea.  A total of over 30,000 soldiers were mobilized, significantly exceeding the number of forces in the Hashiba army stationed in Harima and at Himeji Castle.

Course of events

The breakaway of Bessho Nagaharu and deployment of the main division of the Mōri army prompted Hideyoshi to urgently request reinforcements from Oda Nobunaga.  First, the army of Araki Murashige of Settsu located near Harima converged, but it was not clear where the Mōri army would appear.  During this period, Hideyoshi incessantly looked for the movements of the Mōri while, at the same time, he initiated the capture of Miki Castle and toppled an auxiliary site known as Noguchi Castle.  Meanwhile, with respect to the Mōri army, the main base of the army of Mōri Terumoto was located in Bitchū-Takamatsu Castle, far from the front lines.  The main forces led by Kikkawa Motoharu and Kobayakawa Takakage advanced to Kōzuki Castle and, on 4/18, commenced a siege.  This is known as the Second Siege of Kōzuki Castle.  The garrison defending Kōzuki Castle was comprised of 2,300 to 3,000 men under the command of Yamanaka Yukimori, Amago Ujihisa, Amago Michihisa, and Jinzai Motomichi serving Amago Katsuhisa as the commander-in-chief.

After receiving notice that the Mōri army had headed toward Kōzuki Castle, Hashiba Hideyoshi had the assault against Miki Castle continue while himself leading a battalion to Mount Takakura in support of the Amago army.  After surrounding the castle with an overwhelmingly larger army, the Mōri did not attempt to attack, and, instead, built a fortress, dug deep trenches, strung fences, and installed abatis to fully defend their positions.  For days in a row, the besieging forces trumpeted on conches and beat on war drums to intimidate the defenders.  Further, by severing the supply of provisions to the castle, the Mōri sought to defeat the will of the garrison.

The Oda army was joined by reinforcements with Oda Nobutada as the commander-in-chief along with senior retainers including Takigawa Kazumasu, Sakuma Nobumori, Niwa Nagahide, and Hosokawa Fujitaka.  Nobunaga’s objectives were to capture Miki Castle and strand the Mōri army so he put effort into toppling auxiliary castle to Miki, including Kanki Castle, Shikata Castle, and Takasago Castle.  This focus prevented Hideyoshi from giving attention to Kōzuki Castle and, without the prospect of reinforcements, the Amago army became dispirited.

Unable to just watch, on 6/16, Hideyoshi went to Kyōto to request instructions from Oda Nobunaga.  Nobunaga’s remained firm with his policy to prioritize the pacification of Harima until the end and the Amago army at Kōzuki Castle was in fact treated as a sacrificial pawn.   Having no option other than to vacate his camp on Mount Takakura, Hideyoshi sent a letter to the Amago army urging them to abandon Kōzuki Castle and flee, but the Amago disregarded their subservient relationship and chose to resist to the end.

On 6/25, Takigawa Kazumasu, Niwa Nagahide, and Akechi Mitsuhide set-up an encampment on Mount Mikazuki to prepare for the Mōri army.  The Hashiba and Araki armies moved from a position on Mount Takakura to Mount Shoshya.  At this time, the Mōri army pursued them along the Kumami River causing significant losses for the Hashiba army.  This is known as the Battle of Kumamigawa.  In view of the retreat of the Hashiba army, a retainer of the Mōri named Tamaki Yoshiyasu who participated in this battle later noted in poem that read: “The Hashiba corps standing on the summery mountain disappears in the autumn breeze.”

On 7/1, the Amago agreed to vacate the castle and surrender on the condition that the lives of the soldiers be spared.  On 7/3, Amago Katsuhisa, Amago Ujihisa, and Amago Michihisa, together with Katsuhisa’s lineal heir, Amago Toyowakamaru, all took their own lives.  Yamanaka Yukimori, a central figure in the conduct of the Amago revival army, was apprehended.  Later, while en route to Tomo in Bingo Province, he was murdered at Nariwa in Bitchū Province.  After a period of seventy days, this marked an end to the defense of Kōzuki Castle by the Amago army while the Amago clan as a daimyō family was extinguished.

Surviving remnants of the Amago revival army were led by Kamei Korenori who inherited the headship of the Kamei clan serving as the head of the chief retainers among cadet families of the Amago.  After the Honnō Temple Incident that resulted in the death of Oda Nobunaga, under the command of Hideyoshi, he served in numerous battles including for the capture of Tottori Castle and as a commander of naval forces for the Bunroku-Keichō Campaign on the Korean Peninsula.  After a transfer to Shikano in Inaba Province, he received land in Tsuwano in Iwami Province to found the Tsuwano domain with 43,000 koku which continued until the end of the Edo period.


Owing to this conflict, the Mōri entered into full-scale war against the Oda clan.  In the seventh month of 1578, Araki Murashige betrayed the Oda by aligning forces with the Ishiyama-Hongan Temple, triggering the Siege of Arioka Castle.  Next, Kodera Masamoto defected, and, for a while, the Mōri regained strength in Harima and the surrounding area.  The Mōri army, however, could not engage in offensive operations and instead focused on defensive measures in an effort to preserve their territory.  In the eleventh month, at the Second Battle of Kizugawaguchi, the Mōri navy suffered a major defeat.  In the tenth month of 1579, after the alienation of Ukita Naoie as feared by the Mōri, their sphere of influenced contracted significantly.  That same month, Araki Murashige was defeated while, in 1580, Kodera Masamoto was decimated and the Ishiyama War (which had pinned down the Oda army for many years) came to an end, whereupon the Nobunaga Encirclement Campaign collapsed.

During this period, Hideyoshi maintained a siege that severed supply lines for two years, resulting in the fall of Miki Castle.  He then organized his forces to proceed with the Invasion of Chūgoku.  The Mōri were forced to continue their battle against the Oda clan almost entirely on their own, so they searched for a path to reconciliation, but the conflict persisted until the coup d’état against Nobunaga in the sixth month of 1582.