Siege of Oshi Castle
Date: 6/16 to 7/16 of Tenshō 18 (1590)
Location: The environs of Oshi Castle in the Saitama District of Musashi Province
Outcome: The Narita clan defended an assault by Toyotomi forces against the castle, but ultimately surrendered after the castle was intentionally flooded with the construction of a levee that captured water from the Tone River.
Commanders: Ishida Mitsunari, Ōtani Yoshitsugu, Natsuka Masaie, Sanada Masayuki, Naoe Kanetsugu, Satake Yoshinobu, Utsunomiya Kunitsuna, Hōjō Ujikatsu, Tagaya Shigetsune, Asano Nagamasa, Suzuki Shigetomo, Mizunoya Katsuotoshi
Forces: 20,000 to 50,000
Commanders: Narita Yasusue, Narita Nagachika, Zenshōji Kōyōsai, Kaihime, Honjō Yasunobu, Imamura Sado-no-kami, Shimada Dewa-no-kami, Masaki Toshihide, Sakamaki Yukie, Shibasaki Atsuhide, Beppu Akikiyo, Toshima Yorishige
Forces: 2,000 to 2,100
The Siege of Oshi Castle occurred from 6/16 to 7/16 of Tenshō 18 (1590). This conflict was triggered in regard to the main base of the Narita clan at Oshi Castle in the Saitama District of Musashi Province. Oshi Castle was built around 1478 after Narita Shōtō and Narita Akiyasu (father and son), a gōzoku, or local family of means and influence, eliminated the Oshi clan (members of the Ōgigayatsu-Uesugi family who controlled the area). The following year, the Ōgigayatsu-Uesugi responded by attacking Oshi Castle, but the warring parties settled through mediation by Ōta Dōkan, the head of house affairs of the Ōgigayatsu-Uesugi, and, thereafter, the Narita clan held the territory. After the Nighttime Battle of Kawagoe, the Hōjō clan expanded into the Kantō but the Narita resisted their encroachment.
Course of events
As the multi-generational base of the Narita clan, Oshi Castle was protected by the Motoara and Hoshi rivers that served as natural moats. The site was counted as one of seven strongholds in the Kantō. In the invasions of Shikoku and Kyūshū, Toyotomi Hideyoshi brought the Chōsokabe and Shimazu clans under his command. As he proceeded in his bid to conquer the country, he then turned his attention to the Gohōjō who controlled expansive territories in the Kantō Plains. Hideyoshi sought support via Tokugawa Ieyasu from Hōjō Ujimasa to march upon the capital, but Ujimasa refused, whereupon Hideyoshi decided to attack Odawara. Upon hearing of these plans, Narita Ujinaga and Narita Yasuchika holed-up in Odawara Castle, while Narita Yasusue, Narita Nagachika, and Kaihime holed-up in Oshi Castle.
Prior to the attack by flooding
To attack Tatebayashi and Oshi castles, around 6/5, the Toyotomi army sent Ishida Mitsunari, Ōtani Yoshitsugu, and Natsuka Masaie. On 6/4, Mitsunari moved from Tatebayashi to Oshi, established a base near the Ōmiya entrance to the castle, and initiated an attack, but was met with stiff resistance as the defenders stood their ground. Around 6/8, northern forces including Maeda Toshiie, Uesugi Kagekatsu, and Sanada Masayuki converged with Asano Nagamasa and the Asano division, including Kimura Shigekore and Tokugawa forces, to lead an assault on the castle, but the surrounding swamps and rivers serveed as natural barriers to potential attackers so the Toyotomi army paused. On 6/12, Hideyoshi ordered Ishida Mitsunari to attack the castle by means of flooding. On 6/13, Mitsunari left the northern forces and Asano division to attack Hachigata Castle, relying upon Nagamasa and Shigekore to lead the attack on Oshi Castle.
On 6/14, the defenders at Hachigata Castle surrendered and vacated the site, after which Asano Nagamasa and Sanada Masayuki re-joined the forces laying siege to Oshi Castle. On 6/17, Mitsunari set-up a base on the ancient burial mounds on Mount Maruhaka while forces commanded by Ōtani Yoshitsugu, Natsuka Masaie, Naoe Kanetsugu, Satake Yoshinobu, and Utsunomiya Kunitsuna laid siege to the castle.
Attack by flooding
Ishida Mitsunari’s assault on the castle was not making progress, so he climbed a small mountain nearby to gain a bird’s eye view of the situation> Based on prior experiences at the Battle of Bitchū-Takamatsu Castle, he considered the possibility of an attack by flooding. However, in a letter dated 6/12 to Hideyoshi, Mitsunari expressed pessimism about this method of attack and, instead, recommended a more aggressive assault. Hideyoshi replied with detailed instructions for an attack by flooding. Accordingly, based on historical accounts from the same period, Hideyoshi, rather than Mitsunari, favored an attack by flooding. Hideyoshi did not aim for a conflict of annihilation and Mitsunari simply carried out the orders to attack by flooding.
Based on a letter dated 6/13 from Mitsunari to Asano Nagamasa and Kimura Shigekore, Mitsunari frequently depended upon the instructions of Nagamasa with respect to specific military tactics. To the south of the castle, Mitsunari proceeded to construct a levee in the shape of a semi-circle. He engaged local peasants to work around the clock on the construction, paying them one shō (the equivalent of 1800 cc) of rice and coins with a value of 60 mon (one-thousandth of a kan) for the day shift and one shō of rice and coins with a value of 100 mon for the night shift. The levee was completed over a short period of four or five days. The embankment referred to as the “Ishida Levee” extended for twenty-eight kilometers, enabling an attack by flooding with the water from the Tone River. However, contrary to expectations, the main citadel was not flooded, and appeared to be floating, giving rise to the name of the Floating Castle of Oshi.
On 6/18, owing to a continuing downpour, the flooding approached the main citadel. In an effort to prevent the flooding, Honjō Yasunobu (who was in charge of the defending the rear gate) then dispatched those under his command including Wakimoto Risuke and Sakamoto Hyōe to destroy the levee. The two of them slipped out of the castle at night and destroyed the levee in two locations. This caused the water accumulated from the downpour to rush out, killing approximately 270 of the Toyotomi forces. As a result, the area around Oshi Castle became like a swamp so that even the horses could not gain their footing.
Arrival of reinforcements
At the beginning of the seventh month, Asano Nagamasa, and around 7/6, Uesugi Kagekatsu and Maeda Toshiie, joined those assaulting the castle, but it still did not fall. Nagamasa, rather than Ishida MItsunari, took the lead in ending the campaign as well as the disposition of affairs after the conflict.
Vacating the castle
On 7/5, the defenders of Odawara Castle surrendered and vacated the site, the Gohōjō clan was eliminated, and other outlying castles aligned with the Hōjō fell. Only Oshi Castle remained. Narita Ujinaga responded to a demand from Hideyoshi to have his garrison surrender and, finally, on 7/16, Oshi Castle was vacated. Military chronicles pay attention to Mitsunari’s construction of levees, referring to the siege as the “Ishida Levee” which is regarded as a classic representation of a castle conflict.
Details of participants
Following the sudden death of Narita Yasusue, Narita Nagachika became the commander-in-chief. After the Conquest of Odawara, Nagachika joined Ujinaga to temporarily rely upon Gamō Ujisata of Aizu and then moved to Karasuyama in Shimotsuke Province, but after Nagachika and Ujinaga came into conflict, Nagachika fled and entered the priesthood under the name of Jieisai. In later years, he resided in Owari Province.
Kaihime sewed her own body armor and engaged in the battle. After Oshi Castle was turned over, she became a consort of Toyotomi Hideyoshi, but her whereabouts after the death of Hideyoshi are unknown.
Narita Ujinaga, who holed-up in Odawara Castle and demanded 900 gold taels and 18 helmet decorations, faced the despair of having his territory seized. Thereafter, the castle was assigned to Gamō Ujisato and, in the Edo period, by Tokugawa Ieyasu to the Karasuyama domain. Internal conflicts, however, led to the end of this domain in the generation of their grandchildren.
Owing to his failed assault on Oshi Castle, Ishida Mitsunari was branded as being an ineffective commander and his reputation declined. However, based on orders from Hideyoshi who did not know that the topography of the site caused the assault to fail, the majority of daimyō who participated on behalf of the Toyotomi joined the Western Army at the Battle of Sekigahara. After the death of Hideyoshi, Mitsunari wielded authority as a member of the gobugyō, or Five Commissioners, was defeated by Tokugawa Ieyasu at the Battle of Sekigahara and executed.
After the battle, Sakamaki Yukie settled in the Kamite-Kobayashi area of Musashi, while Shibasaki Atsuhide (Izumi-no-kami) remained in the area and his descendants resided in Gyōda. Meanwhile, Masaki Toshihide sent his wife and children to Aizu while he entered the priesthood, remaining in Gyōda. He founded the Kōgen Temple but died the following year.