Conquest of Aizu
Date: Sixth and seventh months of Keichō 5 (1600)
Location: Uesugi Kagekatsu was based in Aizu in Mutsu Province but Tokugawa Ieyasu returned to Ōsaka before reaching the intended destination
Outcome: After receiving news that Ishida Mitsunari had joined with other daimyō to raise arms in Ōsaka, Ieyasu decided with his commanders to return to confront Mitsunari instead of proceeding to Aizu, averting a direct clash between the Tokugawa and Uesugi armies.
The Conquest of Aizu occurred in months six and seven of Keichō 5 (1600) during which forces led by Tokugawa Ieyasu marched north with the intention of subjugating Uesugi Kagekatsu, a daimyō based in Aizu in Mutsu Province. This conflict is also known as the Conquest of the Uesugi or the Invasion of Aizu. This was the opening chapter in a series of events culminating in the Battle of Sekigahara. However, owing to a threat raised by Ishida Mitsunari in Ōsaka while Ieyasu was en route toward Aizu, Ieyasu made a crucial decision at the Oyama Deliberation to turn back toward Ōsaka, averting a direct conflict between the Tokugawa and Uesugi armies.
Prelude to the battle
Toyotomi Hideyoshi died on 8/18 of Keichō 3 (1598). Prior to his demise, Hideyoshi had entrusted the conduct of political affairs to Tokugawa Ieyasu during the period that Hideyoshi’s eldest son, Toyotomi Hideyori, matured. On 10/25 of 1598, via the Five Elders and Five Commissioners, Tokugawa issued orders for the return of all Japanese soldiers from Korea, which was carried out over the ensuing weeks. Ieyasu then visited the residences of a series of influential daimyō, including Chōsokabe Motochika, Shinjō Naoyori, Shimazu Yoshihisa, Hosokawa Yūsai, and Mashita Nagamori. On 1/3 of 1599, Ieyasu visited Shimazu Yoshihiro (the younger brother of Yoshihisa) and Shimazu Tadatsune (the third son of Yoshihiro).
Ieyasu conducted these visits without notification to the Five Elders and the Five Commissioners, while one of the laws of the Toyotomi administration was not to create cabals among colleagues. Moreover, Ieyasu arranged political marriages of relatives with the families of assorted daimyō without the input of other elders or commissioners. These included: (i) Matsudaira Tadateru (his sixth son) and Irohahime (the daughter of Date Masamune), (ii) Matehime (the daughter of Matsudaira Yasumoto) and Fukushima Masayuki (the adopted son of Fukushima Masanori), (iii) Kyōdaiin (the daughter of Ogasawara Hidemasa) and Hachisuka Yoshishige (the eldest son of Hachisuka Iemasa), (iv) Kana (the daughter of Mizuno Tadashige) and Katō Kiyomasa, and (v) Eihime (daughter of Hoshina Masanao) and Kuroda Nagamasa.
These acts violated one of the laws of the Toyotomi administration prohibiting marital alliances with assorted daimyō so, on 1/19 of 1599, three elders (Ikoma Chikamasa, Nakamura Kazuuji, and Horio Yoshiharu) were dispatched from the Toyotomi administration to investigate the unauthorized marriages. Ieyasu responded to the questioning and, soon thereafter, settled the matter on 2/2 through the exchange of a written pledge with Maeda Toshiie. Toshiie, however, died on 3/3. That same day, seven senior commanders (led by Katō Kiyomasa and Fukushima Masanori) who for some time had been in conflict with Ishida Mitsunari launched an attack against Mitsunari’s residence in Ōsaka. Mitsunari had forewarning of the attack, and, with the aid of Satake Yoshinobu, fled to Yoshinobu’s home on the grounds of Fushimi Castle. Thereafter, Ieyasu mediated between the parties, and, as a result, Mitsunari was removed from his role as one of the Five Commissioners and was confined to Sawayama Castle.
On 9/7, Ieyasu went to Ōsaka and stayed at the residence of Mitsunari. On 9/9, he visited Ōsaka Castle to offer good wishes to mark the change of seasons and on 9/12 stayed at the home of Ishida Masazumi (the older brother of Mitsunari). On 10/2, Ieyasu punished Asano Nagamasa, Hijikata Katsuhisa, and Ōno Harunaga for plotting his assassination. Each one of the them was banished – Nagamasa to Fuchū in Kai Province, Harunaga to Shimōsa, and Katsuhisa to Hitachi. Moreover, a rumor circulated of a conquest of Kaga owing to involvement by Maeda Toshinaga in the plot, so Toshinaga sent his natural mother, Hōshunin, as a hostage to Edo.
Thereafter, Ieyasu allocated the territory under the direct jurisdiction of the Toyotomi clan to assorted daimyō. This included increases of: (i) 50,000 koku in Fuchū in Echizen to Horio Yoshiharu of Hamamatsu in Tōtōmi, (ii) 60,000 koku in Kitsuki in Bungo to Hosokawa Tadaoki of Miyazu in Tango, (iii) 137,000 koku in Nakashima in Shinano to Mori Tadamasa of Kanayama in Mino, and (iv) 10,000 koku to Sō Yoshitoshi of Fuchū in Tsushima.
Origin of the Conquest
As the political influence of Ieyasu increased, one of the Five Elders named Uesugi Kagekatsu ordered his retainer, Naoe Kanetsugu, to strengthen his military capabilities including by the construction of Kōzashi Castle. Neighboring daimyō such as Mogami Yoshiaki and Hori Hideharu informed Ieyasu of these actions by Kagekatsu.
On 3/11 of 1600, Fujita Nobuyoshi, a retainer of Kagekatsu who had been working to restore relations between Kagekatsu and Ieyasu, was ousted from the Uesugi family. On 4/1, Ieyasu then dispatched Ina Akitsuna and Kawamura Nagato (a retainer of Mashita Nagamori) to investigate. At this time, Ieyasu sent a letter of censure through a priest who also served as a diplomat named Saishō Jōtai. The letter criticized the military build-up by Kagekatsu, and further stated that if Kagekatsu did not have treacherous intentions, then he should issue a written pledge and come to Kyōto to explain his position.
Kanetsugu responded by sending a document known as the Naoe Statement to Ieyasu. In this reply, Naoe indicated that to address the reasons for Ieyasu’s distrust based on various rumors would be very important, but this depended upon Ieyasu and his son, Tokugawa Hidetada, leaving the capital to meet him. Ieyasu received this communication on 5/3, and on that same day, he decided to subjugate Kagekatsu. He appointed Fukushima Masanori, Hosokawa Tadaoki, and Katō Yoshiakira to lead the vanguard forces to conquer the Uesugi in Aizu. He further ordered a retainer, Torii Mototada, to remain behind to protect Fushimi Castle.
Upon hearing the decision to proceed with the conquest, retainers including Maeda Geni and Natsuka Masaie appealed to Ieyasu to suspend the plan, but he did not accept their plea. Ishida Mitsunari refused a request from Ieyasu to sojourn at his residence. There is a theory that Mitsunari was preparing to raise arms, and another theory that casts doubt he went that far and instead was seeking a suspension of the campaign.
On 6/2 of 1600, orders were given to regional daimyō in the Kantō to prepare for battle. On 6/6, Ieyasu held a military council in the west quarters of Ōsaka Castle in regard to the planned conquest. On 6/15, Amano Yasukage and Sano Tsunamasa were assigned to the west quarters of Ōsaka Castle during the campaign. Ieyasu assembled a contingent of 55,000 soldiers. On 6/16, Ieyasu deployed from Ōsaka Castle. En route to Edo Castle in the Kantō, Ieyasu departed from Fushimi Castle on 6/18, and from 6/23 to 6/28 stayed ay Hamamatsu, Shimada, Sunpu, Mishima, Odawara, and Fujisawa. On 6/29, Ieyasu visited the Tsuruoka-Hachiman Shrine to pray for victory in battle. On 7/2 he arrived at Edo Castle.
After Ieyasu departed the Kinai region to lead the expedition to Aizu, Ukita Hideie held a deployment ceremony on 7/2, and, on 7/17, Ishida Mitsunari summoned Ōtani Yoshitsugu and Mōri Motonari to launch a rebellion. Ieyasu was in Edo Castle in the Kantō at this time. On 7/19, Ieyasu dispatched forces led by Hidetada as the commanding general toward Aizu, while, on 7/21, Ieyasu himself departed with additional forces from Edo Castle to head toward Aizu. On 7/24, after Ieyasu arrived in Oyama in Shimotsuke Province, an urgent message arrived from Torii Mototada to inform him that Mitsunari had rebelled. Upon hearing the news, Ieyasu immediately suspended the advance and held a military council in Oyama to assess the options with his commanders, an event known as the Oyama Deliberation. As an outcome of this meeting, Ieyasu decided to have forces under Yūki Hideyasu remain to apply pressure against the Uesugi, while Ieyasu would turn around his forces and head west to eliminate the threat from Mitsunari. Meanwhile, changed course and, to address his own anxiety, proceeded to attack Mogami Yoshiaki in Dewa Province. This was known as the Battle of Dewa in the Keichō Era. Consequently, the Tokugawa and Uesugi armies avoided a direct confrontation.
The raising of arms by the Uesugi family could not overcome the inclination of sengoku daimyō to govern their territories together with the residents of those lands. The Uesugi focused on managing their new territory of Aizu, neglecting their response to the new system of governance under Ieyasu. This misreading of the changing political situation affected the prioritization of political policies, serving as a factor in triggering the Conquest of Aizu.