The Sekigahara Campaign was a series of battles in Japan fought between the Eastern Army aligned with Tokugawa Ieyasu and the Western Army loyal to Ishida Mitsunari, culminating in the decisive Battle of Sekigahara. The conflict was sparked by a punitive expedition led by Ieyasu against the Uesugi clan in the northeastern Tōhoku region, providing Mitsunari with an opportunity to denounce Ieyasu in the name of the infant ruling taikō Toyotomi Hideyori while the Tokugawa troops were in the field.
Much of the campaign consisted of a struggle to control key castles on the Tōkaidō and the Nakasendō, the main roads linking Edo and the capital of Kyoto. However, battles and sieges far from these key highways, both in the Tōhoku and in pockets of resistance around the capital, had wide-reaching effects on the manoeuvring and availability of troops for the decisive battle at Sekigahara. The campaign also spilled over briefly into the southern island of Kyūshū, but Ieyasu quickly ordered his forces to stand down following his victory for political reasons.
The campaign dramatically changed the political landscape of Japan, resulting in the ascendancy of the Tokugawa Shogunate over the Toyotomi clan and the shifting of political power between the various daimyō who participated in it.
Background
With the death of Toyotomi Hideyoshi in 1598, his 5 year-old son Toyotomi Hideyori succeeded him formally as the taikō or imperial regent, and rule of Japan fell under a Council of Five Elders appointed by Hideyoshi as regents for Hideyori until he came of age.
Tokugawa Ieyasu, a member of the council, installed himself in Hideyoshi's former residence of Fushimi Castle and arranged political marriages for his children in defiance of the council's restriction on doing so, drawing political ire. Ishida Mitsunari, one of the Five Commissioners appointed to civil administration by Hideyoshi, attempted to approach Maeda Toshiie, a councilman and guardian of Hideyori, at Ōsaka Castle to conspire against Ieyasu; however, the intervention of Hosokawa Tadaoki, a friend of Ieyasu and of the Maeda clan, convinced Toshiie's son Toshinaga to dissuade Toshiie from opposing Ieyasu.
Mitsunari conspired in an assassination plot on Ieyasu, with conflicting narratives on how the plot occurred. By one account, after the plot's failure, several of Ieyasu's generals independently attempted to eliminate Mitsunari. Mitsunari infamously escaped the castle at night disguised as a woman and fled to Fushimi Castle to beg Ieyasu to protect him from retaliation. Ieyasu agreed on the condition that Mitsunari go into exile in his holding at Sawayama Castle in the spring of 1599. Soon after, Maeda Toshiie died in April, and Toshinaga was elected to replace him; however, Ieyasu himself took up guardianship of Hideyori, occupying Ōsaka Castle on 28 October and further enraging the commissioners and council. According to another account, the plot failed when, on 7 September 1599, Ieyasu traveled from Fushimi to Osaka Castle to visit Hideyori and Yodo-dono, mother and daughter, to celebrate the Double Ninth Festival. During this visit, he received a tip-off from Masuda Nagamori, one of the five magistrates, and the plot to assassinate Ieyasu was uncovered. Another collaborator of this plot was revealed to be Maeda Toshinaga, one of the Five Elders, and his accomplices were Hijikata Yuhisa, Ono Harunaga, and Asano Nagamasa, one of the Five Commissioners.Those suspected of being the masterminds of the assassination plot were subsequently severely investigated.
Meanwhile, Tokugawa loyalist Ii Naomasa undertook political initiatives as he formed alliance with Kuroda Yoshitaka and Kuroda Nagamasa. through the Kuroda clan, Naomasa successfully swayed the other military commanders to support the Tokugawa clan. Aside from the Kuroda clan, according to a letter of Naomasa which is preserved in modern-day Hikone Castle Museum, Naomasa also engaged in correspondence with Sanada Nobuyuki to gain his support for the Tokugawa clan in response to a predicted incoming conflict between Ieyasu and his political enemies.
Aizu campaign
Uesugi Kagekatsu, another councilman, defied Ieyasu by militarising, ordering 80,000 men to construct a castle in Aizu and expand its fortifications and road networks. On 7 May 1600, Ieyasu dispatched a letter to Kagekatsu demanding an explanation, Kagekatsu's chief advisor, Naoe Kanetsugu responded with mocking refusal. Subsequent negotiations broke down, with Kagekatsu even attempting to have one of Ieyasu's envoys killed as a spy, and after Kagekatsu refused to present in person at Ōsaka to account for his actions, Ieyasu summoned a war council on 12 July to plan a punitive expedition, referred to as the "Oyama Council", though some Japanese historians dispute the evidence that this council took place. Regardless, Ieyasu gathered various daimyo lords excluding those from western region to discuss the plan to punish the Uesugi clan.
However, when Ieyasu discovered that Katō Kiyomasa was involved in rebellion of Ijuin Tadamasa against his lord, the Shimazu clan, Ieyasu forbade Kiyomasa from taking part in this campaign. This resulted in Kiyomasa later fighting in the Kyushu campaign against the loyalists of Mitsunari, far from the main battle of Sekigahara.
On 24 July, Ieyasu departed from Ōsaka Castle for his punitive campaign, stopping at Fushimi Castle the next night to give his retainer Torii Mototada instructions against a likely Ishida-backed assault before continuing towards Aizu. Ieyasu was wary of a sudden attack by Mitsunari and his allies, and so he marched at a slow pace to monitor his rivals' movements, arriving in Edo on 10 August where he began mustering troops. Date Masamune and his uncle Mogami Yoshiaki, rulers of Sendai and Yamagata respectively on the border with the Uesugi clan's territory, returned swiftly to their home provinces to defend against Kagekatsu. Mitsunari immediately began summoning conspirators to Sawayama Castle following Ieyasu's departure, some even being convinced to abandon their journeys to join Ieyasu's campaign such as Ōtani Yoshitsugu. On 17 August Mitsunari's war council resolved to form the Western Army as a coalition against Ieyasu, with councilman Mōri Terumoto as its nominal commander-in-chief. According to the letters from Mitsunari to Kagekatsu, it was recorded that Mitsunari also sought cooperation with him to plan a joint attack against Ieyasu's army. However, due to lack of coordination, and alleged mistrust from Kagekatsu, in the end he did not move his troops to pursue Ieyasu's army which were on their way returning to Kyoto as Mitsunari intended.
On 22 August 1600, the coalition entered Ōsaka Castle and proclaimed the creation of the Western Army, claiming guardianship of Hideyori. The same day, Mitsunari issued a series of 13 charges against Ieyasu on behalf of the Five Commissioners and three members of the Council of Five Regents. This proclamation effectively marked the declaration of war between the Western and Eastern armies.
Chūbu-Kansai-Kantō theatre
Engagements around Kyoto
The first major assault of the Western Army in the region of the capital was the siege of Fushimi Castle commencing 27 August. Torii Mototada was resolved to defend the garrison to the last, and despite setbacks the defense held until 6 September when Mototada committed seppuku with his remaining forces, majorly setting back Mitsunari's attempts to support his castles along the Nakasendō road. Nine days later, Mitsunari entered Ōgaki Castle uncontested.
Yamaoka Kagetomo, lord of Kōka samurai clan and ally to Tokugawa Ieyasu, already sent his brother, Kageuchi, to Fushimi Castle, before it was being besieged by the Western Army. Upon learning that the Eastern Army had won the main battle of Sekigahara, Kagetomo headed for Sekigahara from Nagashima in Ise, but encountered Natsuka Masaie's forces along the way. Kagetomo then joined the ither Eastern Army generals Ikeda Nagayoshi and Kamei Tsunenori pursuing the fleeing Natsuka's force. They managed to capture the Minakuchi Castle in Ōmi Province, where the Natsuka clan had taken refuge. After that, Kagetomo telling his allies Kuki Moritaka, Ikeda Nagayuki, and Terazawa Hirotaka that they should also capture Kuwana Castle in Ise Province. The cadlte were defended by two brothers Ujiie Yukihiro and Ujiie Yukitsugu, along with Teranishi Naotsugu. However, as Kagetomo and others arrived, the castle immediately surrendered. Furthermore, this Eastern Army contingent advanced further to force surrenders of Takigawa Katsutoshi in Kanbe Castle and Okamoto Munenori in Kameyama Castle. At the end of thus operation, Ieyasu gave permission to Kagetomo to execute Natsuka Genshun, brother of Masaie who was captured during the capitulation of Minakuchi Castle. Kagetomo did so as vengeance for the Kōka samurai whose wives and children had been crucified by Genshun at Fushimi castle. For his service during the war, Ieyasu also awarded Doami 9,000 koku in Omi Province and the command of 10 cavalrymen and 100 samurai from Kōka.
Though the Western Army had attempted to win over the loyalty of Hosokawa Fujitaka, the father of the Eastern Army general Hosokawa Tadaoki, Mitsunari had alienated Fujitaka during the period following Ieyasu's departure from Ōsaka through an abortive hostage-taking ploy resulting in the death of Tadaoki's wife. Fujitaka had instead fortified Tanabe in Tango Province with 500 men in support of Ieyasu. In mid-August, even prior to the official denouncement of Ieyasu, 15,000 Ishida troops under Onoki Shigekatsu had begun a siege of the fortress. However, Fujitaka's influence and prestige as a scholar-poet was so great that the sympathetic Ishida troops gave a weak offense, such as "forgetting" to load cannons before firing on the keep. The siege only ended 19 October following an order by the emperor to surrender to preserve his life and his precious library, following which the Western Army facilitated his retirement to Kyoto.
Ōtsu Castle on the southwest shore of Lake Biwa, garrisoned by Kyōgoku Takatsugu came under siege on 13 October by the 15,000-strong Western forces of Mōri Motoyasu, Tachibana Muneshige and Tsukushi Hirokado, along with a naval blockade by Mashita Nagamori. The staunch resistance of the Eastern Army troops under Takatsugu was only ended on the 21 October after the Western Army bombarded the castle from Mount Nagara, by which point the Western besiegers, like Onoki Shigekatsu's troops at Tanabe, were too late to join the decisive battle at Sekigahara.
Meanwhile, In Mie Prefecture, Furuta Shigekatsu defended Matsusaka Castle from the incursion of the Western Army.
Meanwhile in the other place, in the Battle of Tsu Castle at October 1, the largest prelude to the Battle of Sekigahara in Ise Province, Kuki Yoshitaka served as a commander of the Western Army naval force. He blockaded the sea and prevented reinforcements from the Eastern Army, contributing to the fall of Tsu Castle. Yoshitaka also received reinforcement from the lord of Shingu Castle in Kii Province who also his son-in-law, Ujiyoshi Horiuchi. Together, they seized Toba Castle, which had been left behind by his son, Kuki Moritaka, who sided with the Eastern Army. However, after the Battle of Sekigahara ended with a victory for the Eastern Army, Yoshitaka fled to Toshima Island. Soon, at the urging of his vassal named Toyoda Goroemon, committed suicide on November 17.
After the main battle of Sekigahara ended, Onogi Shigekatsu retreated from Tanabe Castle, but was soon attacked by an army of 5,300 men led by Hosokawa Tadaoki at Fukuchiyama Castle. Afterwards, he surrendered and opened the castle through the mediation of Yamaoka Doami , who was sent as an emissary of Tokugawa Ieyasu . He begged for mercy through Ii Naomasa, but was denied, so he committed suicide at Jusen-in Temple in Kameyama, Tanba Province.
Tōkaidō campaign
On 29 August, Ieyasu had established a base of operations in Oyama, and by 1 September Ieyasu began relocating 50,000 troops mustered in Edo north to Oyama, preparing to strike at Kagekatsu in Aizu but not expecting to do so, believing the Date, Mogami and Maeda forces sufficient to halt the Uesugi advance. With the news of the fall of Fushimi, Ieyasu quickly returned to Edo by 10 September, already planning a westward return. Eager to secure the crossroads near Gifu Castle now threatened by the Western Army at Ōgaki Castle, Ieyasu sent Fukushima Masanori, Kuroda Nagamasa, Honda Tadakatsu, Ii Naomasa and Hosokawa Tadaoki ahead along the Tōkaidō road with 16,000 men, followed by 15,000 men under Ikeda Terumasa, Asano Yoshinaga and Yamanouchi Kazutoyo, to fortify Masanori's holding of Kiyosu and to seize Gifu Castle from Oda Hidenobu. The combined 31,000 troops of this "Tōkaidō Corps" quickly besieged Gifu on 28 September and forced the capitulation of Hidenobu.
At first, Mitsunari wanted to use Gifu Castle and Ōgaki Castle as choke points to impede the Eastern army advances. However, several developments of war forced him to abort the plan as: