Sekigahara Campaign

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 resorted to attempting an assassination plot on Ieyasu, but when the plot failed, 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.

Mobilisation

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.

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.

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 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.

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.

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.

Meanwhile in Ōsaka, Mōri clan retainer Kikkawa Hiroie was enraged by Mitsunari confining Mōri Terumoto, nominal commander-in-chief of the Western Army, to the castle, and in response to the insult Hiroie secretly communicated with the Tōkaidō Corps promising to keep the 36,000 Mōri troops from engaging the Eastern Army. Mōri Hidemoto, Terumoto's cousin and deputy, also urged Terumoto to defect to the Eastern Army before any engagement occurred.

On 29 September, the Tōkaidō Corps occupied the highlands of Akasaka to threaten Ōgaki Castle as Nabeshima Naoshige besieged the Eastern Army stronghold of Matsuoka Castle. The following day, 30,000 Western troops under Mori Hidemoto, Chōsokabe Morichika, and Nabeshima Katsushige besieged An'nōzu Castle, held by Fukuda Nobutaka and his 1,700-strong garrison. Ishida Mitsunari left Ōgaki Castle on 1 October for Sawayama Castle, where he relented and summoned Mōri Terumoto, who had returned to his own domain to raise troops, to return; one messenger was intercepted, and though a second messenger succeeded in prompting Terumoto to depart with his 30,000 troops, they were delayed from reaching the major confrontations.

Ieyasu moved quickly towards the capital region, reaching Shimada in Suruga Province on 12 October and Nakaizumi in Totomi Province the following day. At the same time, Mōri Hidemoto and Kikkawa Hiroie entered Mino Province and fortified Mount Nangu. During this march, Ieyasu received a letter from Western Army commander Kobayakawa Hideaki offering to defect to the Eastern Army. On 17 October Ieyasu reached Kiyosu Castle, intending to bypass Ōgaki entirely to threaten Sawayama Castle or Ōsaka, and 2 days later he had arrived at Gifu Castle.

Ieyasu arrived in Akasaka around noon on 20 October, meeting up with the main body of the Tōkaidō Corps, and encamped on the hill at Okayama. Western Army generals convinced Mitsunari to permit a testing raid, and Shima Sakon and Akashi Masataka led a small force to attack the Eastern Army at the Kuisegawa river near Ōgaki, mauling the Eastern Army and forcing them to retreat to Sekigahara to regroup. That night, Shimazu Yoshihiro with Ukita Hideie's support suggested a night assault on the exhausted Eastern Army troops, but was opposed by Shima Sakon who convinced Mitsunari to decline the proposal, insulting Yoshihiro. Mitsunari instead ordered a general withdrawal in the evening to the valley of Sekigahara after learning of Kobayakawa Hideaki's occupation of Mount Matsuo, emptying Ōgaki Castle of most of its garrison.

On 21 October, the Western Army arrayed itself in the valley with Mitsunari's headquarters at the foot of Mount Sasao facing the Eastern Army, while Mōri Hidemoto led a separate formation at the Eastern Army's left flank at the foot of Mount Nangu. While both sides became aware of each others' positions at dawn, a thick fog halted plans initially and battle was not commenced until 8am when the fog cleared. The subsequent battle of Sekigahara saw a decisive defeat for the Western Army, with Mitsunari's contingent annihilated by Kobayakawa Hideaki defecting to the Eastern Army and rolling down the formation's right flank, while after a smaller engagement and the treachery of Kikkawa Hiroie forced the eastern formation under Mōri Hidemoto to withdraw rather than join battle.

From 21 to 30 October, Ōgaki Castle was subsequently besieged and taken by the Eastern Army, while on 22 October Kobayakawa Hideaki began besieging Sawayama, ostensibly as a sign of penance and loyalty to Ieyasu; Ishida Masazumi, Mitsunari's brother and commander at Sawayama, capitulated and committed suicide on the second day of the siege. Ishida Mutsnari and Ankokuji Ekei were captured fleeing north from Sekigahara, and were subsequently executed in Kyoto on 9 November.

Nakasendō campaign

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