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.^[7]

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.^[8]

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.^{[lower-alpha 1]} This proclamation effectively marked the declaration of war between the Western and Eastern armies.^[9]

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.^[10]

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.^[11]

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.^[12][13]

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.^[14]

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.^[15]

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.^[16][17] 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.^[18]

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.^[18]

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. ^[19]

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.^[20]

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 Mitsunari and Ankokuji Ekei were captured fleeing north from Sekigahara, and were subsequently executed in Kyoto on 9 November.^[21]

Nakasendō campaign

On 25 August, Ōtani Yoshitsugu had made contact with the Sanada clan at Ueda Castle and recruited Sanada Masayuki and his son Yukimura; the eldest son, Sanada Nobuyuki, had already been sent by Masayuki to join the Tokugawa, with whom he already had marriage ties through his wife Komatsuhime^{[lower-alpha 2]} in an attempt to preserve the Sanada clan. After dispatching the Tōkaidō Corps, Ieyasu sent his son Tokugawa Hidetada along the Nakasendō road with 36,000 men towards Mino, ordering him to threaten Ueda Castle while continuing on to meet the Tōkaidō Corps. He reached Karuizawa on 7 October and subsequently arrived in Komoro on 12 October; however, there he disobeyed his father's orders and instead marched on Ueda, encaping at Sometani.^[22] Komatsuhime herself had a brief but bloodless confrontation at Numata Castle with Masayuki and Yukimura before the siege of Ueda began in earnest.^[23]

Hidetada, being unexperienced in warfare at 21 years old, besieged Ueda Castle and its commander Sanada Masayuki unsuccessfully until 16 October, at which point he realised his delay and broke the siege to continue westward along the Nakasendō. He would still, however, only rendezvous with his disappointed father's forces late in the day on 21 October, after the Battle of Sekigahara had been fought and decided.^[24]

Keichō Dewa Campaign

The northern campaign in and around Dewa province, known as the Keichō Dewa Campaign, was the key flashpoint providing the casus belli for the Sekigahara Campaign at large, and largely consisted of actions by the Uesugi clan against the combined forces of Mogami Yoshiaki and Date Masamune in an unsuccessful attempt to draw Ieyasu's troops northward from Edo.

In the course of responding to the Uesugi clan's mobilisation prior to the declaration of war, Date Masamune seized Shiroishi Castle south of Sendai from the Western Army-aligned Uesugi army in a short siege from the 18–19 August. Masamune's 20,000 troops under Katakura Kagetsuna, Oniniwa Tsunamoto and Yashiro Kageyori attacked Matsukawa, defended by 6,000 defenders under Honjo Shigenaga and Suda Nagayoshi, on October 6, but were repelled.^[26][27][28]

Uesugi Kagekatsu nominated Naoe Kanetsugu to lead the campaign against the Date and Mogami. The three-division invasion of Mogami territory was launched on 14 October with the aims of taking Yamagata Castle, the 10,000-strong headquarters of Mogami Yoshiaki. Kanetsugu advanced from Yonezawa on 14 October with 20,000 troops of the first division westward towards Yamagata Castle; en route, the Uesugi army besieged Hataya Castle on 18 October, where the garrison of 300 led by Eguchi Gohei fell defending it to the last man.^[12] Simultaneously, the 4,000-strong second division under Honmura Chikamori and Yokota Munetoshi besieged Kaminoyama Castle south of Yamagata, held by the Mogami retainer Satomi Minbu; the Uesugi forces conquered it but suffered setbacks thanks to a Mogami counterattack, losing Chikamori in battle and delaying the second division's approach towards Yamagata. The third division, comprising 3,000 men under Shida Yoshihide and Shimo Yoshitada, also advanced towards Yonegawa from the north via Shōnai.^[29]

Kanetsugu arrived at Hasedō Castle on 21 October, the day of the Battle of Sekigahara, commencing the Siege of Hasedō. During the course of the siege, Date Masamune and Rusu Masakage crossed the mountain passes to encamp 2.5 km east of Yamagata on 24 October,^[30] and Rusu Masakage led an attempt to relieve the siege,^[31] prompting Kanetsugu to attempt to storm Hasedō; the assault was repelled, and Kanetsugu retreated leaving a holding force to threaten Hasedō. On November 5, news finally reached Kanetsugu of the defeat at Sekigahara and orders were given to lift the siege entirely, withdrawing the skeleton force to Yonezawa.^[32] Date Masamune capitalised on this withdrawal by attempting to besiege Fukushima Castle on 12 November,^[30] but withdrew when Uesugi Kagekatsu led a force to relieve the defenders.^[33] The Uesugi conflict with the Date and Mogami would outlast the Sekigahara Campaign proper, with a subsequent abortive attempt by Date Masamune to take Fukushima Castle defeated in battle at Matsukawa on 28 May 1601.^[34]