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.
[7] 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.
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.
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.
However, due to lack of coordination, and alleged mistrust from Kagekatsu, in the end he did not move his troops to pursue the Ieyasu's army which were on their way returning to Kyoto as Mitsunari intended.
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.
[13] 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 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.
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.
[34] 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.
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,[41] and Rusu Masakage led an attempt to relieve the siege,[42] prompting Kanetsugu to attempt to storm Hasedō; the assault was repelled, and Kanetsugu retreated leaving a holding force to threaten Hasedō.
[43] Date Masamune capitalised on this withdrawal by attempting to besiege Fukushima Castle on 12 November,[41] but withdrew when Uesugi Kagekatsu led a force to relieve the defenders.
[45] In the run-up to hostilities, Nabeshima Naoshige of Saga Domain intended to support the Tokugawa; however, his son Katsushige was present at Mitsunari's war council on 17 August and swayed him towards joining the Western Army.
[9] However, while the Nabeshima forces participated in some operations on Honshu including the assaults on Fushimi and Annotsu, Naoshige eventually recalled Katsushige to Kyūshū prior to the decisive battle at Sekigahara and switched allegiance to the Eastern Army.
[46] Kiyomasa besieged Udo Castle before meeting up with Kuroda Yoshitaka, who had already conquered several minor locations on Kyūshū, and Nabeshima Katsushige, whose family had defected to the Tokugawa.
[47] The Tachibana troops continued to hold out in the castle, with Ginchiyo organising an ad-hoc militia of nuns to slow the advance of the Eastern Army, donning armour herself to serve as Muneshige's rearguard.