The Oyama Deliberation (Oyama hyōjō) was a consequential war council held on 7/25 of Keichō 5 (1600) in the village of Oyama in Shimotsuke Province. This occurred while Tokugawa Ieyasu was leading a large army toward Aizu in Mutsu Province with the intention of subjugating Uesugi Kagekatsu, an event known as the Conquest of Aizu serving as a prelude to the Battle of Sekigahara.
Following the demise of Toyotomi Hideyoshi in 1598, Ieyasu had experienced a gradual falling out with Ishida Mitsunari, both of whom had served as senior commanders under Hideyoshi. Around this time, Uesugi Kagekatsu, the seventeenth head of the Yamanouchi-Uesugi clan in Iwashiro Province, began to secretly collaborate through his channels with Mitsunari for assistance to build-up his military. Ieyasu traveled to Kyōto seeking explanations for Kagekatsu’s actions but was refused. In the sixth month of 1600, Ieyasu departed from Fushimi Castle with a huge contingent of 55,000 soldiers, having the intended destination of Aizu in Iwashiro Province with the aim of subjugating Kagekatsu.
Following a stay at Edo Castle in the Kantō, the army split into two groups. Ieyasu’s third son, Tokugawa Hidetada (who later became the second shōgun of the Edo period) arrived with the first contingent in Utsunomiya, while Ieyasu led the second contingent to the village of Oyama in Shimotsuke Province. An urgent message sent by Torii Mototada (a retainer of Ieyasu based at Fushimi in Kyōto) arrived, whereupon Ieyasu learned that Mitsunari had gathered forces in Ōsaka with the intention of toppling Ieyasu.
Ieyasu then immediately summoned his generals for a war council known as the Oyama Deliberation. The meeting was held in the Suga Shrine on the southern grounds of Oyama Castle. The key options on the table included: (i) continue with the plan to attack the Uesugi, (ii) change course by turning west to challenge Mitsunari, or (iii) ally with Mitsunari. Most of the generals serving Ieyasu came from families that had been retainers of the Toyotomi for generations, with wives and children in Ōsaka. The decision whether to proceed as planned or return to Ōsaka represented a crucial turning point for Ieyasu’s future.
In addition to Ieyasu and Hidetada, those in attendance included retainers of Ieyasu (Honda Tadakatsu, Honda Masanobu, Ii Naomasa) and generals who were patrons of the Toyotomi (Fukushima Masanori, Yamauchi Kazutoyo, Kuroda Nagamasa, Asano Yukinaga, Hosokawa Tadaoki, Katō Yoshiakira, Hachisuka Yoshishige).
During the meeting, Fukushima Masanori, the lord of Kiyosu Castle in Owari Province, offered that even if hostages were lost, he would serve in the vanguard of a force to head west. Yamauchi Kazutoyo, the lord of Kakegawa Castle in Tōtōmi Province, pledged cooperation with Ieyasu by proposing to compel daimyō with castles and territory in the Tōkai region to vacate their holdings. The spirit shown by these men encouraged the other generals, driving consensus in support of Ieyasu. Ieyasu praised Kazutoyo for making the greatest of all contributions. As a result, Ieyasu made a final decision for the Eastern Army to turn west to attack Mitsunari in Ōsaka.
Several weeks later, a total of approximately 200,000 forces from the Eastern and Western armies faced-off at Sekigahara in Mino Province in a climactic battle to determine the future course of Japan. The Eastern Army prevailed. One of the direct causes for the outcome owed to a decision at the last moment by Kobayakawa Hideaki and others to betray the Western Army in favor of the Eastern Army. Moreover, the western army was challenged from the onset to coordinate its actions, whereas the Eastern Army was better aligned. The decisions made at the Oyama Deliberation played a role in forging this unity of action. In this regard, the Oyama Deliberation was a critical point of departure for Ieyasu’s ascension.