Since the spring of 1600, 上杉景勝 (Uesugi Kagekatsu) in the northeast and 石田三成 (Ishida Mitsunari) in the west had been coordinating toward a single objective: to catch 徳川家康 (Tokugawa Ieyasu) between two armies simultaneously. By late August, the trap had sprung.

Ieyasu was at Oyama, in present-day Tochigi Prefecture, his army marching northeast toward Aizu to confront Uesugi Kagekatsu's forces — when the intelligence arrived from the west. Mitsunari had mobilized. The Western Army was gathering at Osaka. Two coordinated enemies, one to the northeast and one hundreds of miles to the southwest, with Ieyasu's army pointed in exactly the wrong direction.

He was fifty-seven years old. He had spent a lifetime in this kind of moment.

Ieyasu convened a 評定 (hyōjō) — a formal war council — at Oyama, gathering the assembled daimyo who had joined his northeastern campaign. In the traditional account, he presented the situation plainly: the Western Army was moving. Every man in this room must choose a side. And then, one by one, the lords spoke.

Fukushima Masanori (福島正則 (Fukushima Masanori)) went first, in the traditional telling. He had quarreled bitterly with Mitsunari for years; his contempt for the man was well known. His pledge was easy. The others followed. The mechanism was not just moral — it was social. Once a daimyo had spoken his loyalty before witnesses in that room, defecting to Mitsunari meant not only political risk but visible personal dishonor. Ieyasu had transformed a military briefing into a binding ceremony.

Modern historians are careful here. The dramatic specifics — Fukushima pledging first, the sequential declarations, the theater of public loyalty — are drawn from chronicles composed decades after the event. Some scholarship questions whether a formal meeting occurred on the terms described. Fukushima Masanori may already have been traveling west before any council took place. The loyalty declarations may be a later narrative construction imposed on a series of more prosaic decisions.

What is not disputed is the outcome. Ieyasu pivoted. By September 1, he was back in Edo with approximately 50,000 men, and the western campaign was underway. The Uesugi, deprived of their western partner's coordination, were left to a holding action. Ieyasu spent the next six weeks writing letters, securing defections, negotiating with wavering daimyo — a campaign of persuasion conducted in parallel with the military advance.

On October 21, 1600, at 関ヶ原 (Sekigahara), Ieyasu's Eastern Army destroyed the Western coalition in a single day. Mitsunari was captured and executed. The Tokugawa shogunate was formally established in 1603 and would govern Japan for 265 years.

The 評定 (hyōjō) at Oyama — real or partly legendary — became one of the founding stories of Tokugawa rule, told and retold as evidence of Ieyasu's supreme political intelligence: that he could take a trap, turn it inside out, and emerge from it with a stronger coalition than he entered with. Whether the scene unfolded exactly as the chronicles describe almost does not matter. The man who arrived caught between two armies left with fifty thousand men at his back. That part is not in dispute.