The army had been heading north. Uesugi Kagekatsu had fortified his domain at Aizu, ignored Tokugawa Ieyasu's demands for an explanation, and had his advisor Naoe Kanetsugu fire back a letter so contemptuous it was practically a declaration of war. The so-called Naoe Letter listed Ieyasu's political crimes one by one and closed with what amounted to a public dare: come and do something about it. So Ieyasu marched. The daimyo of the east fell in behind him.

At Oyama in Tochigi Province, on the twelfth and thirteenth of July, the army paused. Traditional accounts describe what followed as a 軍議 (gungi) — a war council — but it was something stranger and more consequential than a planning meeting. Ieyasu had engineered a situation where neutrality was simply not available. Every warlord in the room understood that attendance itself was a declaration. To march east was to put your name on one side of a ledger that Ieyasu was quietly and permanently keeping.

The assembled cast was formidable and fractious. Date Masamune, the one-eyed warlord of Sendai, stood alongside his uncle Mogami Yoshiaki — men who despised each other nearly as much as they despised the Toyotomi faction. Together they would fight their own sub-war against the Uesugi in the northeast while the main drama played out to the west. Each man was calculating odds on who would rule Japan for the next century.

Some Japanese historians have questioned whether the Oyama Council unfolded exactly as later Tokugawa chronicles describe. The dramatic "loyalty audition" framing — every warlord forced to choose in a single room on a single afternoon — may owe something to the victors writing the history. The broad outline, however, is not seriously disputed: Ieyasu assembled eastern daimyo, Mitsunari raised his coalition in the west, and what began as a punitive march against the Uesugi became the opening movement of the Sekigahara campaign.

The deeper irony was already in motion before Oyama. Mitsunari had coordinated with Kagekatsu precisely to lure Ieyasu east, away from the power centers of the west, while the western coalition assembled. It may have been the cleverest trap of the era. Ieyasu may have suspected it. He marched anyway — and when the news of the western uprising arrived, he did not panic. He turned the entire army around and aimed it straight at Sekigahara. The trap had sprung, and Ieyasu was already walking through it in the direction he intended.