The armies had already turned around. That was the remarkable thing about September 10, 1600. Fifty thousand men who had spent all summer marching north toward the Uesugi domains — preparing for what looked like a war of succession against the defiant lord Uesugi Kagekatsu — had reversed direction almost overnight. They pivoted toward a completely different enemy, a hundred miles to the west, before their general had taken a single step in that direction himself.

When Tokugawa Ieyasu arrived back at Edo Castle on September 10, 1600, he had just completed a swift return from Oyama, where he had been encamped with roughly 50,000 assembled troops. The news arrived in late August: Ishida Mitsunari, the sharp-tongued bureaucrat whom half the daimyo in Japan despised personally, had raised the 西軍 (Seigün, Western Army) in Osaka. The five great magistrates of the Toyotomi regency council were invoking Hideyoshi's legacy against Ieyasu. Fushimi Castle — held by Ieyasu's loyal retainer Torii Mototada — had just fallen after a desperate resistance. Everything Ieyasu had built over three decades was suddenly at stake.

On September 1, Ieyasu had called a council at Oyama. The eastern daimyo, many of them personally hostile to Mitsunari, committed to the eastern cause. Fukushima Masanori, Kuroda Nagamasa, Hosokawa Tadaoki — men who had despised Mitsunari at Hideyoshi's court — barely needed persuading. The 東軍 (Tōgun, Eastern Army) was born. The vanguard turned west immediately. Ieyasu returned to Edo.

What he did there in the following weeks was not rest. It was the true campaign.

Messages flew outward in every direction. Lords along the mountain roads were contacted. Intelligence about the Western Army's movements poured in. Most critically, certain lords who had publicly committed to Mitsunari's coalition were quietly working back channels — and Ieyasu's agents were meeting them. The defection of Kobayakawa Hideaki, who commanded a crucial position on the eastern flank at Sekigahara and whose last-minute betrayal would decide the battle entirely, was engineered during precisely this period.

Meanwhile, the vanguard was already racing west. Fukushima Masanori's corps of approximately 16,000 men and Ikeda Terumasa's corps of approximately 15,000 — a combined force of roughly 31,000 — were driving along the Tokaido at a pace that startled even their allies. Ieyasu sent messages ahead: slow down, don't overextend, wait for coordination. Fukushima paid limited attention. The man was running on years of suppressed fury toward Mitsunari.

Ieyasu himself would not depart Edo until October 7, when he marched with approximately 32,000 additional men. By then, his vanguard had already seized Gifu Castle, shattered the Western Army's eastern screen, and demoralized the coalition that was supposed to stop him.

This is what separated Ieyasu from other Sengoku commanders. His patience was a weapon as deadly as any army. While Mitsunari scrambled to assemble his forces at Ogaki, Ieyasu sat at his castle, pulled strings from hundreds of miles away, and ensured that when the decisive battle came, the outcome was already determined. The battle of Sekigahara, fought on October 21, 1600, lasted only a few hours. Six weeks of preparation at Edo made it so.