He was marching the wrong way. That was the trap, and 石田三成 (Ishida Mitsunari) had set it perfectly. In the summer of 1600, Tokugawa Ieyasu led a massive army northeast across the Kantō plain, headed for the domain of 上杉景勝 (Uesugi Kagekatsu), who had refused to answer the council's summons. It was supposed to be a punitive expedition — a demonstration of Tokugawa authority over a defiant lord.

Then the letters arrived. Mitsunari, the brilliant and widely hated administrator who had served 豊臣秀吉 (Toyotomi Hideyoshi) as his chief bureaucrat, had activated a coalition of western lords. While Ieyasu's army pointed northeast, the Western Army was assembling at Osaka, behind him. His supply lines. His allies. His vulnerability.

The trap was elegant. If Ieyasu continued to Aizu, the Western Army would seize Osaka, cut him off, and surround him with two enemies at once. If he turned around immediately, he might look panicked — and daimyo following a panicking commander had a habit of reconsidering their loyalties.

Ieyasu, characteristically, did neither. He was 57 years old — an old man by the brutal arithmetic of his era — and he had spent a lifetime reading moments exactly like this one. He continued to Edo, his castle town on the Kantō plain, arriving approximately around August 10. There he stopped, gathered his forces, and waited.

What followed in those August days was not battle but something perhaps more decisive: the accumulation of commitment. Ieyasu sent letters to every lord of the east. He called a conference at Oyama, in present-day Tochigi Prefecture, where the assembled daimyo were invited to declare themselves. By some accounts, 福島正則 (Fukushima Masanori) — who despised Mitsunari personally — pledged for the Tokugawa first, and the room followed. The pivot was complete.

Before departing Edo, Ieyasu made one more calculation. He left 鳥居元忠 (Torii Mototada) at Fushimi Castle with a small garrison and explicit instructions: the castle would fall, and Mototada should die defending it. Both men understood the bargain. Mototada's last stand would slow the Western Army and — more importantly — demonstrate to every wavering daimyo that the Tokugawa demanded, and inspired, absolute loyalty. Fushimi fell on September 8. Mototada died with it.

The army that finally marched west from Edo numbered 75,000 men. On October 21, 1600, at a place called 関ヶ原 (Sekigahara), it met the Western Army of 120,000 — and won before noon, when key western generals defected mid-battle. The 戦国時代 (Sengoku Jidai) era, a century of civil war, ended in a single morning. The decisions that made it possible were made quietly, in August, in a castle town on the Kantō plain, by an old man reading letters and deciding who he could trust.