He was in Edo. The battle was three weeks away. And Tokugawa Ieyasu (徳川家康) had already won.

By early October 1600, the campaign that would define Japan for the next 265 years was visibly accelerating — but Ieyasu, 57 years old, was in no hurry. He had spent nearly a month in Edo after reversing his march north, consolidating his forces, finalizing promises, and letting his vanguard commanders do the aggressive work. Fukushima Masanori, Ii Naomasa, and others had swept through the Mino region; Gifu Castle had fallen September 29. The eastern forces had effectively cut off the heart of central Japan.

The western coalition assembled by Ishida Mitsunari to defend the legacy of Toyotomi Hideyoshi was enormous — perhaps 80,000 to 120,000 men on paper, drawn from daimyo across western Japan who had prospered under Hideyoshi and resented the Tokugawa's rising power. The eastern army was smaller, perhaps 70,000 to 90,000. On paper, it was nearly balanced.

But Ieyasu had not been spending his weeks in Edo resting. He had been writing letters.

The most consequential letters went to Kobayakawa Hideaki, a young general of wavering loyalties commanding 15,000 men. Kobayakawa was nominally part of the western coalition. His position, atop Mount Matsuo overlooking the narrow Sekigahara valley, would make him the fulcrum of any battle fought there. Ieyasu's agents — particularly Kuroda Nagamasa — had been in contact with Kobayakawa for months. By the time the armies faced each other, Kobayakawa had almost certainly already committed to switching sides.

He was not alone. Tens of thousands of "western" troops had privately pledged not to fight. They would sit on their hills and watch. The battle's outcome had been scripted in a hundred secret letters before a single sword was drawn.

On October 21, 1600, in heavy morning fog, the two armies met in the Sekigahara valley and went to work. For several hours the fighting was genuine and fierce. Then Kobayakawa's 15,000 men — sitting motionless on Mount Matsuo while the battle raged below them, doing nothing when every western commander needed them to move — finally charged. According to one famous account, Ieyasu, impatient with Kobayakawa's hesitation, ordered muskets fired at Kobayakawa's position as a live warning. Whether or not that happened exactly as told, the effect was the same: Kobayakawa charged downhill into the Ōtani lines. The western forces watching from other hillsides — who had their own secret arrangements — joined the attack on their own supposed allies. The western line collapsed within hours.

Ishida Mitsunari fled the battlefield. He was captured in hiding days later and executed in Kyoto on November 6, 1600. The Toyotomi loyalists who escaped that day would survive only until the Siege of Osaka in 1615, when Ieyasu erased the last of them.

There is a painful footnote about Ieyasu's own family. His son and designated heir, Tokugawa Hidetada, had marched the alternate Nakasendō route with 38,000 men — the largest single force in the eastern army. He never arrived. Besieging Ueda Castle, held by the brilliant tactician Sanada Masayuki, Hidetada got stuck. He missed the entire battle. That Ieyasu won Sekigahara without his largest contingent — that the pre-arranged defections were so thoroughly reliable that a third of the eastern army's manpower was simply absent and it didn't matter — tells you everything about how the battle had truly been won.

Ieyasu departed Edo for the west on October 7. He would enter Sekigahara on October 21 and exit it the master of Japan. In 1603, the emperor granted him the title of 征夷大将軍 (Sei-i Taishōgun) — Shogun. Two years later, he passed the title to his son Hidetada, establishing beyond any doubt that Tokugawa rule would be a dynasty, not a single man's achievement.

天下統一 (tenka tōitsu) — the unification of the realm under heaven — was complete. It had been decided not on the foggy field of Sekigahara, but in the letters written in the weeks before it.