The gates of 大垣城 (Ōgaki-jō) opened sometime in the night of October 20, 1600, and an army began to move.

Note on dates: The Japanese lunar calendar places this event in the 9th month, 14th day of Keichō 5. The Gregorian equivalent is October 20, 1600.

Approximately eighty thousand men filed into the rain, torches and lanterns catching the lacquer of commanders' armor as they turned west. Their destination was fourteen kilometers away — a mountain pass at 関ヶ原 (Sekigahara), a name that, within twenty-four hours, would mean the end of an era.

Their commander was 石田三成 (Ishida Mitsunari), forty-one years old, a brilliant administrator who had served as one of the five bugyo (commissioners) under 豊臣秀吉 (Toyotomi Hideyoshi) and who had spent the months since Hideyoshi's death maneuvering against 徳川家康 (Tokugawa Ieyasu) for control of Japan's future. He had assembled a Western Army that — on paper — outnumbered Ieyasu's forces. He held a coalition of clans. He had chosen terrain. He believed, marching out of Ōgaki in the rain, that he was choosing the battlefield on his own terms.

He was not.

At the council of war earlier that evening, 島津義弘 (Shimazu Yoshihiro) — commanding 1,500 Satsuma troops — had proposed a night attack. Strike Ieyasu's Eastern Army while it was rain-soaked and exhausted, camped in the dark. It was an aggressive, tactically sound idea. Mitsunari's advisor 島左近 (Shima Sakon) opposed it as dishonorable. Mitsunari sided with Sakon. Shimazu retired in silent fury — and the next morning, his troops would sit idle on the battlefield while the battle raged, a passive act of vengeance for the slight.

The decision to march was driven partly by intelligence — possibly misinformation — suggesting that Ieyasu planned to bypass Ōgaki entirely and strike Sawayama, Mitsunari's home castle, threatening 豊臣秀頼 (Toyotomi Hideyori) and the entire Toyotomi legacy Mitsunari was fighting to protect. Whether true or planted, the intelligence worked: Mitsunari abandoned the security of the castle walls and chose the open field.

What he did not know:

小早川秀秋 (Kobayakawa Hideaki) — twenty-two years old, commanding 15,600 men on 松尾山 (Matsuo-yama), the critical high ground that anchored Mitsunari's northern flank — had already sent Ieyasu a letter. The letter offered to change sides at the decisive moment. Modern historians, including Jun Shiramine and Junji Mitsunari, have found pre-battle correspondence confirming Kobayakawa had committed to Ieyasu before the first shots were fired.

吉川広家 (Kikkawa Hiroie), commanding the advance elements of the 毛利 (Mōri) clan's enormous force, had secretly promised Eastern Army commanders that the Mōri would not advance. On the day of battle, asked repeatedly to move forward, he would reply each time that his men were eating lunch.

Ieyasu almost certainly knew the Western Army was moving. He may have even helped cause it.

The army arrived at 関ヶ原 (Sekigahara) in the pre-dawn hours and took up their positions in the fog and rain. The high ground was theirs. The positions looked excellent. Mitsunari set up his headquarters on 笹尾山 (Sasao-yama) — a command position that later critics would note was too far from the fighting for effective battlefield command.

The battle began at dawn on October 21, 1600. It lasted approximately six hours.

When 小早川秀秋 (Kobayakawa Hideaki) finally committed his 15,600 men — not to the Eastern flank as promised to Mitsunari, but crashing into the Western Army's center — the coalition fractured instantly. The 毛利 (Mōri) did not move. Shimazu sat watching. The Western flanks, one by one, turned or fled. Mitsunari's center collapsed under the combined pressure of the Eastern Army's advance and Kobayakawa's attack from the rear.

Mitsunari fled the field in disguise, hiding in a peasant's house in the mountains of 近江 (Ōmi) before being betrayed once more — captured by farmers, handed to Ieyasu's men, brought to Kyoto. On November 6, 1600, he was executed publicly in the capital he had fought to protect.

徳川家康 (Tokugawa Ieyasu) established the 徳川幕府 (Tokugawa Bakufu) in 1603. It governed Japan for 265 years.

The night march from 大垣 (Ōgaki) — Mitsunari's bold gamble, his aggressive choice, the decision he made with eighty thousand men in the rain — was the last clear moment when the outcome was still uncertain. By the time the fog lifted the next morning over 関ヶ原 (Sekigahara), the answer was already written. He had marched his army into a position that three of his commanders had already ceded to the enemy. The battlefield he chose was the battlefield his betrayers had chosen for him.