The gates of Ogaki Castle opened after midnight. No torches. No drums. Horses' mouths were muzzled against any sound that might carry across the dark valley to the Tokugawa scouts camped at Akasaka. One by one, then in columns that stretched beyond sight, the Western Army poured out into the autumn rain and vanished into the mountains.

Ishida Mitsunari, approximately 41 years old, had spent the last several days watching his options close. Tokugawa Ieyasu stood in Gifu Castle, 30 kilometers to the east. The Eastern Army's outlying forces were pushing against his perimeter — that very day, a skirmish at Sone Castle had gone badly for the Western side, with Shimazu Yoshihiro's (Shimazu Yoshihiro's) men driven back and Ogaki's outer earthworks partially razed. If Ieyasu chose to bypass Ogaki entirely and march directly for Osaka Castle — the seat of the young Toyotomi heir Hideyori — the political foundation of the Western cause would collapse without a battle being fought.

Mitsunari had to get ahead of him. He had to block the roads west.

There was one place where the geography of Japan funneled all movement between east and west into a single narrow valley: the plain of 関ヶ原 (Sekigahara), "Meadow of the Barrier." The name itself came from an ancient barrier station — a 関 (seki), a checkpoint — that had stood at this mountain crossing since the seventh century. Three great roads converged here: the Nakasendo (Nakasendō), the Ise road, and the Hokkoku road. The valley was flanked by mountains on every side. A force that held Sekigahara could not be outflanked from the west. A force that sat waiting at Ogaki could be bypassed entirely.

So Mitsunari chose the night march.

Earlier, Shimazu Yoshihiro had proposed another option: a surprise night attack on the Eastern Army at their camp at Akasaka. It was a bold idea — the kind of gamble that occasionally decides wars. Mitsunari's chief strategist, Shima Sakon, refused it as dishonorable. Mitsunari backed him. The Shimazu, warriors of the far south with a tradition of ferocious independence, were already suspicious of the mainland daimyo (daimyo) coalition. Being dismissed that way — especially by a man they privately considered a bureaucrat who had turned general — left a wound that did not close. Their resentment would shape what happened the next morning.

The march covered roughly 14 kilometers, through rain, on mountain roads, in total darkness, with no lights and no noise. October nights in the mountains of Gifu Prefecture are cold — well below 10 degrees Celsius — and the rain made the road a channel of mud. Men arrived at their assigned positions exhausted, wet, and unable to see. It was somewhere between impressive and chaotic, and it worked: the Eastern Army did not intercept them.

What Mitsunari did not fully know — what he perhaps chose not to believe — was that the heights he was counting on were not fully his.

Kobayakawa Hideaki, approximately 18 years old, occupied Mount Matsuo on the southeastern flank with 15,600 men — a strong position commanding the entire valley floor. He had already made a private arrangement with Tokugawa Ieyasu. He would wait. He would watch. And at the right moment, he would charge downhill — not into the Eastern Army's flank as Mitsunari expected, but into the Western Army's own lines.

On Mount Nangu, the combined Mori forces — roughly 28,000 men — occupied the ridge that would have put the Eastern Army in a deadly vise from behind. But Kikkawa Hiroie, commanding the vanguard of that force, had made his own deal with Ieyasu. When Mori Hidemoto gave the order to advance, Hiroie's troops physically blocked the path. The great western flanking force would not move.

None of this was visible in the fog.

Otani Yoshitsugu — one of the Western Army's most capable and loyal commanders — sensed what others did not. He was gravely ill, probably with leprosy or a severe skin disease that had disfigured his face; he was carried to his position in a palanquin, unable to ride. Despite this, he insisted on being present. He positioned his forces at the southeast edge of the valley floor, directly below Mount Matsuo, and then did something unusual: he rotated half his men to face uphill. Facing the man who was supposed to be their ally. Just in case.

When dawn came on October 21, the valley was still dark with fog — 霧 (kiri), thick autumn mountain mist that settled into the basin and blotted out the ridgelines. The Eastern Army marched in from the east. Neither side could clearly see the other. Both armies found themselves in the same narrow valley, groping toward each other through the gray murk. The fog would not lift until around 8 a.m.

The battle began when someone could finally see well enough to charge.

The 霧 (kiri) of Sekigahara is one of the most famous atmospheric details in Japanese military history — the armies of a divided nation assembling blind, in the same valley, in the dark and cold, before the most decisive morning in centuries. Classical Japanese poets use 霧 (kiri) as an autumn word, a 季語 (kigo): fog carries the resonance of obscured vision, of the unknown, of things that cannot be clearly seen. No word was ever better chosen for the night before Sekigahara.