He rode into Ōgaki Castle (大垣城, Ōgaki-jō) without resistance. The gates opened. The garrison stood aside. The town, the walls, the river crossings — all were his. For Ishida Mitsunari (石田三成), the unopposed entry on September 15, 1600, must have felt like confirmation that the campaign was proceeding as designed.

The castle's position was genuinely formidable. Ōgaki sat in the flat agricultural plain of western Mino Province, ringed by the Ibi and Kuise rivers, at the junction of the 中山道 (Nakasendō) — one of Japan's five great highways (五街道, gokaidō) — and the pass leading west through Sekigahara toward Kyoto. Whoever held Ōgaki held a key to the road. If Tokugawa Ieyasu's Eastern Army wanted to reach the capital, they would have to either besiege Ōgaki (slow, expensive, giving western allies time to concentrate) or march around it (exposing their flank and supply lines to a western garrison at their back). Mitsunari also positioned satellite forces on the surrounding mountains — most significantly at Nangū-san Castle, designed to block the eastern approaches entirely.

The strategy was sound. This was the problem with being a brilliant administrator at war: Mitsunari could analyze terrain, calculate supply lines, and design defensible positions. What his surveys could not measure was loyalty.

Mitsunari was 41 years old. He had served the Toyotomi house his entire career — a loyalist by nature, by training, by every professional instinct he possessed. He assumed, not unreasonably, that the lords who had signed onto the Western coalition shared some fraction of this disposition.

They did not.

At Nangū-san, the vanguard of the Mōri forces — the coalition's largest constituent — would eventually stand completely immobile, because their commander Kikkawa Hiroie (吉川広家) had secretly promised Ieyasu they would not fight. On the heights above Sekigahara, Kobayakawa Hideaki (小早川秀秋) commanded 15,000 men in the most strategically decisive position on the battlefield — and had already been in private correspondence with Ieyasu for months.

None of this was visible from inside Ōgaki Castle on the evening of September 15.

What Mitsunari could see: the castle was secured, the road was controlled, his forces were assembled. What he would learn fourteen days later: on September 29, Ieyasu's vanguard captured Gifu Castle (岐阜城) to the north, directly threatening his communication and supply lines. The Eastern Army was not going to wait for a siege. They were maneuvering.

The endgame arrived on October 20, when Ieyasu's full Eastern Army appeared before Ōgaki's walls and surrounded the castle. That evening, Mitsunari held a war council. The options were bleak: be besieged in Ōgaki and wait for allies who had secretly promised not to come, or march west in darkness to the open field at 関ヶ原 (Sekigahara) and fight in the open.

He chose to fight. He marched his army out of Ōgaki after nightfall — a few hours' march to the valley west of the castle. Ieyasu had anticipated the decision exactly. When dawn came on October 21, the two armies were arranged across the field at Sekigahara, and the battle that had actually been decided months earlier, in private correspondence and whispered promises, played out in six hours of steel and fire.

On September 15, standing inside the castle he had entered without resistance, Mitsunari could not have known any of this. The walls had not yet closed. The plan was still intact. From inside Ōgaki on that evening, the war still looked like it might be won.

It was the last day that illusion held.