The letters had already gone out. For weeks, Tokugawa Ieyasu (徳川家康, Tokugawa Ieyasu) had been writing to daimyo on the other side — quiet promises, careful assurances, the machinery of betrayal laid like rails before the locomotive even moved. Now, in the final days of September 1600, that locomotive was moving.

The Eastern Army's vanguard — tens of thousands of men under Fukushima Masanori (福島正則, Fukushima Masanori) and Ikeda Terumasa (池田輝政, Ikeda Terumasa) — had already pushed deep into the heartland. Observers along the Tōkaidō (東海道, Tōkaidō) road reported that the column took hours to pass a single point. Not since Toyotomi Hideyoshi (豊臣秀吉, Toyotomi Hideyoshi) had Japan seen an army move this fast, this far, with this much confidence.

Ishida Mitsunari (石田三成, Ishida Mitsunari) had counted on Ieyasu being cautious. The man was 57 years old — a survivor of Oda Nobunaga's (織田信長, Oda Nobunaga) patronage, of decades under Hideyoshi's thumb, of a hundred alliances made and broken and remade. Surely he would proceed carefully, feel out the terrain, give the western coalition time to concentrate its forces and establish strong defensive positions. Instead, the eastern forces swept forward like water finding its level — inevitable, unstoppable, and already past the point where intervention could help.

Ieyasu himself would not leave Edo Castle (江戸城, Edo-jō) until October 7. He did not need to hurry. His vanguard commanders knew their jobs: push west, hit hard, take Gifu. By September 29, Fukushima and Ikeda's forces crossed the Kiso River (木曽川, Kiso-gawa) in a pincer from north and south, crushed a western force at the village of Komeno (米野, Komeno), and drove the defenders back toward Gifu Castle (岐阜城, Gifu-jō). On September 30, the castle fell — in less than a single day.

Gifu Castle was not just any fortress. Perched 330 meters above the city on the sheer face of Mount Kinka (金華山, Kinka-san), it was Nobunaga's own fortress, the symbol of his domination of central Japan. Its young lord, Oda Hidenobu (織田秀信, Oda Hidenobu), was Nobunaga's grandson, in his early twenties, inheritor of a great name and a great castle but not the old warlord's terrifying instincts. When his fortress fell, he prepared himself for seppuku (切腹, seppuku). Ikeda Terumasa talked him out of it — Hidenobu's surrender was more useful than his death.

News of Gifu's fall reached the western coalition commanders at Ogaki Castle (大垣城, Ogaki-jō) like a crack of thunder. Their outer defenses were collapsing faster than anyone had imagined possible. The daimyo who had been quietly wondering whether to defect began quietly accelerating those plans.

Ieyasu received the news in Edo and ordered his own departure. He was in no particular rush. The army moving west was enormous — his personal force on the Tōkaidō, roughly 30,000 men, and his son Tokugawa Hidetada (徳川秀忠, Tokugawa Hidetada) pushing another 38,000 along the Nakasendō (中山道, Nakasendō) mountain road. Total Eastern Army strength approached 75,000 to 89,000 men by the time they converged. The western coalition nominally matched that number, but numbers on paper meant nothing if the men holding weapons had already decided which side they were really on.

This was not merely a military campaign. It was 天下取り (tenka-dori) — the taking of the realm — executed with the patience of a man who had spent an entire lifetime waiting for exactly this moment. Nobunaga had attempted it and died in a burning temple. Hideyoshi had achieved it and left it unfinished. Now Ieyasu, at 57, was completing the work — not by conquest alone, but by patient, methodical elimination of every alternative.

When the eastern army began to move in late September 1600, it was not just marching toward a battle. It was marching toward 268 years of history that had not yet happened.