The plan had a name: Operation Woodpecker. The idea was elegant, almost beautiful in its geometric logic. Yamamoto Kansuke — Takeda Shingen's (Takeda Shingen's) chief strategist, a man in his early sixties whose reputation had been built on exactly this kind of thinking — had designed a two-phase trap.

Phase one: 8,000 Takeda troops under Kosaka Masanobu would climb Mount Saijoyama at night and 'tap' — like a woodpecker — at the rear of Uesugi Kenshin's (Uesugi Kenshin's) camp. Kenshin, with the mountain turning dangerous behind him, would have no choice but to descend to the Hachimanbara plain below.

Phase two: Shingen waited on the plain with his main force in 挟撃 (kyōgeki) — a pincer, or crane's wing formation designed to catch and crush the Uesugi between two closing arms. The woodpeckers drove them down; Shingen's jaws closed. Clean, decisive, over.

Kenshin was not on the mountain.

Sometime in the pre-dawn hours of the autumn of 1561, the Uesugi army descended from Saijoyama and materialized on the Hachimanbara plain — in the same dense fog that was supposed to be their disadvantage — before Shingen expected them. The two armies practically stumbled into each other in the mist rising off the Chikuma River. The woodpecker troops were still climbing an empty mountain.

For hours, Kenshin's roughly 13,000 men hammered Shingen's 8,000. Kenshin, who was 31 years old, led from the front in the most literal sense: at some point during the chaos, he rode directly into Shingen's command position and attacked the Takeda lord with a drawn sword. Shingen — 39 years old, caught sitting — raised his iron 軍配 (gunbai, war fan) and deflected the blows. A retainer drove Kenshin back before a fatal strike could land. It was the kind of duel that belongs to legend rather than recorded history, and it has been painted, dramatized, and argued over ever since.

Meanwhile, Yamamoto Kansuke watched his plan come apart.

The Takeda's brilliant strategist understood what had happened with terrible clarity. His trap had been read. His army was fighting at a catastrophic numerical disadvantage because of a plan he had conceived. The men dying on that plain were dying because of him. When this understanding settled, Kansuke made his decision: he would not wait for the woodpecker troops to return and rescue the situation. He charged into the Uesugi ranks alone. According to some accounts he received more than 80 wounds before he finally fell — charging again and again until his body had nothing left to give. He chose to die for the error rather than live past it.

The woodpecker troops did eventually return and the battle stabilized, but the damage was done. Among the Takeda dead was Shingen's younger brother Takeda Nobushige, approximately 37 years old, commanding the Takeda vanguard at Hachimanbara. He was killed in hand-to-hand combat with the Uesugi vanguard commander. His head was taken by Uesugi forces before retainers managed to recover it. Shingen is said to have wept openly over his brother's body when it was brought to him.

The armies eventually withdrew. No territory changed hands. The Takeda kept Shinano Province, as they already had. The Uesugi achieved nothing they had not already possessed. Shingen spent the rest of his life insisting the battle had been a victory — he even sent letters to neighboring domains claiming as much. Most historians call it a catastrophe for both sides, a bloody draw in which estimates of casualties range from roughly 3,000 Uesugi and 4,000 Takeda dead (moderate estimates) to figures as high as 70% of each force (figures that most historians now consider exaggerated). What is not disputed is this: the Fourth Battle of Kawanakajima produced higher casualties as a percentage of forces engaged than virtually any other battle of the Sengoku period.

And at the end of it, both armies went home.