The fire started at approximately dawn. In early March 1582 — most likely the 3rd, though the rapid cascade of events makes the precise dates hard to pin down — 武田勝頼 (Takeda Katsuyori) set fire to 新府城 (Shinpu-jō), his own headquarters, and walked away from it.

He had built Shinpu himself. That was the point of it. His father, 武田信玄 (Takeda Shingen) — the Tiger of Kai (甲斐の虎, Kai no Tora) — had conducted campaigns from the old Tsutsujigasaki mansion in Kōfu, a fortress-palace that spoke of an older era. Katsuyori wanted something new: a proper stone castle, a modern seat of power, a place that was his rather than inherited. Shinpu was that statement. Now the statement was burning.

The collapse had begun on March 2, when 高遠城 (Takato-jō) — a Takeda stronghold in Shinano Province held by Katsuyori's son-in-law — fell to Oda Nobutada's forces in a single day. The speed of it was the message. Three separate armies were moving: Oda Nobutada from the north, 徳川家康 (Tokugawa Ieyasu) from the south, and another Oda force pressing from the west. Together they numbered forty to fifty thousand warriors. Katsuyori's force inside Shinpu numbered perhaps fifteen thousand — on paper. In practice, every general and soldier in the castle performed the same calculation the moment Takato fell: What can I do for my family? Each man's answer was to leave.

The desertion was not cowardice, exactly. It was the mathematics of a broken dynasty. Takeda Shingen had been dead for nine years. His reputation — the fear he had generated in Nobunaga, the near-mythic aura of the cavalry charges, the legend of the Twenty-Four Generals (二十四将, Nijūshi-shō) — had outlasted him just long enough to become a liability. Katsuyori had been fighting in his father's shadow rather than with his father's army. After 長篠 (Nagashino) in 1575, when Nobunaga's volley lines cut down the charging Takeda cavalry in waves, the dynasty's invincibility became a memory. The soldiers who stayed after that had stayed out of obligation, out of loyalty to a name. When the name visibly failed, the obligation dissolved.

By the time Katsuyori lit the torch, perhaps five hundred to six hundred men remained. He left the castle with them, heading east on mountain paths toward Iwadono Castle — held by 小山田信茂 (Oyamada Nobushige).

Oyamada was not merely a retainer. He was one of the celebrated Twenty-Four Generals of Takeda Shingen — a designation so famous in samurai lore that it was commemorated in paintings, in poetry, in the stories that circulated across Japan about the Takeda clan's glory years. He had served the family his entire adult life. More than that: just a week before Katsuyori fled Shinpu, Oyamada had sent personal word inviting him to take refuge at Iwadono. The offer had been explicit, generous, and warm.

The gates were closed.

Oyamada had already sent messengers to the Oda camp. The calculation he had made — that the Takeda name was finished, that survival required accommodation with Nobunaga — was not irrational. But he had gone further than necessary. He had not simply failed to answer the call; he had invited Katsuyori in and then barred the door. He had used the invitation as a tool to strand his lord in the mountains with nowhere to go.

The 裏切り (uragiri) — the betrayal — was total.

With Iwadono closed, Katsuyori had no shelter left. His band continued to shrink. Every mountain path offered another opportunity for a man to slip away, to vanish into the forest with his horse and his weapons and whatever future he might carve out. By the time Katsuyori made his final stand at 天目山 (Tenmokuzan) on March 11, only 41 warriors remained — along with approximately 50 women, family members and retainers' wives who had followed because they had nowhere else to go, or because they chose not to go anywhere else.

Those 41 men held off the pursuing Oda forces long enough for the three who mattered to die on their own terms. Katsuyori, his wife Hoshunin, and his son committed 切腹 (seppuku) at 天目山 (Tenmokuzan). The Battle of Tenmokuzan lasted perhaps an hour. When it was over, the Takeda clan was over. The male line — descending from one of the oldest and most celebrated samurai dynasties in Japan, tracing lineage back to the Minamoto — was extinguished.

The names of those 41 warriors are recorded in the chronicles. They stayed because they were kin, or because they had no families left to protect, or because walking away from their lord was simply not something they were capable of doing. The clan's ultimate defenders were not its generals. Its generals had gone home.

As for Oyamada: he went to the Oda camp expecting recognition, perhaps reward, perhaps a domain. What he received was an executioner. 織田信長 (Oda Nobunaga) had Oyamada killed — according to the accounts, specifically because he had refused shelter to his lord. Nobunaga did not want treacherous retainers serving him. The man who betrayed the Takeda to survive was killed by the men he betrayed them for, on the grounds of the betrayal itself.

The broader consequence of the Takeda collapse extended far beyond Kai Province. The last major obstacle to Nobunaga's unification of Japan was gone. Three months later — on June 21, 1582 — Nobunaga himself was dead, assassinated by 明智光秀 (Akechi Mitsuhide) in the 本能寺の変 (Honnōji no Hen). The power vacuum created by the destruction of both the Takeda and then Nobunaga opened the path for 豊臣秀吉 (Toyotomi Hideyoshi) and, eventually, Tokugawa Ieyasu — who had helped destroy Katsuyori — to complete the unification. Ieyasu would later claim descent from the Minamoto, echoing the legitimacy claims the Takeda themselves had used. History absorbed the destroyed and erased the name of the destroyer.

The Sengoku period's (Sengoku Jidai) harshest lesson is visible in the ashes of Shinpu: a military reputation is only as durable as the last battle. Shingen's ghost gave Katsuyori's clan a borrowed credibility long after Nagashino should have ended it. When the credit ran out, fifteen thousand men walked away in forty-eight hours. What remained was forty-one.