The charter was 200 years old, and it had no exit clause.

In 1668, Hoshina Masayuki — the founder of the Aizu-Matsudaira domain and a half-brother of the third Tokugawa shogun — wrote fifteen house precepts for his descendants. Article 1 commanded absolute, unconditional loyalty to the Tokugawa shogunate. It was not a political alliance or a calculated arrangement. It was an oath with no conditions, no time limit, and no provision for the possibility that the Tokugawa dynasty itself might fall. The precepts were displayed in the domain's great hall; every lord of Aizu memorized them as a child.

Matsudaira Katamori was born into that oath on February 15, 1836. By the time he was 32, it had consumed his life.

From 1862 to 1868, Katamori had served as 京都守護職 (Kyōto Shugoshoku) — the Tokugawa shogunate's Military Commissioner of Kyoto — a post so politically dangerous that several lords had refused it before Katamori accepted. He accepted because Article 1 of his house code said he must. For six years he enforced the shogunate's authority in the imperial capital, deploying the 新選組 (Shinsengumi) — the sword-bearing police unit — against 尊王攘夷 (sonnō jōi), loyalist, and 浪人 (rōnin) activists. He made enemies of every faction that would eventually become the Meiji government. He was one of the most hated men in the imperial loyalist universe.

When the 戊辰戦争 (Boshin Sensō) — the Boshin War — broke out in January 1868 and the Tokugawa shogunate collapsed within months, Katamori had only one option: go home to Aizu and hold. The domain's charter permitted nothing else.

By early September 1868, imperial forces from Satsuma, 長州 (Chōshū), and Tosa — domains that had equipped their armies with Western rifles and artillery — were pouring into the mountains of 会津 (Aizu) domain in northern Honshu. They eventually numbered more than 30,000. The defenders inside 鶴ヶ城 (Tsurugajō) — Tsuruga Castle, also called Wakamatsu Castle — numbered in the hundreds, supplemented by the domain's remaining able-bodied men and, eventually, its women and elderly as well.

The town at the foot of the castle was burning.

Tsuruga Castle was an unusual structure — its five-story keep bore distinctive red tiles, the only castle in Japan with this feature. In the smoke and chaos of the burning town below, those red tiles were visible from the mountain above and from the surrounding fields: a blood-colored landmark that told everyone, defender and attacker alike, where the last stronghold stood. The castle's concentric inner and outer 曲輪 (kuruwa) — baileys — were designed to confuse and slow invaders, and the stone walls were thick. But the castle had been built for the wars of the seventeenth century, not for the Armstrong guns that the imperial forces were now hauling through mountain passes.

Among the defenders was Niijima Yae — 22 years old, the daughter of the domain's gunnery instructor, who had cut her hair, dressed in male clothing, and taken a position on the castle walls with a Spencer repeating carbine. She could fire seven rounds without reloading. In the weeks ahead she would become one of the most famous defenders of the castle, later called the "Joan of Arc of the 幕末 (Bakumatsu)."

What made the September situation unique — and distinct from the Byakkotai tragedy on October 8, the death of Nakano Takeko on September 26, and the formal surrender on November 6 — was its particular cruelty of timing and irony. On this same date, September 3, 1868, an imperial decree far to the south renamed the old shogunate capital: 江戸 (Edo) was now officially 東京 (Tōkyō) — the Eastern Capital. While the men and women inside Tsuruga Castle bled and burned for the old order, the city that had been the symbol of that order for 265 years was being renamed to serve the new one.

Katamori was fighting to preserve a Japan that was already being erased — not just militarily, but linguistically, cartographically, name by name.

He had not chosen this. His ancestor's charter had chosen it for him. The house precept that commanded loyalty to the Tokugawa shogunate had been written as a moral statement, a statement of identity, a definition of what the Aizu-Matsudaira lords were. It had not been written to be a death warrant. But that is what it had become.

The formal 籠城 (rōjō) — the castle holdout — did not begin in earnest until October 6, when the 母成峠 (Bonari-tōge), the mountain pass that guarded the approach to Aizu-Wakamatsu, fell to the imperial forces. But in the early September period, the trap had already closed. There was nowhere to go. The charter said hold. So they held.