Why Was Tsurugajo Castle Besieged? The Aizu War Across the Northeastern Front

On the twenty-third day of the eighth month in the old calendar, the Aizu domain withdrew into Tsurugajo Castle and entered a siege against the new imperial government forces. Today, the Aizu War is often remembered through the Byakkotai or the bombardment of the castle. Yet the closing of the castle gates cannot by itself explain why Aizu had been driven into such a position.

Before the siege came political conflict in Kyoto, the defeat at Toba-Fushimi, and negotiations over the punishment of Aizu. Fighting also continued on several fronts in northeastern Honshu and the Hokuriku region, including Shirakawa, Echigo, and Nihonmatsu. Aizu's troops and its relationships with supporting domains were stretched across this broad area rather than concentrated around the castle.

Tsurugajo was not besieged simply because its walls were weak, nor had the castle always been the single target of the campaign. Outer fronts retreated, and the defence at Bonari Pass, an eastern route into the Aizu Basin, was broken. That breakthrough allowed war that had covered a large region to move rapidly into the castle town.

This article therefore turns back from the moment of encirclement. It follows the political background, petitions and alliance, the changing fronts across the northeast, the pass breakthrough, entry into the town, the siege, and surrender. Documented events are kept distinct from the article's interpretation of how those events formed a causal sequence.

The lines of troops, smoke, and burning town shown in the image are not archival photography. They are AI-generated reconstructions intended to clarify relationships among events. They do not present precise violence against named individuals or turn personal death and suffering into spectacle.

On the eighth day of the ninth month of Keio 4, the era name changed to Meiji and was applied retroactively to the first day of that year. This is why the events of the Aizu War may be dated either Keio 4 or Meiji 1. Old-calendar dates are identified here so that the order of events is not obscured by mixing calendar systems.

The battle of Toba-Fushimi began on the third day of the first month of Keio 4. Troops from Aizu and Kuwana were among the former shogunate forces, but the new imperial government side won, and Tokugawa Yoshinobu withdrew to Edo. What had begun as fighting near Kyoto developed into a civil war aimed at settling the power of former-shogunate forces across the country.

Katamori returned to Aizu. In the second month he transferred the headship of the domain and expressed an intention to submit. This sequence matters because it shows that Aizu did not choose only one unchanging strategy of retreating into the castle. While reforming its military preparations, the domain also looked for a route through which punishment and submission might be negotiated.

From the new government's perspective, however, Aizu had supported the shogunate's security order in Kyoto and had fought at Toba-Fushimi. Even after Edo Castle was surrendered in the fourth month, the treatment of Aizu and Shonai in the northeast remained unresolved.

A National Diet Library account explains that early surrender terms included conditions such as Katamori's execution, the surrender of the castle, and confiscation of territory, which Aizu found very difficult to accept. Questions remain about how particular formulations and conditions were transmitted, so this article does not reconstruct a complete ultimatum. It records the large distance between the positions in negotiation.

An intention to submit and a growing willingness to resist conditions of disarmament or punishment are not mutually exclusive. Aizu itself should not be represented as a single mind whose choice was fixed from the beginning. The evidence instead reveals a narrowing range of political options.

Nor should the record be simplified into either claim that submission made war automatically avoidable or that military preparation proved submission insincere. Statements of intent, conditions demanded by an opponent, and defensive preparations are different kinds of evidence. They were unfolding at the same time and must be assessed separately.

Aizu could not defend only one border route. Intense fighting continued around Shirakawa from the intercalary fourth month, while the Hokuriku campaign included contests over Nagaoka Castle. Within the territory of present-day Fukushima Prefecture, Nihonmatsu, the Pacific coastal area, and other localities also became battlefields.

A map of the Aizu War held by the National Museum of Japanese History marks castles and forces across a broad space that includes Dewa, Echigo, and Mutsu. Its reported troop numbers and results cannot automatically be treated as exact modern statistics. It does show, however, that contemporary representation did not confine the conflict to the Aizu Basin.

A wide theatre dispersed troops, ammunition, information, and command. Aizu units operated toward Shirakawa, Echigo, Nikko, and other approaches instead of remaining concentrated close to the castle town. Supporting domains also withdrew or redirected forces as their own positions deteriorated, weakening coordination within the alliance.

The new imperial government forces entered the northeast by sea and over multiple land routes, adjusting their fronts as domains surrendered or left the alliance. A single arrow would oversimplify that advance. The accompanying visual therefore shows several lines approaching Aizu at different times rather than one simultaneous movement.

To understand the later siege, the outer fronts matter before the strength of the castle walls. Once those fronts collapsed, even a strong fortress could lose reinforcement and supply and become isolated from the military geography that had supported it.

Lines drawn on a modern explanatory map do not mean that every force shared one continuous boundary at the same moment. Roads, passes, castles, and ports saw movement and battle on different dates. The reconstruction represents the scale of the theatre while avoiding the false image of a complete, simultaneous ring.

Two days after the breakthrough at Bonari Pass, the siege of Tsurugajo began on the twenty-third day of the eighth month in the old calendar. New imperial government forces entered the castle town, encircled the fortress, and bombarded it. Fighting and fires spread through the town, which became covered in smoke.

Tsurugajo had stone walls, moats, and turrets and did not fall immediately. The National Diet Library account states that roughly five thousand people gathered inside, including approximately one thousand older people, women, and children. These figures are treated as estimates preserved in compiled historical materials, not a complete modern census.

The castle became both a military stronghold and a place holding wounded people and noncombatants. Under bombardment, those inside had to maintain treatment, cooking, firefighting, and defence. As time passed, shortages of food, ammunition, sanitation, and rest became more severe.

Nearly a month of resistance should not be converted into a simple legend of an impregnable castle. Walls could continue to stand while external fronts and supply disappeared. In that situation, endurance also meant an increasing burden on the people confined within the enclosure.

The present keep was reconstructed in 1965 and is not the surviving building of 1868. The artwork does not copy a modern photograph. It uses a non-photoreal arrangement suggesting late-Tokugawa red roof tiles, stonework, bombardment, and smoke without claiming architectural precision.

A title such as the battle of Tsurugajo can draw attention toward the castle alone, but the surrounding town was also a battlefield. A responsible account keeps the defence inside the castle connected to civilian harm outside it while refusing to invent the fate of an individual household or resident where no supporting record has been established.

The siege lasted for nearly a month. On the twenty-second day of the ninth month in the old calendar, Aizu surrendered and handed over Tsurugajo. The National Diet Library gives the corresponding Gregorian date as 6 November 1868. The two dates are presented together without treating the old calendar and Gregorian calendar as interchangeable.

Bombardment severely damaged the castle, which remained in a ruined condition after the war. In 1874, buildings within the enclosure were demolished while the stone walls remained. The present keep was rebuilt in 1965, and its roof was changed to red tiles in 2011.

The Aizu domain was abolished, and Katamori and others were placed under confinement. The Matsudaira house was later permitted to re-establish itself in the Tonan domain, and many former Aizu retainers and their families moved north. Some later became involved in settlement and security in Hokkaido. The end of fighting therefore marked the beginning of difficult efforts to rebuild lives.

Why, then, was Tsurugajo besieged? The answer cannot be found in the castle's defences alone or in political hostility dating from Kyoto alone. Negotiations over the treatment of Aizu did not reach a settlement; the alliance's broad fronts retreated; Bonari Pass was broken; and a war losing reinforcement and supply converged on the castle town.

This answer makes the Aizu War a long process in which political negotiation, military geography, and regional society were connected, rather than a single day of tragedy. The memories held by the surviving stone walls mark an endpoint in that process, not its beginning.

Nor did the experience of Aizu's people end with surrender. Confinement, migration, separation from former lands, demolition of the castle, and later reconstruction belong to different periods of postwar history and require their own evidence. After explaining the siege, we must still ask how its consequences persisted in livelihoods and memory beyond the language of victory and defeat.