The Battle of Bonari Pass

Background — The Fall of Nihonmatsu and Disagreement over the Advance Route

In the 7th month of 1868 (Keiō 4), the New Government army captured Nihonmatsu Castle, the stronghold of Nihonmatsu domain, breaking apart one pillar of the Ōuetsu Reppan Dōmei alliance. Following this victory, the leadership of the New Government army split over the choice of their next target. Staff officer Ōmura Masujirō argued for pressing on toward Sendai and Yonezawa, while Itagaki Taisuke of Tosa and Ijichi Masaji of Satsuma argued for a direct assault on Aizu domain, the core of the alliance. Itagaki and Ijichi's argument ultimately prevailed, and the New Government army set its sights on Aizu.

Two main routes led into Aizu domain: Nakayama Pass, the main highway approach from the Kōriyama direction, and Bonari Pass, a secondary route from Adachi District. The defenders, centered on Aizu domain, concentrated their forces at Nakayama Pass, leaving the comparatively lightly defended Bonari Pass as the route the New Government army identified as its point of breakthrough.

The Formidable Terrain of Bonari Pass

Bonari Pass lay on the border between Aizu and Nihonmatsu domains in Iwashiro Province, a mountain pass road that served as a relatively inconspicuous secondary route. The defenders constructed three tiers of gun emplacements at the pass, making the most of the terrain despite their limited numbers. The garrison totaled roughly 800 men — about 200 Aizu domain troops, some 400 men of the Denshūtai (an elite corps of the shogunate army), about 100 Sendai domain troops, about 100 Nihonmatsu domain troops, and remnants of the Shinsengumi. Command fell to Ōtori Keisuke, Infantry Commissioner of the shogunate army.

The Fierce Fighting of August 21 — The Fall of the Three Tiers of Emplacements

On October 6, 1868 (the 21st day of the 8th month, Keiō 4), the New Government army staged a diversion suggesting an attack toward Nakayama Pass to draw Aizu's attention there, while directing its main force at the lightly defended Bonari Pass. The attacking force, roughly 7,000 strong under Itagaki Taisuke and Ijichi Masaji, was centered on Satsuma and Tosa troops, joined by contingents from Chōshū, Satō, Ōgaki, and Ōmura domains.

The battle began early in the morning and continued throughout the day in thick fog. The three tiers of emplacements built by the defenders were overrun one after another by the New Government's assault, which held an overwhelming advantage in both numbers and equipment. Against a defending garrison of only about 800, the attacking force numbered roughly 7,000 — nearly a ninefold advantage — and was also equipped with modern Western-style arms far superior to the defenders'.

The Rout and the Blow to the Denshūtai

After fierce fighting, the defenders' line collapsed completely by the afternoon, and the former shogunate and Aizu defenders under Ōtori Keisuke were routed. The Denshūtai, supposedly an elite unit of the shogunate army, is recorded as having suffered especially heavy losses in this defeat. Though the defenders had built a three-tiered defensive position making the most of the terrain, they could not overcome the overwhelming disparity in numbers and equipment.

This defeat completely undermined Aizu domain's strategic decision to concentrate its forces at Nakayama Pass. With the lightly defended Bonari Pass broken through, the entire defensive line guarding the approach to Aizu territory collapsed at once.

The Advance on Inawashiro, the Rush toward Aizu-Wakamatsu, and the Tragedy of the Byakkotai

Having broken through Bonari Pass, the New Government army pressed onward toward Inawashiro without pause. Aizu domain was forced to abandon Inawashiro Castle, and on the following day, August 22, urgent word reached Aizu-Wakamatsu that the New Government army was closing in on Tōnokuchihara. The domain lord, Matsudaira Katamori, ordered the Byakkotai's Shichū Nibantai, under the command of Hyūga Naiki, to move out to Tōnokuchihara, but the unit suffered heavy losses in the fighting there and was scattered. Of the 42 cadets in the unit, only 20 managed to reach Mount Iimori by August 23. What they saw from the mountain was black smoke rising over the castle town, which they mistook for the fall of the castle, and the young cadets took their own lives on the spot. This mass suicide of the Byakkotai at Mount Iimori occurred just two days after the defeat at Bonari Pass, and remains one of the most widely known tragedies symbolizing the Aizu War.

Its Place within the Aizu War

The Battle of Bonari Pass is regarded not merely as an isolated engagement but as the battle that effectively decided the outcome of the entire Aizu War. The defeat here allowed the New Government army to advance to the outskirts of Aizu-Wakamatsu within a matter of days, forcing Aizu domain into a full siege defense. Aizu-Wakamatsu Castle (Tsuru-ga-jō) withstood roughly a month of siege, but the losses in troops and the breach in the defensive line caused by the defeat at Bonari Pass could never be made good, leading to the castle's surrender the following month.

Sites That Survive Today

Today, Bonari Pass, on the border of Kōriyama City and the town of Inawashiro in Fukushima Prefecture, is marked with monuments and interpretive panels recounting the battle, and the site draws history enthusiasts as a place that preserves the memory of the opening battle of the Aizu War. The mountain paths around the pass still retain much of their original terrain, continuing to bear witness to the fierce battle in which an overwhelmingly outnumbered defense collapsed within half a day.