The Battle of Bonari Pass

Background——The Fall of Nihonmatsu and the Dispute over the New Government Army's Route

In the 7th month of Keiō 4 (1868), the New Government army captured Nihonmatsu Castle, the main base of Nihonmatsu domain, breaking apart one member of the Ōuetsu Reppan Dōmei alliance. Following the fall of Nihonmatsu, opinions divided within the New Government army's leadership over its next target. Staff officer Ōmura Masujirō advocated advancing toward Sendai and Yonezawa, whereas Itagaki Taisuke of Tosa domain and Ijichi Masaji of Satsuma domain argued that Aizu domain, the core of the Ōuetsu Reppan Dōmei, should be attacked directly. In the end, the position of Itagaki and Ijichi was adopted, and the New Government army set Aizu as its next target.

There were two main routes into Aizu: Nakayama Pass, the main highway from the Kōriyama direction into Aizu, and Bonari Pass, a secondary road from Adachi District into Aizu. The defenders concentrated their forces at Nakayama Pass, so the comparatively thinly defended Bonari Pass came to be noticed by the New Government army as a point of breakthrough.

Bonari Pass as a Natural Fortress

Bonari Pass was a mountain pass on the district border between Aizu and Nihonmatsu domains in Iwashiro Province. As a route it was a secondary road, a relatively inconspicuous path. The defenders built three tiers of batteries (gun emplacements and breastworks) at the pass and, although their numbers were limited, set up a defense that made use of the terrain. The garrison consisted of about 200 Aizu domain troops, about 400 of the Denshūtai, an elite unit of the shogunate army, about 100 Sendai domain troops, and about 100 Nihonmatsu domain troops, plus remnants of the Shinsengumi and others, for a total of roughly 800 men. Command was held by Ōtori Keisuke, then Infantry Commissioner of the shogunate army.

The Fierce Battle of the 21st Day of the 8th Month——The Fall of the Three Tiers of Batteries

On the 21st day of the 8th month of Keiō 4, the New Government army staged a diversion feigning an attack toward Nakayama Pass to draw Aizu's attention in that direction, while directing its main force at the thinly defended Bonari Pass. The New Government army numbered roughly 7,000 in total, but the force that made for the pass on the 21st was about 2,200 under Itagaki Taisuke and Ijichi Masaji. Centered on Satsuma and Tosa troops, it was joined by contingents from Chōshū, Satō, Ōgaki, and Ōmura domains.

The battle began early in the morning and continued throughout the day in dense fog. The three tiers of batteries built by the defenders were breached one after another in the face of the fierce assault of the New Government army, which held an overwhelming advantage in both numbers and equipment. Against a defending force of only 800, the New Government army attacking the pass alone numbered nearly three times that. In addition, the New Government army possessed the latest Western-style equipment, whereas the defenders' equipment was relatively inferior.

The Rout of the Former Shogunate Forces and the Blow to the Denshūtai

After fierce fighting, by the afternoon the defenders' line had collapsed completely, and the former shogunate and Aizu defensive force led by Ōtori Keisuke was routed. The Denshūtai, which was an elite unit of the shogunate army, is said to have suffered a heavy blow in this battle. Although the defenders had built a three-tiered defensive position that made use of the terrain, they could not overturn the overwhelming disparity in numbers and equipment.

As a result of this defeat, Aizu domain's strategic judgment to concentrate its forces toward Nakayama Pass completely backfired. Once the thinly defended Bonari Pass was breached, the defensive line leading into Aizu territory collapsed at once.

The Advance into Inawashiro, the Rapid Push toward Aizu-Wakamatsu, and the Tragedy of the Byakkotai

The New Government army, having broken through Bonari Pass, continued its advance toward Inawashiro without a pause. Aizu domain was forced to abandon Inawashiro Castle, and on the following day, the 22nd day of the 8th month (October 7), an urgent report reached the castle town of Aizu-Wakamatsu that the New Government army was closing in on Tonokuchihara. The former domain lord, Matsudaira Katamori, ordered the Byakkotai Shichū Nibantai (Second Shichū Unit), led by Hyūga Naiki as captain, to deploy to Tonokuchihara, but the unit suffered a heavy blow in the fighting there and was routed. The cadets who went out were scattered, and on the 23rd day of the 8th month (October 8) only 20 had finally reached Iimori-yama. What they saw from the mountaintop was black smoke rising over the castle town of Aizu-Wakamatsu. The young men, who are also said to have mistaken it for the fall of the castle, took their own lives on the spot, a tragic end. The mass suicide of the Byakkotai at Iimori-yama occurred just two days after the defeat at Bonari Pass, and it is widely known to this day as a tragedy that symbolizes the Aizu War.

Its Position within the Entire Aizu War

The Battle of Bonari Pass is evaluated not merely as an isolated local battle but as the battle that effectively determined the outcome of the entire Aizu War. Because of the defeat in this battle, the New Government army advanced to the outskirts of Aizu-Wakamatsu within just a few days, and Aizu domain was forced into a full-scale castle defense. Aizu Wakamatsu Castle (Tsuru-ga-jo) endured the siege for about one month, but the troops lost and the gap in the defensive line caused by the defeat at Bonari Pass could never be filled, leading to its surrender in the 9th month of the same year.

Historic Sites Preserved Today

Today, Bonari Pass has monuments and explanatory boards that convey the traces of the battle. Located on the border between Kōriyama City and Inawashiro Town in Fukushima Prefecture, the pass is a destination for history enthusiasts as a historic site that conveys the opening battle of the Aizu War to the present day. The mountain paths around the pass still largely retain the terrain of that time and continue to pass on the memory of the fierce battle, in which the defenders, amid an overwhelming disparity of forces, collapsed within just half a day.