The Battle of Bonari Pass (Japanese:母成峠の戦い) was part of the Boshin War, and occurred on the 6 October 1868 (Gregorian Calendar), or the 21st day of the Eighth Month, Keiō-4 year (Japanese calendar). The Bonari pass was a strategic access at the limit of the fief of Aizu.
The battle opposed a mixed force of 700 troops- including former shogunate soldiers (Denshutai and Shinsengumi) led by Otori Keisuke and Hijikata Toshizo, Aizu forces, and soldiers of various northern domains- to 2,000 troops favourable to the Imperial government. Outnumbered, the shogunal troops had to retreat to the North to Sendai, where the fleet of Enomoto Takeaki was ready to evacuate them to Hokkaidō. This left Aizu exposed, and facilitated the imperial advance.
Three days later, the Imperial troops reached the castle of Wakamatsu and led siege to it, in the key action of the Battle of Aizu.