The Shinsen-gumi masterless warrior party, including SAITO, came under the command of the domain of Aizu and fought in the Battle of Shirakawaguchi on the 26th of intercalary month of May, as well as the Battle of Bonari-toge on October 6.
斎藤をはじめとする新選組は会津藩の指揮下に入り、閏4月5日には白河口の戦いに参加、8月21日の母成峠の戦いにも参加した。