Byakkotai was part of Aizu's four-unit military, set up in the domain's drive to finalize its military modernization, in the wake of the Battle of Toba-Fushimi. The other three units were Genbutai, Seiryutai, and Suzakutai. Each of the four was named after the protecting gods of compass directions. Byakkotai was meant to be a reserve unit, as it was composed of the young teenage 16 to 17 year old sons of Aizu samurai in a group around 350, who fought in the Boshin War (1868–1869)