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, Seiryūtai, 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, 16 to 17 year old sons of Aizu samurai. It was subdivided further, along the lines of rank within the domain's samurai population: two squads were from the upper (shichū) rank, two from the middle rank (yoriai), and two from the lowest (ashigaru). Twenty of the members of the 2nd shichū squad, cut off from the rest of their unit in the wake of the Battle of Tonoguchihara, retreated to Iimori Hill, which overlooked the cas