The Byakkotai was part of Aizu's four-unit military, formed in April 1868 in the domain's drive to finalize its military modernization, in the wake of the Battle of Toba–Fushimi. The other three units were the Genmutai (Black Tortoise Unit), the Seiryūtai (Azure Dragon Unit), and the Suzakutai (Vermilion Bird Unit). 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 castle town. From there, they saw what they thought was the castle on fire, and committed seppuku (with one failed attempt) in desperation, believing their lord and families dead. However these 20 Byakkotai members were mistaken in their assessment of defeat, as the castle defenses had not actually been breached; the castle town surrounding the inner citadel was aflame. As the majority of the town was between Iimori Hill and the castle, the boys saw the rising smoke and assumed that the castle itself had fallen.