Following his defeat at the hands of Tsukishima Hana during the Freshman War, Hisashi Amachi transferred to Rindow High. Upon enrolling here as a student, Amachi used the former Kurotaki Alliance territory as a home base to centralize his future operations.
Initially comprised of the students of Rindow, the Amachi Army eventually expanded to include dangerous fellows, such as Kagawa Kazuya (a.k.a. Gaga), the Arson Brothers Muroto Zenmei and Muroto Koumei, Daitou Takashi, and the Naitou Twins Naitou Ichigo and Naitou Ichie - all of whom would become officers for the faction. The Army's rapid ascent to power scared most major factions in the Town and among the schools they conquered included: Tsukamoto High, Takiya Commerce, Nanamori Industrial, and Kurosaki Industrial. At the peak of their strength, more than 300 men would move under Amachi's order.
After losing a hard fought War to the Hana-Gumi, Amachi's army was reduced to just five of the previously mentioned officers. Muroto Zenmei bowed out of the gang in an effort to reform and turn over a new leaf. With renewed spirit, they decided to rebuild their group from scratch and promised each other that their defeat against the Hana-Gumi would be the only defeat they would ever experience.
By the time of the war with The Manji Empire, it's unknown just how strong Amachi's crew has become, but they were able to eliminate Manji's Renjou Group. The attack on officers Gaga and Muroto Koumei, however, was seen as the chief cause of Amachi's anger and participation in the War.