Sadao Araki
During the 1920s Koiso joined the relatively moderate "Tōseiha" (Control Faction) led by General Kazushige Ugaki, along with Gen Sugiyama, Yoshijirō Umezu, Tetsuzan Nagata, and Hideki Tōjō as opposed to the more radical "Kōdōha" (Action Faction) under Sadao Araki.