The Army of the Isthmus was composed of six divisions under the command of Hugo Österman.

The III Corps with the II Corps formed the Army of the Isthmus (Kannaksen armeija).

Army of the Isthmus (Kannaksen Armeija, KannA) under Lieutenant General Hugo Österman was located on the Karelian Isthmus.

When the Winter War started Österman was made commander of the Army of the Isthmus, but was dismissed on 19 February 1940 after the Finnish lines had been breached.

The II and III Corps were organised into the Army of the Isthmus which was located on the Karelian Isthmus, the likely location for the main Soviet attack.