Then, it participated in the Polish–Soviet War, halting the advance of the Soviet 1st Cavalry Army led by General Semyon Budyonny.
It engaged the enemies of newly restored Poland in several battles: Dubno, Jelnica, Zamosz, Szpanow, Klewan and Jaroslawicze.
After some additional changes in the structure of the unit, the divisional headquarters were located in Kraków, together with the 20th Infantry Regiment and elements of other subunits.
According to Plan West, the 6th Division, commanded by General Bernard Mond was part of Bielsko Operational Group, which belonged to Kraków Army.
It defended the positions in the villages of Brzezce and Wisla Wielka, attacked by German 5th Panzer Division.
Several German tanks managed to break through the 20th Infantry Regiment and destroy the Third Battalion, together with divisional artillery, located in the rear.
Only 17 light and 2 heavy cannons were saved, and under the conditions, two battalions of the 16th Regiment assaulted German tanks in a suicide mission near the village of Cwiklice.
Despite heavy losses, the division managed to hold its positions until September 4, when, together with elements of Operational Group Boruta, it withdrew towards the Dunajec river.
The Reserve Center of the Sixth Division, located in Kraków, was in mid-September 1939 evacuated to the so-called Romanian Bridgehead, where it was incorporated into the Operational Group Dniestr of Karpaty Army.
Tasked with the defense of the area of Zaleszczyki, it crossed the Romanian border after Soviet Invasion of Poland.