During the invasion of Poland in 1939, General Biernacki commanded the Army "Prusy", which was supposed to be the strategic reserve that would be used to launch a concentrated counter-attack as the enemy approached Warsaw.

He made an attempt to stop the German XVI Army Corps who had broken through the front near Częstochowa and pushed towards Warsaw.

On 10 September he become commander of Northern Front, consisted of remaining fragments of Polish Army.

After his defeat at Battle of Tomaszów Lubelski, he managed to evacuate to France through Hungary, where he was immediately arrested by order of the Commander-in-Chief General Wladyslaw Sikorski for political reasons.

When General Kazimierz Sosnkowski took over the position of Commander-in-Chief in 1943, Dąb-Biernacki was released from prison and went to Ireland, where he was engaged in beekeeping.