During the Polish Defensive War, in the battle of the Border the German Army Group South struck between Poznań and Łódź Armies, penetrating Polish defenses and forcing Polish armies to retreat. The Poznań Army itself was not heavily engaged during those early days but was forced to retreat due to danger of being flanked. Later the Poznań Army strengthened by the remains of the Pomorze Army took part in the Polish counteroffensive Battle of Bzura; finally remaining units withdrew towards Warsaw and took part in its defense.