The largest battle during this campaign, the Battle of Bzura, took place near the Bzura River, west of Warsaw, and lasted from 9 to 19 September. The Polish armies "Poznań" and "Pomorze", retreating from the border area of the Polish Corridor, attacked the flank of the advancing German 8th Army, but the counterattack failed despite an initial success. After the defeat, Poland lost its ability to take the initiative and counterattack on a large scale. The German air power was instrumental during the battle. The offensive of the "Luftwaffe" broke what remained of the Polish resistance in an "awesome demonstration of air power". The "Luftwaffe" quickly destroyed the bridges across the Bzura River. Then, the Polish forces were trapped out in the open and were attacked by wave after wave of "Stukas", dropping light bombs, which caused huge numbers of casualties. The Polish anti-aircraft batteries ran out of ammunition and retreated to the forests but were then smoked out by the Heinkel He 111 and Dornier Do 17s dropping incendiaries. The "Luftwaffe" left the army with the task of mopping up survivors. The "Stukageschwaders" alone dropped of bombs during the battle.