The Battle of the Bzura river took place from 9 to 12 September 1939 during the German invasion of Poland, which started World War II. On 9 September the Poznań Army ("Armia Poznań") operational group, led by General Edmund Knoll-Kownacki, attacked the German 8th Army led by General Johannes Blaskowitz. On 11 September Poland's Pomorze Army reinforced the Polish troops in battle. At first, the Polish assault was successful but the Germans reinforced their troops on 12 September and started to counterattack. General Tadeusz Kutrzeba ordered Knoll to retreat beyond the Bzura river. Kutno remained outside the battle area. Despite the ultimate defeat of Polish army, the Battle of the Bzura river made the Germans change their strategy and regroup and also delayed the capitulation of Warsaw.