The Battle of the Bzura (or Kutno or Battle of Kutno — German name) was a battle in the opening campaign of World War II during the 1939 German invasion of Poland, fought between 9 and 19 September, 1939, between Polish and German forces. Finally, the Germans outflanked the Polish forces and took all of western Poland. It was the single largest battle in the 1939 September campaign and took place to the west of Warsaw, near the Bzura River. In it, a Polish breakout attack gained initial success but eventually faltered after a concentrated German counterattack.