In 1939, the Luftwaffe opened the German attack on Poland with operation Wasserkante, an air attack on Warsaw on 1 September. This attack by four bomber groups was of limited effectiveness due to low-lying cloud cover and stout Polish resistance by the PZL P.11 fighters of the Pursuit Brigade, which claimed down 16 German aircraft for the loss of 10 of their own. However, heavy losses in Polish fighter aircraft meant that by 6 September the air defense of Warsaw was in the hands of the 40 mm (24 guns), 75 mm (72 guns) anti-aircraft guns and many anti-aircraft machine guns of the Warsaw Defense Command.[4]