The aftermath of the bombing of Warsaw, October 1939. In the previous month, 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 shot 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 and 75 mm anti-aircraft guns of the Warsaw Defense Command. As the German Army approached Warsaw on 8 September 1939, 140 Junkers Ju-87 Stukas attacked the portions of the city on the east bank of the Vistula River and other bombers bombed the Polish Army positions in the western suburbs. On 13 September Luftwaffe level and dive bombers caused widespread fires. Further resistance was followed by propaganda leaflet drops. Finally, starting at 08:00 on 25 September, Luftwaffe bombers under the command of Major Wolfram Freiherr von Richthofen conducted the first major city attack of World War II, dropping 500 tons of high explosive bombs and 72 tons of incendiary bombs, in coordination with heavy artillery shelling by Army units. The center of Warsaw was badly damaged. Approximately 1,150 sorties were flown by a wide variety of aircraft, including even obsolescent Junkers Ju-52/3m bombers, which dropped 13 percent of the incendiary bombs dropped on the day! Although commonly portrayed as being absolutely decisive, the Black Monday air attack was a mixed success. Smoke from fires and large amounts of dust obscured targets and greatly reduced accuracy. As a result, Luftwaffe bombers dropped a significant amount of their bomb loads on German infantry positions in the northwest suburbs of the city, leading to acrimonious discussions between Luftwaffe and Army commanders. The tonnage dropped combined with only approximate delivery on target and the short duration does not begin to approximate the intensity of attacks major European cities were subsequently to suffer. However, on 26 September three key forts in the city defenses were captured, and the Polish garrison offered its surrender - on 27 September German troops entered the city. Overall, Warsaw suffered approximately 25,800 civilian deaths, with 40 percent of the buildings in the city were damaged and 10 percent of the buildings destroyed. However, to attribute this destruction completely to aerial bombardment would be an error; damage included that resulting from intense street fighting between German infantry and armor units and Polish infantry and artillery, as well as from constant bombardment by German artillery.