The bombing of Warsaw in World War II started with the aerial bombing campaign of Warsaw by the German Luftwaffe during the siege of Warsaw in the invasion of Poland in 1939. It also included German bombing raids during the Warsaw Uprising in 1944. During the course of the war, approximately 85% of the city was destroyed due to German mass bombings, heavy artillery fire, and a planned demolition campaign. Siege of Warsaw (1939) 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. 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 0800 on 25 September, Luftwaffe bombers under the command of Major Wolfram Freiherr von Richthofen conducted the largest air raid ever seen by that time, dropping 560 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 obsolescent Junkers Ju 52/3m bombers, which dropped 13 percent of the incendiary bombs dropped on the day. Only two Ju 52 bombers were lost. Although commonly portrayed as being absolutely decisive, the Black Monday air attack was a mixed success. While the bombing lowered Polish morale, it did not cause the Polish surrender. 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,