One of the main reasons for the large aircraft losses in the initial period of war with Germany was not the lack of modern tactics, but the lack of experienced pilots and ground support crews, the destruction of many aircraft on the runways due to command failure to disperse them, and the rapid advance of "Heer" troops, forcing the Soviet pilots on the defensive during Operation Barbarossa, while being confronted with more modern German designs. In the first few days of the invasion of the Soviet Union, the "Luftwaffe" destroyed some 2,000 Soviet aircraft, most on the ground, at a loss of only 35 (of which 15 were non-combat-related).