By the end of 1943 the Allied Combined Bomber Offensive had taken a heavy toll on Germany. Under the leadership of Generalleutnant Josef Kammhuber, the Luftwaffe night fighter force expanded and adapted to the threat. The development of and eventual widespread use of airborne radar in the Luftwaffe enabled the German air defences to inflict heavy losses on Bomber Command, but they could not prevent widespread destruction of industrial cities.[lower-alpha 4] Only once did the British lose as many as nine percent of the attacking force over Germany—during an attack on Pilsen, Czechoslovakia on 17 May 1943.