 Complete the following phrase:  Over the next three years the Combined Bomber Offensive had forced the Luftwaffe to invest heavily in air defence. In the campaign against the RAF, the German air defence system, named the Kammhuber Line, had become increasingly sophisticated. In 1942 the introduction of Lichtenstein radar, despite its early teething difficulties, and greater improvements in the armament and capability of German night fighters produced a force capable of inflicting heavy losses on British bomber streams. Though British losses rarely reached more than ten percent of a raidconsidered a minimum target to irreparably damage British combat powerthe night fighter force grew in size and potency. British losses in the Battle of the Ruhr MarchJuly 1943 and most notably in the 