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 raid—considered a minimum target to irreparably damage British combat power—the night fighter force grew in size and potency. British losses in the Battle of the Ruhr (March–July 1943) and most notably in the Battle of Berlin (November 1943 – March 1944) reached an all-time high; 569 bombers in the latter campaign.