In November 1943 Arthur Harris began the "Battle of Berlin" in the belief the destruction of the German capital would end the war without Operation Overlord, the planned amphibious invasion of France. The battle would end in March 1944, in a defensive victory for the German night fighter force. The introduction of SN-2 radar and the passive Flensburg radar detector helped the crews. Flensburg had been able to detect the Monica radar emissions which warned RAF crews of an approaching night fighter. radar detector and the SN-2 radars had proved their worth along with Flensburg. The Monica radar gave only warning of a fighter within in a 45 degree cone, while Flensburg could detect the bomber from away. The capture of Monica and Boozer early warning receivers in March 1943 allowed the Germans to develop Flensburg. The Monica set was captured a week after its introduction in a major blow for Bomber Command. The capture of a Ju 88 night fighter that landed in England in July 1944 exposed these developments to the British who then developed counter-measures; mainly by stripping the tail radars from their bombers or switching them off.