The adoption of radar by German forces happened before the war; in December 1939 British Wellington bombers were detected by radar on the way to attack the German fleet at Wilhelmshaven, resulting in more than half of them being shot down in the "Battle of Heligoland Bight". Following the invasion of France, radar stations were built across the continent.

The British intelligence efforts to detect and learn more about German radar capabilities are well detailed in Most Secret War by R V Jones, one of the most notable incidents being the Bruneval Raid, in which British paratroops seized radar equipment and even a technician from a coastal radar site. An additional benefit of the raid was that barbed wire and other defences were then built around other radar stations, making them easier to pick up via aerial reconnaissance.

Missions over Europe by units such as [138 and 161 Special Duties Squadrons] (http://www.tempsford-squadrons.info/index.htm) of the RAF and the ['Carpetbaggers'] (http://www.harringtonmuseum.org.uk/801st492nd.htm ) of the USAAF were flown at low altitude to avoid radar detection (and indeed sound detection), avoiding known flak defences as far as possible. The main German defences, the Kammhuber Line, focused on defending Germany itself, so night fighters were not as much of a threat as flak.

See also [The Secret Air War Over France: USAAF Special Operations Units in the French Campaign of 1944] (http://aupress.maxwell.af.mil/digital/pdf/paper/t_moore_secret_air_war_over_france.pdf)