The British started a strategic bombing campaign in 1940 that was to last for the rest of the war. Early British bombers were all twin-engined designs, and the 1939 Battle of the Heligoland Bight had shown the vulnerability of bombers to fighter attack. Therefore, RAF Bomber Command turned to a policy of area bombing at night. Later in the war pathfinder tactics, radio location, ground mapping radar, and very low-level bombing enabled specific targets to be attacked.