Fact File : Ruhr Air Offensive
5 March to 24 June 1943 (some sources say the offensive went on until the Gelsenkirchen raid of 9/10 July)
Theatre: Western Europe
Location: The Ruhr valley, Western Germany.
Air Marshal Sir Arthur Harris's RAF Bomber Command.
Outcome: Extensive damage to the industrial centres of the Ruhr.
Avro Lancaster I Bomber in Flight©
Over objections from Arthur Harris, who feared dilution of Bomber Command's esprit de corps
, a specialist Pathfinder Force was established in the summer of 1942. Pathfinder aircraft flew ahead of the main force, located targets and marked them with special flares.
As well as Pathfinder tactics, the 1943 raids on the Ruhr used three navigational aids: GEE, a radar-based guidance system with a range of 720km (450 miles) from Britain, introduced early in 1942 and first jammed by the Germans a year later; Oboe, a more precise radio-based system with a range of 400km (250 miles), introduced in December 1942 and jammed ten months later; and H2S, an air to surface radar system.
The Ruhr campaign began with a raid on Essen on 5 March 1943. After Pathfinder craft guided by Oboe had dropped flares on the Krupps armaments factory, three waves of bombers passed over in 38 minutes, aiming their bombs at the flares.
While only 40 per cent of the force got within three miles of the target, the Krupps works and a large part of Essen were destroyed. This was followed by another 21 raids following the same pattern, targeting industrial centres throughout the Ruhr.
Area bombing of city centres was achieving far more visible results than bombing of industrial targets. The synthetic oil refinery at Gelsenkirchen was bombed twice, but was back in production within weeks; by contrast, a single raid on Düsseldorf left 140,000 people homeless.
However, German air defences were increasing in efficiency; Bomber Command lost 1,000 aircraft during the campaign. Harris was forced to switch targets and send out diversionary raids, limiting the devastation caused by each raid.
The fact files in this timeline were commissioned by the BBC in June 2003 and September 2005. Find out more about the authors who wrote them.