The Ruhr had been attacked by Bomber Command from 1940; its defences and the amounts of industrial pollutants produced a semi-permanent smog that hampered bomb aiming.

In 1942 some answers to the chronic problems of night navigation and target finding began to emerge but the number of bombers had stagnated.

Despite its problems, Bomber Command had been able to achieve some spectacular results but these had been isolated events and due to favourable circumstances as well as judgement.

The signals were encoded to prevent German use but this made it harder for Bomber Command navigators to get Gee fixes.

Anti-jamming devices were short-lived in effectiveness as the Germans quickly overcame them but Gee Mk II was easier for navigators to use.

[8] Oboe was a blind-bombing device controlled by two ground stations in England which measured the distance of an aircraft from them with radar pulses.

An aircraft flying at 28,000 ft (8,500 m) could receive Oboe transmissions at about 270 mi (430 km), enough to mark targets in the Ruhr, which led to the device being installed in fast, high-flying Mosquito bombers, which usually navigated with the usual aids until beginning an Oboe run about 10 mi (16 km) from the target.

Cat and Mouse stations could handle only one aircraft at a time and a marking run took ten minutes, allowing six bomb- or marker-runs per hour.

The new devices and the increase in the number of heavy bombers promised a large improvement in the quantity of bombs dropped and in accuracy of aim.

Gee remained useful as a means of navigation on return journeys but required development to overcome German jamming.

[11] The Target Indicator bomb (TI) was an aerodynamic metal case which ejected coloured pyrotechnic candles at a set height by a barometric fuze.

After the Fall of France, a belt of Freya radar stations was built to give early warning of aircraft entering German-controlled airspace, from Denmark south to Switzerland.

The German system had not been centralised to sift the information provided by radar, searchlights, wireless interception and direction finding to co-ordinate FlaK and night-fighters.

Kammhuber used the new equipment to revise the night defence system by increasing the width of Henaja from 25–60 mi (40–97 km) with Dunaja in front of them.

[18] The British resorted to a deliberate campaign of area bombing which immediately increased the amount of destruction achieved by Bomber Command.

[19] In 1942, Bomber Command had been able to inflict considerable damage on several occasions but had failed consistently to disrupt the German war economy.

Despite the weight of the apparatus and aerodynamic penalty of its aerial array causing a loss of at least 25 mph (40 km/h) in speed, night-fighter interceptions increased to the extent that searchlight illumination was made redundant and the lights were transferred to the local FlaK units around cities.

Kammhuber refused to allow night-fighters to roam freely but made the line more flexible, by deepening the Dunaja zone to 124 mi (200 km) either side of the Henaja to exploit the increased range of Freya and Würzburg, which created the Himmelbett system.

[22] Despite the big increase in the German anti-aircraft effort, such concern for the future as existed did not prevent 150 FlaK batteries from being transferred to Italy.

Only the Generalluftzeugmeister, Erhard Milch, in charge of Luftwaffe aircraft production, foresaw the crisis that would ensue if fighter output was not given greater emphasis.

From 1940, many foreign workers, prisoners of war and slave labourers were brought to the Krupp works at Essen, already a city of 670,000 people.

There were 1,294 civilian casualties, 859 occurring at Neheim-Hüsten where 493 female Ukrainian slave labourers were drowned; another 58 bodies were found around the Eder Dam.

The British dispatched 18,506 sorties against targets marked by Oboe, accurate to 200 yd (180 m), dropping about 34,000 long tons (35,000 t) of bombs.

In May, Kammhuber made a claim for a night-fighter force of 2,160 aircraft, especially if the US bombers turned to night bombing but Hitler refused to listen.

Contemporary sources show that the bombing was a watershed in the development of the German war economy, which has been severely underestimated by later accounts.

[65] Having reorganised the steel allocation system, the German planners were forced into a large cut in the ammunition production programme to compensate.

All over Germany the destruction in the Ruhr caused shortages of parts, castings and forgings, a Zulieferungskrise (sub-components crisis), which reached far beyond heavy industry.

[66][67] In 1961, Webster and Frankland, in their official history The Strategic Air Offensive against Germany 1939–1945 (volume II), recorded that in February 1943, Bomber Command had an average of 593 crews and aircraft available for operations and 787 in August.

[69] A spare pumping system had been built for the Ruhr and Organisation Todt rapidly mobilized workers from the construction of the Atlantic Wall to make repairs.

Destruction of the Sorpe Dam would have caused significantly more damage but it was a stronger design and less likely to be breached and it had been made a secondary target.