Before the Second World War, the British Air Ministry had identified the industrialised Ruhr Valley, especially its dams, as important strategic targets.

[6] Wallis devised a 9,000 lb (4,100 kg) bomb (more accurately, a mine) in the shape of a cylinder, equivalent to a very large depth charge armed with a hydrostatic fuse, designed to be given a backspin of 500 rpm.

Dropped at 60 ft (18 m) and 240 mph (390 km/h) from the release point, the mine would skip across the surface of the water before hitting the dam wall as its forward speed ceased.

A subsequent test suggested that a charge of 7,500 lb (3,400 kg) exploded 30 ft (9.1 m) under water would breach a full-size dam; crucially this weight would be within the carrying capacity of an Avro Lancaster.

[10][11] In February 1943, Air Vice-Marshal Francis Linnell at the Ministry of Aircraft Production thought the work was diverting Wallis from the development of the Vickers Windsor bomber (which did not become operational).

[13] On 26 February 1943, Portal over-ruled Harris and ordered that thirty Lancasters were to be allocated to the mission and the target date was set for May, when water levels would be at their highest and breaches in the dams would cause the most damage.

Led by 24-year-old Wing Commander Guy Gibson, a veteran of more than 170 bombing and night-fighter missions, twenty-one bomber crews were selected from 5 Group squadrons.

(The BBC documentary Dambusters Declassified (2010) stated that the pronged device was ultimately not used, owing to problems related to vibration, and that other methods were employed, including a length of string tied in a loop and pulled back centrally to a fixed point, in the manner of a catapult.)

Gibson also had VHF radios (normally reserved for fighters) fitted to the Lancasters so that he could control the operation while over the target,[15] an early example of what became the master bomber role.

3 was a mobile reserve consisting of aircraft piloted by Flight Sergeant Cyril Anderson, Flt Sgt Bill Townsend, Flt Sgt Ken Brown (RCAF), P/O Warner Ottley and P/O Lewis Burpee (RCAF), taking off two hours later on 17 May, either to bomb the main dams or to attack three smaller secondary target dams: the Lister, the Ennepe and the Diemel.

2 flew further north, cutting over Vlieland and crossing the IJsselmeer before joining the first route near Wesel and then flying south beyond the Möhne to the Sorpe River.

Flight Sergeant George Chalmers, radio operator on "O for Orange", looked out through the astrodome and was astonished to see that his pilot was flying towards the target along a forest's firebreak, below treetop level.

2 did not fare well: Munro's aircraft lost its radio to flak and turned back over the IJsselmeer, while Rice[17] flew too low and struck the sea, losing his bomb in the water; he recovered and returned to base.

1 lost Astell's bomber near the German hamlet of Marbeck when his Lancaster hit high voltage electrical cables and crashed into a field.

Hopgood's aircraft was hit by flak as it made its low-level run and was caught in the blast of its own bomb, crashing shortly afterwards when a wing disintegrated.

[19] The Eder Valley was covered by heavy fog, but the dam was not defended with anti-aircraft positions as the difficult topography of the surrounding hills was thought to make an attack virtually impossible.

Due to various problems, only two Lancasters reached the Sorpe Dam: Joe McCarthy (in T for Tommy, a delayed aircraft from the second wave) and later Brown (F for Freddie) from the third formation.

This attack differed from the previous ones in two ways: the 'Upkeep' bomb was not spun, and due to the topography of the valley the approach was made along the length of the dam, not at right angles over the reservoir.

Anderson (Y for York) never arrived having been delayed by damage to his rear turret and dense fog which made his attempts to find the target impossible.

[31] Initial German casualty estimates from the floods were 1,294 killed, including 749 French, Belgian, Dutch and Ukrainian prisoners of war and labourers.

[39] According to an article by German historian Ralf Blank [de],[40] at least 1,650 people were killed: around 70 of these were in the Eder Valley, and at least 1,579 bodies were found along the Möhne and Ruhr rivers, with hundreds missing.

Of the bodies found downriver of the Möhne Dam, 1,026 were foreign prisoners of war and forced labourers in different camps, mainly from the Soviet Union.

Worst hit was the city of Neheim (now part of Neheim-Hüsten) at the confluence of the Möhne and Ruhr rivers, where over 800 people perished, among them at least 493 female forced labourers from the Soviet Union.

[44] Barnes Wallis was also of this view; he revealed his deep frustration that Bomber Command never sent a high-level bombing force to hit the Möhne dam while repairs were being carried out.

He argued that extreme precision would have been unnecessary and that even a few hits by conventional HE bombs would have prevented the rapid repair of the dam which was undertaken by the Germans.

The considerable amount of labour and strategic resources committed to repairing the dams, factories, mines and railways could not be used in other ways, on the construction of the Atlantic Wall, for example.

The pictures of the broken dams proved to be a propaganda and morale boost to the Allies, especially to the British, still suffering from the German bombing of the Baedeker Blitz that had peaked roughly a year earlier.

[28] Even within Germany, as evidenced by Gauleiters' reports to Berlin at the time, the German population regarded the raids as a legitimate attack on military targets and thought they were "an extraordinary success on the part of the English" [sic].

Work on the earthquake bombs resulted in the Tallboy and Grand Slam weapons, which caused damage to German infrastructure in the later stages of the war.

They rendered the V-2 rocket launch complex at Calais unusable, buried the V-3 guns, and destroyed bridges and other fortified installations, such as the Grand Slam attack on the railway viaduct at Bielefeld.