'Chastise' was the British air attack by Avro Lancaster four-engined heavy bombers of No. 617 Squadron of Air Chief Marshal Sir Arthur Harris’s RAF Bomber Command, under the command of Wing Commander Guy Gibson, against the Möhne, Eder and Sorpe dams in north-western Germany (16/17 May 1943).

'Chastise' was schemed on the basis of the specially developed 'bouncing bomb' invented and developed by Barnes Wallis of the Vickers company. The raid succeeded in breaching the Möhne and Eder dams, causing catastrophic flooding of the Ruhr river valley and many villages in the Eder river valley; the Sorpe dam sustained only minor damage. Two hydroelectric power stations were destroyed and several more were damaged, factories and mines were also either damaged or destroyed, and it is believed that some 1,600 people were drowned. However, the significance of the damage was lessened by the speed with which the Germans effected repairs, and local industrial manufacture returned to normal during September.

Before the start of World War II, the British Air Ministry had identified Germanys heavily industrialised Ruhr river valley, and especially its dams, as important strategic targets. These provided hydro-electric power and pure water for steel-making, and also supplied drinking water and water for the canal transport system of the area. Methods for effective attacks on the dams had been considered and worked out with care, and calculations suggested that repeated hits with large bombs could be effective, but needed a degree of accuracy which RAF Bomber Command could not currently provide, especially in the face of the German defences.

'Chastise' emerged from the concept for a bomb designed by Barnes Wallis and developed by his team at Vickers, where Wallis was assistant chief designer. Wallis had worked on the company’s Wellesley and Wellington bombers, for whose geodetic structures he was largely responsible, and while working on the company’s Windsor he had also begun work, with support of the Admiralty, on a bomb designed initially for attacking ships, although dams were soon added as another type of target for which the bomb would be suitable.

Wallis’s initial concept was based on the release of a 22,000-lb (9979-kg) bomb from an altitude of about 40,000 ft (12190 m). This idea was part of Wallis’s own earthquake bomb concept: the bomb was designed to accelerate to transonic speed under the force of gravity so as to penetrate deep into the earth before detonating and causing shock waves possessing the destructive effect of an earthquake. At that time there was no bomber capable of flying at the required altitude with so great a payload, however, so Wallis’s thoughts turned of necessity to a bomb with a considerably lighter explosive payload, though this would have to be detonated directly against the dam wall below the waterline. The most obvious weapon for this task was the heavyweight anti-ship torpedo, but the important German reservoir dams were protected by anti-torpedo nets to prevent such an attack, however.

Wallis’s breakthrough overcame this difficulty by the creation of a drum-shaped bomb (for practical purposes specially designed heavy depth charge), which would be spun backward at more than 500 rpm before being dropped at a sufficiently low altitude and at the correct speed to skip for some distance along the surface of the water in a series of bounces before reaching the dam wall, where its residual spin would run the bomb down the side of the dam to the depth set on the hydrostatic fuse would detonate the weapon at 30 ft (9.1 m), with the water containing much of the blast and ensuring that this was directed into the dam wall. Thus an accurate drop would bypass the dam’s defences and ensure that the bomb exploded right against the dam.

Initial evaluation of the basic concept included the blowing up a plaster model dam at the Building Research Establishment, Watford, during May 1942 and then the breaching of the disused Nant-y-Gro dam in Wales in July of the same year. The first full-scale trials were carried out at Chesil Beach in January 1943, and demonstrated that a bomb of sufficient size could be carried by the Lancaster, which was RAF Bomber Command’s current mainstay heavy bomber, and that it was therefore not necessary to await the advent of a larger bomber such as the Windsor.

Air Vice Marshal Francis Linnell at the Ministry of Aircraft Production thought that the 'bouncing bomb' work was diverting Wallis from the development of the Windsor, and persuaded Sir Charles Worthington Craven, the chairman of Vickers, to speak about the matter to Wallis, who resign. As a result of a briefing from Linnell, Harris, commanding RAF Bomber Command, also opposed the use of his bombers in this mission. Wallis had written to an intelligence officer, Group Captain Frederick Winterbotham, about the matter, however, and Winterbotham ensured that the Chief of the Air Staff, Air Chief Marshal Sir Charles Portal, heard of the project. Portal watched the film of the Chesil Beach trials, was convinced and, overriding Harris, on 26 February ordered the allocation of 30 Lancaster bombers to the mission, with a target date in May as the water levels behind the dams would then be at their highest and breaches in the dams would cause the most damage. With eight weeks to go, the larger 9,250-lb (4196-kg) 'Upkeep' bomb (filled with 6,600 lb/2994 kg of Torpex explosive) needed for the mission, and the modifications to the Lancaster bombers, had yet to be designed.

The implementation of 'Chastise' was allocated to Air Vice Marshal E. A. B. Rice’s No. 5 Group, which formed a new squadron to undertake the operation. This was initially called Squadron X, as the speed of its formation outstripped the RAF process for naming squadrons, but later became No. 617 Squadron. Led by Gibson, a veteran of more than 170 bombing and night-fighter missions, the squadron came into existence with 21 crews selected from the existing squadrons of No. 5 Group. These crews included RAF personnel of several different nationalities, as well as members of the Royal Australian Air Force, Royal Canadian Air Force and Royal New Zealand Air Force, who were frequently attached to RAF squadrons. The new squadron was based at RAF Scampton, about 5 miles (8 km) to the north of Lincoln.

The targets selected were the Möhne and Sorpe dams, which were the two key dams upstream from the Ruhr industrial area, with the Eder dam on the Eder river, which feeds into the Weser river, as a secondary target. While the loss of hydro-electric power was important, the loss of water supply to industry, cities and canals would have greater effect, and there was also the potential for devastating flooding if the dams broke.

The aircraft were Lancaster B.Mk III bombers modified to the Lancaster B.Mk III Special (Type 464 Provisioning) standard. To reduce weight, much of the internal armour was removed, as was the dorsal machine gun turret. The size and unusual shape of cylindrical bomb meant that the weapon bay doors had to be removed, and the bomb itself was hung, partially exposed to the airstream, in the lower fuselage between two outward-hingeing crutches, and before being dropped was spun up to speed by an auxiliary motor.

Bombing from an altitude of 60 ft (18 m), at a speed of 240 mph (390 km/h) and at a pre-selected distance from the target called for expert crews, and intensive night-time and low-altitude flight training began.

There were also technical problems which had to be solved, the first being a method to fix the moment when the bomber was at the optimum distance from its target. Both the Möhne and Eder dams had towers at each end, and this allowed the simple creation of a special targeting device with two prongs, making the same angle as the two towers at the correct distance from the dam, and this showed when to release the bomb. The second problem was how to determine the bomber’s exact height above the water, for the barometric altimeters then in use lacked sufficient accuracy. Two spotlights were therefore mounted, one under the bomber’s nose and the other under the fuselage, so that at the correct height their light beams converged on the surface of the water. The crews practised at the Eyebrook Reservoir, near Uppingham in Rutland; Abberton Reservoir near Colchester in Essex; Derwent Reservoir in Derbyshire; and Fleet Lagoon on Chesil Beach. The bomb itself was first tested at the Elan Valley reservoirs.

The bombs were delivered to the squadron on 13 May, after the final tests on 29 April. At 18.00 on 15 May, Gibson and Wallis briefed four key officers: the squadron’s two flight commanders, Squadron Leader Henry Maudslay and Squadron Leader H. M. 'Dinghy' Young; Gibson’s deputy for the Möhne attack, Flight Lieutenant John Hopgood; and the squadron bombing leader, Flight Lieutenant Bob Hay. The rest of the crews were told at a series of briefings on the following day, which began with a briefing of pilots, navigators and bomb-aimers at about 12.00.

The squadron was divided into three formations. Formation No. 1 comprised nine aircraft in three groups: listed by pilot, these were Gibson, Hopgood and Flight Lieutenant H. B. 'Micky' Martin (an Australian serving in the RAF); Young, Flight Lieutenant David Maltby and Flight Lieutenant David Shannon (RAAF); and Maudslay, Flight Lieutenant W. Astell and Flying Officer L. Knight (RAAF). Formation No. 1’s mission was to attack the Möhne dam, and any aircraft still with a bomber were then attack the Eder dam.

Formation No. 2, numbering five aircraft piloted by Flight Lieutenant Joseph McCarthy (an American serving in the RCAF), Pilot Officer Vernon Byers, Flight Lieutenant Robert Barlow (RAAF), Pilot Officer Geoffrey Rice and Flight Lieutenant Les Munro (RNZAF), was to attack the Sorpe dam.

Formation No. 3 was the mobile reserve and comprised the aircraft flown by Flight Sergeant Cyril Anderson, Flight Sergeant William Townsend, Flight Sergeant Ken Brown (RCAF), Pilot Officer Warner Ottley and Pilot Officer Lewis Burpee (RCAF). Taking off two hours later on 17 May, the aircraft of Formation No. 3 were to attack either the main dams or three smaller secondary target dams: the Lister, Ennepe and Diemel dams.

Two crews were unable to make the mission as a result of illness.

The operations room for 'Chastise' was at the headquarters of No. 5 Group in St Vincents Hall, Grantham, Lincolnshire. The mission codes, transmitted in Morse, were 'Goner' meaning bomb dropped, 'Nigger' meaning that the Möhne dam had been breached and 'Dinghy' meaning that the Eder dam had been breached. Nigger was the name of Gibson’s dog, a black labrador retriever, which had run over and killed on the morning of the attack. Dinghy was Young’s nickname, a reference to the fact that he had twice survived crash landings at sea and he and his crew had been rescued from from an inflatable rubber dinghy on each occasion.

For thee outward leg of their mission, the bombers used two routes, carefully selected to avoid known Flak concentrations, and were timed to cross the coast of German-occupied Europe simultaneously. The first aircraft, those of Formation No. 2 and flying the longer, northern route, took off at 21.28. McCarthy’s bomber developed a coolant leak and he took off in the reserve aeroplane 34 minutes late. Formation No. 1 took off in groups of three at 10-minute intervals from 21.39, and the reserve formation did not begin taking off until 00.09 on 17 May.

Formation No. 1 crossed the coast into occupied Europe between Walcheren and Schouwen, flew over the Netherlands, skirted the air bases at Gilze-Rijen and Eindhoven, curved around the Ruhr defences, and turned north to avoid Hamm before turning south to head for the Möhne river.

Formation No. 2 flew farther to the 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.

The bombers flew at very low height, at an altitude of only about 100 ft (30 m), to avoid radar detection and thus the attentions of the night-fighters of General Josef Kammhuber’s XII Fliegerkorps defending this area of Germany.

The first casualties were suffered soon after the bombers reached the Dutch coast. Formation No. 2 did not fare well: Munro’s aeroplane lost its radio to Flak and turned back over the IJsselmeer, while Rice flew too low and struck the sea, losing his bomb in the water; he recovered and returned to base. Barlow’s and Byers’s bombers crossed over the coast in the area of Texel island. Byers’s bomber was shot down by Flak soon after this, and crashed into the Waddenzee. Barlow’s aeroplane hit electricity pylons and crashed some 3.1 miles (5 km) near Haldern, to the east of Rees. Only the delayed bomber flown by McCarthy survived to cross the Netherlands. Formation No. 1 lost Astell’s bomber near the German hamlet of Marbeck when it flew into high-voltage electrical cables and crashed into a field.

The aircraft of Formation No. 1 arrived over the Möhne reservoir and Gibson’s G-for-George bombed first with Hopgood’s M-for-Mother attacking second. Hopgood’s aeroplane was hit by Flak as it made its low-level run and was then caught in the blast of its own bomb and destroyed. Martin’s P-for-Peter bombed third, and though hit made a successful attack. Then Young’s A-for-Apple made a successful run and after him came Maltby’s J-for-Johnny, and then, finally, the dam was breached.

Gibson then led Young, Shannon, Maudslay and Knight to the Eder dam. The valley of the Eder river was foggy but undefended. The tricky topography of the surrounding hills made the approach difficult and the first aeroplane, Shannon’s L-for-Leather, made six runs before taking a break. Maudslay’s Z-for-Zebra then attempted a run but the bomb struck the top of the dam and the Lancaster was caught in the blast. Shannon made another run and successfully dropped his bomb. The final bomb of the formation, from Knight’s N-for-Nut, breached the dam.

McCarthy’s T-for-Tom reached the Sorpe dam alone. This was the dam considered the least likely to be breached as it was a vast earth structure rather than the concrete construction like the two dams which had been attacked successfully. Despite the mist and ill-placed hills, McCarthy’s aeroplane successfully dropped its bomb but did not breach the dam.

Three of the reserve aircraft were directed to the Sorpe dam. Burpee’s S-for-Sugar did not reach the dam, having been shot down over the Netherlands. Brown’s F-for-Freddy reached the dam and in increasingly dense mist finally dropped his bomb without breaking the dam. Anderson’s Y-for-Yorker arrived last, but by this time the mist was too dense for him even to attempt the run.

The remaining two aircraft were sent to subsidiary targets: Ottley’s C-for-Charlie was shot down en route, and Townsend’s O-for-Orange successfully dropped its bomb on the Annexe.

On the way back, flying again at tree-top level, the attacking force lost another of its aircraft, when Young’s machine was hit by Flak and crashed into the North Sea just off the coast of the Netherlands.

In all, 53 of the 133 aircrew were killed, and three bailed out to be taken prisoner.

The two direct hits on the Möhne dam resulted in a breach about 250 ft (76 m) wide and 292 ft (89 m) deep. The destroyed dam poured around 330 million tons of water, into the western Ruhr region. A torrent of water some 32.5 ft (10 m) high swept at about 15 mph (24 km/h) through the valleys of the Möhne and Ruhr rivers. A few mines were flooded; 11 small factories and 92 houses were destroyed; and 114 factories and 971 houses were damaged. The floods washed away about 25 roads, railways and bridges as the flood waters spread for around 50 miles (80 km) from the source. Estimates show that before 15 May 1943 water production on the Ruhr was 1 million tonnes, but this dropped to a quarter of that figure after the raid.

The Eder river drains toward the east into the Fulda river, which runs into the Weser river and thus into the North Sea. The main purpose of the Edersee was to serve as a reservoir to keep the Weser and the Mittellandkanal navigable during the summer months. The wave from the breach was not strong enough to result in significant damage by the time it hit Kassel, about 21.75 miles (35 km) downstream of the breach.

The greatest impact on the armaments production of the Ruhr region was the loss of hydroelectric power. Two power stations (producing 5100 kW) associated with the dam were destroyed and seven others were damaged. This resulted in a loss of electrical power in the factories and many households in the region for two weeks. In May 1943 coal production dropped by 400,000 tons which German sources attributed to the effects of the raid.

Recent research indicates that at least 1,650 people were killed: around 70 in the Eder river valley, and at least 1,579 bodies were found along the Möhne and Ruhr rivers, with hundreds more missing. Some 1,026 of the bodies found downriver of the Möhne dam were foreign prisoners of war and forced labourers in different camps, mainly from the USSR. Worst hit was the town of Neheim at the confluence of the Möhne and Ruhr rivers, where more than 800 people died, among them at least 493 female forced labourers from the USSR.

In overall terms, though, 'Chastise' did not exercise the military effect believed at the time. By 27 June, full water output had been restored as a result of an emergency pumping scheme which had been inaugurated only in the previous year, and the electricity grid was again producing power at full capacity.

The value of 'Chastise' can perhaps best be seen as a very real boost to British morale, though its effects on German food production was significant, with many square kilometres of arable land washed away and effectively unusable until the 1950s. There was also a great loss of farm animals bred for food.

Like many British air raids, 'Chastise' had been undertaken with a view to keep drawing a sizeable proportion of the German defensive effort back into Germany and away from actual and potential theatres of ground war, a policy which culminated in the Berlin raids of the winter of 1943/44. In May 1943 this meant drawing the Luftwaffe and anti-aircraft defence forces away from the USSR, and early in 1944 it meant clearing the way for the aerial side of the forthcoming 'Overlord'. The very considerable labour and strategic resources committed to repairing the dams, factories, mines and railways could not be used for other tasks such as the construction of the 'Atlantic Wall', for example.

The aerial reconnaissance photographs of the broken dams proved to be a propaganda and morale boost to the Allies, especially to the British. An associated, but equally major, effect was that Wallis’s ideas on earthquake bombing, previously rejected, now became accepted by Harris. Before 'Chastise', it had been British practice to 'area bomb' with many light bombs, in the hope that one would hit the target. Work on the earthquake bomb theory now resulted in the 12,000-lb (5443-kg) 'Tallboy' and 22,000-lb (9979-kg) 'Grand Slam' weapons, which caused unprecedented damage to German infrastructure in the later stages of the war. They rendered V-2 assembly buildings unusable, buried the V-3 guns, sank the battleship Tirpitz and destroyed many bridges, canals and hardened installations. Notable amongst their successes were the U-boat pens at Brest, where they penetrated 20 ft (6.1 m) thick roofs of reinforced concrete, the Saumur tunnel and the Dortmund-Ems canal.