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Operation Chastise was an attack on German dams carried out on 16–17 May 1943 by Royal Air Force No. 617 Squadron, later called the Dam Busters, using a purpose-built "bouncing bomb" developed by Barnes Wallis. The Möhne and Edersee dams were breached, causing catastrophic flooding of the Ruhr valley and of villages in the Eder valley; the Sorpe Dam sustained only minor damage. Two hydroelectric power stations were destroyed and several more damaged. Factories and mines were also damaged and destroyed. An estimated 1,600 civilians – about 600 Germans and 1,000 mainly Soviet forced labourers – died. Despite rapid repairs by the Germans, production did not return to normal until September.

Before the Second World War, the British Air Ministry had identified the industrialised Ruhr Valley, and especially its dams, as important strategic targets. In addition to providing hydroelectric power and pure water for steel-making, they supplied drinking water and water for the canal transport system. Calculations indicated that attacks with large bombs could be effective but required a degree of accuracy which RAF Bomber Command had been unable to attain when attacking a well defended target. A one-off surprise attack might succeed but the RAF lacked a weapon suitable for the task.

The mission grew out of a concept for a bomb designed by Barnes Wallis, assistant chief designer at Vickers. Wallis had worked on the Vickers Wellesley and Vickers Wellington bombers and while working on the Vickers Windsor, he had also begun work, with Admiralty support, on an anti-shipping bomb, although dam destruction was soon considered.

At first, Wallis wanted to drop a 10-long-ton (10 t) bomb from an altitude of about 40,000 ft (12,000 m), part of the earthquake bomb concept. No bomber aircraft was capable of flying at such an altitude or of carrying such a heavy bomb. A much smaller explosive charge would suffice if it exploded against the dam wall under the water, but German reservoir dams were protected by heavy torpedo nets to prevent delivery of an explosive warhead through water.

Wallis then devised a 9,000 lb (4,100 kg) bomb in the shape of a cylinder, equivalent to a very large depth charge armed with a hydrostatic fuse, but designed to be given backward spin of 500 rpm. Dropped at 60 ft (18 m) and 240 mph (390 km/h) from the release point, the bomb would skip across the surface of the water before hitting the dam wall. The residual backspin would cause the bomb to submerge, running it down the side of the dam towards its base.

The first trials were at Chesil Beach in January 1943, which demonstrated that a bomb of sufficient size could be carried by an Avro Lancaster, rather than waiting for a larger bomber such as the Windsor to come into service. Avro Chief Designer Roy Chadwick adapted the Lancaster to remove the mid-upper (dorsal) gun turret to reduce drag, increase speed and save fuel, bearing in mind that a large, ungainly weapon would be suspended below the fuselage. He also worked out the design and installation of controls and gear for the carriage and release of the bomb in conjunction with Barnes Wallis.

Air Vice-Marshal Francis Linnell at the Ministry of Aircraft Production thought the work was diverting Wallis from the development of the Windsor. Pressure from Linnell via the chairman of Vickers, Sir Charles Worthington Craven, caused Wallis to resign. Sir Arthur Harris, head of Bomber Command, after a briefing by Linnell also opposed the allocation of his bombers. Wallis had written to an influential intelligence officer, Group Captain Frederick Winterbotham, who ensured that the Chief of the Air Staff, Air Chief Marshal Charles Portal, heard of the project. Portal saw the film of the Chesil Beach trials and was convinced. 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. With eight weeks to go, the larger Upkeep bomb that was needed for the mission and the modifications to the Lancasters had yet to be designed.

Three aircrew from one aircraft parachuted but one later died from wounds and the others were captured. A crewman in another aircraft survived its crash. In total, therefore, 53 of the 133 aircrew who participated in the attack were killed, a casualty rate of almost 40 percent. Thirteen of those killed were members of the RCAF and two belonged to the RAAF.

Of the survivors, 34 were decorated at Buckingham Palace on 22 June, with Gibson awarded the Victoria Cross. There were five Distinguished Service Orders, 10 Distinguished Flying Crosses and four bars, two Conspicuous Gallantry Medals, eleven Distinguished Flying Medals and one bar.

Initial German casualty estimates from the floods were 1,294 killed, including 749 French, Belgian, Dutch and Ukrainian prisoners of war and labourers. Later estimates put the death toll in the Möhne Valley at about 1,600, including people who drowned in the flood wave downstream from the dam. After a public relations tour of America, and time spent working in the Air Ministry in London writing the book published as Enemy Coast Ahead, Gibson returned to operations and was killed on a Mosquito operation in 1944.

Following the Dams Raid, 617 Squadron was kept together as a specialist unit. A motto, Après moi le déluge ("After me the flood") and a squadron badge were chosen. According to Brickhill there was some controversy over the motto, with the original version Après nous le déluge ("After us the flood") being rejected by the Heralds as having inappropriate provenance (having been coined, reportedly, by Madame de Pompadour) and après moi le déluge having been said by Louis XV in an "irresponsible" context. The motto having been chosen by King George VI, the latter was finally deemed acceptable. The squadron went on to drop the Tallboy and Grand Slam bombs and attacked the German battleship Tirpitz, using an advanced bomb sight, which enabled the bombing of small targets with far greater accuracy than conventional bomb aiming techniques.

In 1977, Article 56 of the Protocol I amendment to the Geneva Conventions, outlawed attacks on dams "if such attack may cause the release of dangerous forces from the works or installations and consequent severe losses among the civilian population". There is however an exception if "it is used for other than its normal function and in regular, significant and direct support of military operations and if such attack is the only feasible way to terminate such support".