Known as Operation Chastise, the mission destroyed a number of dams, power stations, factories and mines in Germany, causing the deaths of 1,600 civilians and 53 RAF crew members. 

The importance of the targets involved – in particular, the Ruhr Valley’s dams – had been known to the British since before the war began. However, technology had not yet developed a large enough bomb that would also be able to accurately hit such a specific, and well-guarded, target. 

Several solutions were developed in the early years of the war, including the use of a gigantic bomb dropped from a high altitude. The RAF, however, did not have any planes that could either transport a bomb of the necessary weight nor reach the right height – estimated to be around 12,000 metres – to make such an attack viable. Barnes Wallis, assistant chief designer at Vickers, an engineering and armament firm, realised that a smaller bomb would manage to destroy a dam should it be exploded against the wall from beneath the water as opposed to being dropped on the wall from a height. 

German dams like those on the Ruhr Valley had been protected against such bombs through torpedo nets that sat in the water. If the British plan was to succeed, they would need to effectively skim the surface of the reservoir before hitting the wall, sinking to some depth, and exploding. 

By May 1942, work on such a bomb was underway and tests could begin. Later in the summer, it was clear that a bomb weighing 3,400kg would be big enough to destroy a dam should it sink to around nine metres below the surface. This, at last, was a viable weight for a bomber craft. 

The mission itself wasn’t to take place until the following May. A new squadron, Squadron X, was formed to undertake the operation, with crew members from Australia, Canada, New Zealand and the UK. 

On the night of 16 May 1943, the aircraft used two separate routes to reach their targets: the Möhne, Sorpe and Edersee dams. Not all the planes made it to their destinations, with one hitting the sea as a result of flying too low, another two striking electricity pylons or cables, and one being shot down. 

The bombing began, and despite flak from below, the Möhne Dam fell. Eder, which was less well-defended, took several attempts before being destroyed. The Sorpe Dam, by comparison, was always considered particularly difficult to demolish. In the end, it was only partially damaged. Two more planes were lost on the return flight. 

The human cost of Operation Chastise was heavy. Some 133 aircrew were involved, and only 80 made it home alive. Around 1,600 civilians in the valleys affected by the dams were killed, including hundreds of prisoners of war. Indeed, as of the 1970s, it is now considered a breach of the Geneva Convention to attack a dam if it will release “dangerous forces … and consequent severe losses among the civilian population”.