Seventy years ago an RAF bomber raid destroyed important German dams. At the time many argued it was only a propaganda victory. It was much more than that, writes historian Dan Snow.
At 9.28pm on 16 May 1943, the first of 19 Lancaster heavy bombers lifted off the runway into a clear, still early summer night.
It was another British raid on the Ruhr region of Germany. The industrial heartland of Hitler's war machine was straining to produce tanks, ammunition and aircraft for a final, titanic assault on the Soviet Red Army on the Eastern Front.
British aircraft had been levelling entire neighbourhoods, blasting and incinerating homes, factories and people in a series of massive but clumsy blows.
This raid was different. This was a raid aimed with astonishing precision against a choke point in Germany's production chain. As such it was the ancestor of today's "smart bombs" and surgical strikes.
It was a raid sent to destroy a series of mighty dams, wreaking havoc with the Ruhr's vital water supplies. Known as Operation Chastise to its planners, it is remembered simply as the Dambusters raid.
The story of the Lancasters that left RAF Scampton that night is utterly remarkable for so many reasons. There was the ingenuity of the weapon they carried - a purpose-built bomb, codenamed Upkeep, designed by the brilliant Barnes Wallis to bounce along the surface of water like a skimming stone to avoid obstacles placed in its way.
The skill and bravery of the pilots who flew at night, at 100ft (30m) or less over enemy territory is breathtaking. They flew so low that one hit the sea, which tore off the underslung bomb, and scooped up seawater into the fuselage, while another was engulfed in flames as it ploughed straight into high voltage electricity cables.
The aircraft that did make it to the dams pressed home their attacks with a reckless disregard for their own safety. The results certainly impressed the world at the time - two dams were breached, and a third damaged.
As flood water surged down the valleys, factories and infrastructure were badly affected. The combination of science, flying skill, grit and the obvious impact of the raids made it front page news around the world and turned the Dambusters into celebrities.
The post-war film, enduringly popular, cemented the raid in the popular consciousness. Yet this celebration of the raid provoked a backlash. Experts such as Sir Charles Webster and Noble Frankland - the official historians of the Strategic Air Offensive - believed that it was oversold, its achievements exaggerated and other Bomber Command raids unfairly ignored.
These voices point to the speed at which the dams were repaired, and production of energy, steel and other armaments resumed. British planners had known that the success of the raid largely depended on the German ability to rebuild the dams in time to store up the autumn rains.
The Germans certainly rose to the challenge: the dams, which had taken five years to build, were repaired by armies of forced labourers working around the clock in just five months.
A major hydroelectric power station at Herdecke was out of action for weeks, not months, thanks to a similarly Herculean effort. Thousands of troops, Hitler youth, prisoners of war and enslaved workers were thrown at the task.
Canals were dredged, factories rebuilt, river banks reinstated, bridges replaced. Britain's bomber supremo, Sir Arthur Harris, who had opposed the raid as harebrained all along, with some justification, wrote later: "I have seen nothing... to show that the effort was worthwhile except as a spectacular operation."
Senior Nazis downplayed the damage after the war. Albert Speer, the German armaments minister, expressed amazement that the repair operations were left untroubled by further bombing raids which would have delayed the vital reconstruction and turned a nuisance into a major crisis.
Time has thrown up a wealth of information about the impact of the raids, much of it unavailable to an earlier generation of historians.
In James Holland's recent book, Dam Busters: The Race to Smash the Dams, he states that "it is time to put the record straight". He insists that the damage was "absolutely enormous" and it was "an extraordinary achievement".
He points out that every bridge for 30 miles below the breached Mohne dam was destroyed, and buildings were damaged 40 miles away. Twelve war production factories were destroyed, and around 100 more were damaged. Thousands of acres of farmland were ruined.
Germans instantly referred to it after the raid as the "Mohne catastrophe". Even the cool Speer admitted that it was "a disaster for us for a number of months". German sources attribute a 400,000-tonne drop in coal production in May 1943 to the damage caused.
Another German report into the effects of the raid talked about "considerable losses of production" caused by "the lack of water" and that "many shaft mines, coking plants, smelting works, power stations, fuel plants and armaments factories were shut down for several days".
The fact that a titanic effort was made to repair this damage shows how high a priority the dams were, and it meant resources were shifted from elsewhere. Nowhere was this costlier to the Third Reich than on the beaches of Normandy.
Hitler had ordered the construction of a massive network of defences against an Allied invasion. Now thousands of workers who should have been toiling in France were redirected to the Ruhr to repair the dams. A year later allied troops would have faced far more significant defences had it not been for the Dambusters raid.
No raid mounted by so few aircraft had ever caused such extensive material damage. It did not bring German war production to a permanent halt, but nobody had expected it to.
Its critics talk of its propaganda impact as if wars are fought by dispassionate robots rather than soldiers, workers and politicians with all the irrational cauldron of human emotions. Propaganda, as Churchill knew so well, is as much a part of war as killing enemy soldiers.
The most important impact of the Dambusters raid may indeed have been in convincing people on both sides that the Allies were winning, and that, often, is how wars are won and lost.