The scene was apocalyptic in its scale of devastation. As the RAF bombers released their cargo of incendiaries on the night of 27 July 1943, the northern German port of Hamburg was engulfed in one of the worst firestorms in history. The flaming mass of bombs sent warm air soaring thousands of feet into the sky, creating a vacuum at ground level that was filled by winds gusting at 150mph. These tornadoes not only fanned the conflagration, but also scythed through almost everything in their path.
Trees were uprooted, buildings were destroyed and people were hurled through the air. Some burned to death in the street or in their homes from the sheer intensity of the heat. Other victims, screaming in agony, were stuck in the roadways as the asphalt turned to boiling liquid.
Thousands were asphyxiated by lack of oxygen, or died of smoke inhalation as they sought shelter in cellars. Those who made it to the rivers or canals fared little better: the scorching heat continued to suck oxygen out of the air, while the fire was spread across water surfaces by exploding oil tankers and the debris of burning coal barges.
The next morning, long after the bombers had left, much of Hamburg was a smouldering wilderness of death. The smoke was so thick that it blotted out the sun. There were corpses everywhere, many of them little more than twisted, blackened remnants of humanity. Occasionally, to the anguish of the rescue parties, a naked figure, charred beyond recognition, might emit a faint sound of life.
The raid that caused this inferno was one of a series of Allied attacks carried out against Hamburg in late July 1943, leaving roughly 45,000 people dead, including 21,000 women and 8,000 children. In addition, 1.2 million refugees left the city in the immediate aftermath of the attacks, many of them mentally and physically scarred. One distraught mother was found to be carrying her dead infant in her suitcase.
In terms of civilian slaughter, the Hamburg campaign of mid-1943 was the most destructive assault by the Royal Air Force in the entire Second World War. The death toll was higher than that inflicted in the Dresden raid of February 1945, where as many as 25,000 people were estimated to have been killed. ΣΤΗΝ ΔΡΕΣΔΗ ΟΙ ΝΕΚΡΟΙ ΕΙΝΑΙ 600.000 {ΕΞΑΚΟΣΙΕΣ ΧΙΛΙΑΔΕΣ}! 
Yet, for all their historic lethalness, neither the Hamburg nor the Dresden raid was a departure from the RAF bombing strategy that prevailed from early 1942 until the end of the war. Far from being unfortunate or freak occurrences, they represented the ultimate fruition of British air policy. Bomber Command's entire strategic offensive seems to have been based on the belief that the Nazi regime could be destroyed through wholesale, indiscriminate killing of Germany's urban population.
Night after night, the RAF pounded the residential districts of German cities, its deadly effectiveness increasing as the bomber fleet, led by the mighty, four-engined Avro Lancasters, grew in size and the Reich's defences weakened. On the night of 23 February 1945, 367 Lancasters dropped 1,825 tons of bombs on the town of Pforzheim, causing another firestorm that killed 17,600 people, a quarter of the population. This was proportionately a higher casualty rate than at Nagasaki in Japan, where the second atomic bomb was dropped a few months later. In March 1945, 225 Lancasters dropped 1,127 tons of bombs on the medieval city of Würzburg in a raid lasting just 17 minutes. More than 5,000 people died, 66 per cent of them women and 14 per cent children. "The inner core of Würzburg had become a cauldron of fire. The roar was deafening and the smoke suffocating," wrote one witness.
Both during and after the war, the government maintained that it was never Britain's policy to carry out carpet bombing of civilian targets. "We have always adhered firmly to the principle that we attack none but military objectives," declared Archibald Sinclair, the secretary of state for air, in the Commons in October 1943. The mounting toll of civilian deaths was presented as a regrettable consequence of raids against factories, energy plants, transport networks or military installations, not as an end in itself.
Even after victory was achieved, this unconvincing line was maintained. In one lecture, Charles Portal, the chief of the Air Staff for most of the war, said that it was "a curious and widespread fallacy that our bombing of the German cities was really intended to kill and frighten Germans and that we camouflaged this intention by the pretence that we would destroy industry. Any such idea is completely and utterly false. The loss of life, which amounted to some 600,000 killed, was purely incidental." But as a study of wartime archives demonstrates, both Sinclair and Portal were being dishonest with the public.
 Urban destruction through "concentrating bomb-loads on the densest and most vulnerable areas of cities", to quote one Air Staff paper, was the primary goal of Brit ain's air offensive over Germany.
It had not always been that way. After war had been declared on 3 September 1939, RAF Bomber Command mounted only occasional, sporadic raids against German military or naval targets, and pilots were under strict instructions not to put civilian lives at risk. "Indiscriminate attacks on civilian populations as such will never form part of our policy," the RAF's director of plans, John Slessor, pronounced with an air of moral certitude. The depths of caution were well illustrated by an episode in the war cabinet in 1939, when one minister suggested mounting a raid on a German armaments works. Kingsley Wood, then the air secretary, professed his outrage, saying that he would not countenance an attack on "private property".
There were a number of factors that led Britain's military and political leaders privately to adopt the policy of urban bombing from early 1941, even if the strategy was never openly declared in public. One was the Luftwaffe's Blitz against British cities, which began in September 1940 and was symbolised by the notorious raid on Coventry in November that left nearly 600 people dead.
In a sense, the bombing of Germany was a sustained act of revenge for these atrocities. Arthur Harris, the head of RAF Bomber Command, frequently justified the air offensive by adapting the Old Testament quotation, whereby the Germans had "sown the wind" and therefore deserved to "reap the whirlwind". Another factor was the disastrous failure of Britain's armed forces on almost every front, from continental Europe to the Far East. Bombing Germany's cities seemed the only way to hit back at the Reich.
Because the bombers had to use the darkness of night as protective cover against German defences, their aiming could not be accurate, so the government made a virtue of necessity by declaring that whole cities, rather than specific sites, were targets. The arrival of the Lancaster, which first went into service in December 1941, transformed the threat of the bomber fleet with its phenomenal ability to carry huge loads. Designed by the far-sighted Avro engineer Roy Chadwick, it was a weapon of unique menace in the European theatre, capable of lifting a bomb weighing ten tons in its enormous bomb bay, stretching two-thirds of the way along the fuselage. Without the Lancaster, it would never have been possible to mount an effective offensive at all.
Arthur Harris, who first took up his post at Bomber Command in February 1942, was also vital in adopting an aggressive bombing policy. Ferociously independent, stubborn and dogmatic, Harris was a veteran of the First World War and his experience of the Western Front had filled him with loathing for the idea of another land campaign in Europe. He passionately believed that the way to defeat Germany was not by invasion, but by smashing its economy with repeated hammer blows at its industrial workforce.
“The cities of Germany, including their working populations, are literally the heart of Germany's war potential," he once wrote. With his hatred of the Reich, he rarely had the slightest hesitation about inflicting carnage. "What we want to do, in addition to the horrors of fire, is to bring masonry crashing down on top of the Boche, to kill the Boche and to terrify the Boche," he told the Air Staff. Sometimes this belligerent spirit revealed itself in a morbid sense of humour. On one occasion he was being driven at high speed in his black Bentley to London from Bomber Command headquarters in High Wycombe when his car was stopped by the police.
“You could have killed someone," said the aggrieved constable through Harris's window. "Young man, I kill thousands of people every night," was his laconic reply.
Harris was ruthless in implementing the bombing strategy but, until the last stages of the war, he had the full support of his political and military superiors. Indeed, as unpublished papers show, the air ministry and Air Staff had taken an uncompromising stance towards the German population long before Harris's appointment. Typical was a paper, now in the archives of Cambridge University, written in August 1941 by the bombing operations directorate of the air ministry. 
This argued that the focus of future British attacks must be "the people in their homes and in factories, also the services such as electricity, gas and water upon which the industrial and domestic life of the area depends".
Warming to this theme, the directorate then found support for such theories in the Luftwaffe's bombing of Coventry. To most Britons, this attack had been an outrage. To the Air Staff, it was an inspiration. The assault on Coventry, argued the paper, was "one of the most successful raids carried out by the German Air Force on this country", with a ton of high explosive and incendiaries for every 800 citizens. "If Bomber Command could carry out a raid on the Coventry scale every month, the result would be a complete state of panic in the industrialised west of Germany", as well as "considerable loss of life and limb, widespread destruction and damage to the houses of workers".
This was the outlook that existed throughout the top of the RAF and the government, contrary to public denials. In September 1941, Norman Bottomley, the deputy chief of the Air Staff, urged "saturation by incendiaries" to achieve the twin objectives of breaking "the morale of the population" and making "people conscious of constant physical danger". His boss, Charles Portal, was just as hardline, telling Winston Churchill in late 1941 that if Bomber Command were provided with a force of 4,000 planes, huge damage could be inflicted on Germany, including the destruction of six million homes and "civilian casualties estimated at 900,000 killed". Already in 1941, wrote Portal to the prime minister: "We have caused death and injury to 93,000 civilians. The result was achieved with a fraction of the bomb-load we hope to employ in 1943."
Portal largely got his wish, as bomber production was hugely stepped up. By the end of the war, no fewer than 7,377 Lancasters had been produced, operating in a force of 56 squadrons. In private, the Air Staff had no compunction about using the term "terror bombing" to describe their strategy, as is clear in one paper, from the bombing operations directorate in January 1945, which set out the case for a "spectacular catastrophe" from the air to break the will of the German population in the east. Code-named Operation Thunderclap, such an assault would adhere to "the basic principle of true morale bombing", which was "to provoke a state of terror by air attack". It was this mentality that led, a few weeks later, to the raid by 796 Lancasters on Dresden.
The fierce controversy over Dresden, which flared up in the final months of the war, did not perturb Harris. With characteristic bullishness, he remarked: "I do not personally regard the whole of the remaining cities of Germany as worth the bones of one British Grenadier." But what had angered him about the Air Staff was the refusal to be open with the public about strategic bombing. Brutally frank himself, he despised the euphemisms and evasions that his superiors used to cover up the reality.
“The aim of Bomber Command should be unambiguously and publicly stated," he wrote in 1943. "That aim is the destruction of German cities, the killing of German workers and the disruption of civilised life throughout Germany." He wanted the government to declare its commitment to "the creation of a refugee problem on an unprecedented scale", and he expressed his contempt of the official eagerness "to downplay the obliteration of German cities and their inhabitants".
One of Harris's arguments in favour of greater openness was that the cover-up was an insult to the heroic men who flew in his command. They risked their lives in the hostile night skies over Germany, he said, yet the government seemed to regard their missions as so "disreputable" that their purpose could not be mentioned in public. The failure to be clear about the RAF's objective "to eliminate entire cities" would, he warned, "inevitably affect adversely the morale of crews and I would urge that this rather than the appeasement of sentimental and humanitarian scruples should be our primary consideration". But Harris never got the unequivocal statement he wanted from the government.
Whatever the debate about the morality of urban bombing, Harris was absolutely correct in his praise for the Bomber Command aircrews, which showed the most remarkable courage in the face of daunting odds. Of the 125,000 men who served in the command during the war, no less than 44 per cent were killed in action. When the campaign was at its most dangerous in early 1944, the chances of a bomber crewman surviving were lower than those for a British soldier at the Battle of the Somme in 1916. Yet even in the face of anti-aircraft fire, blinding searchlights and menacing fighters, the aircrews continued on their missions. "All war is brutal," Harris told his group commanders soon after taking over at High Wycombe. "It is going to be a damned sight more brutal still. The fact remains that if there are any weaker brethren who cannot stomach it, the sooner we dispose of them, the better." Hardly any of his recruits showed that they did not have the stomach for the job.Yet the tragedy of the bomber war is that there could have been an alternative strategy, one made possible by the brilliance of the Lancaster and the reliability of its crews. In a host of daring raids, most notably the Dambusters attack of May 1943, the Lancaster proved that it could be a rapier as well as a bludgeon.
In the Dams raid, led by Guy Gibson, the Lancasters succeeded in dropping their bouncing bombs from just 60 feet above the water to an accuracy of a few yards, even while under fierce anti-aircraft fire. Other precision attacks, such as those on the French railway system in the run-up to D-Day in 1944, also achieved striking results, the Lancaster's effectiveness enhanced by the introduction of ever more sophisticated navigational technology and bombsights. For all its unparalleled lifting capacity, the Lancaster was also highly manoeuvrable and fast, capable of over 300mph in a dive. "You couldn't break that aircraft. You could pull it about and do steep turns and all that. The Lancaster was magnificent," said one pilot.
But neither the virtues of Chadwick's design nor the skills of the bomber crews were ever fully exploited, because of Harris's myopic obsession with the mass killing of German civilians. The bomber chief never had any enthusiasm for precision raids on specific sites, such as oil plants, railway depots and army barracks, which he dismissed as "panacea targets". He even privately said that the Dams raid "achieved nothing", and in early 1945 threatened to resign when Portal, who had begun to see the ineffectiveness of continued mass urban destruction, urged him to concentrate more on Germany's oil supplies.
Portal should have forced the issue much earlier. If the Lancaster fleet had been allowed to focus on targets of real military and economic value, it is likely that the German war machine would have collapsed more quickly and the conflict may have finished months earlier. Urban bombing might have been Britain's only way of fighting back against Nazi Germany in the middle years of the war but, by 1945, Bomber Command's strategy had descended into gruesome futility.

Leo McKinstry's most recent book is "Lancaster: the Second World War's Greatest Bomber" (John Murray, £20)