From 13 to 15 February 1945, British (and some American) heavy bombers dropped 2,400 tons of high explosives and 1,500 tons of incendiary bombs onto the ancient cathedral city of Dresden. In just a few hours, around 25,000 to 35,000 civilians were blown up or incinerated.

Victor Gregg, a British para captured at Arnhem, was a prisoner of war in Dresden that night who was ordered to help with the clear up. In a 2014 BBC interview he recalled the hunt for survivors after the apocalyptic firestorm. In one incident, it took his team seven hours to get into a 1,000-person air-raid shelter in the Altstadt. Once inside, they found no survivors or corpses: just a green-brown liquid with bones sticking out of it. The cowering people had all melted. In areas further from the town centre there were legions of adults shrivelled to three feet in length. Children under the age of three had simply been vaporised.

It was not the first time a German city had been firebombed. “Operation Gomorrah” had seen Hamburg torched on 25 July the previous year. Nine thousand tons of explosives and incendiaries had flattened eight square miles of the city centre, and the resulting inferno had created an oxygen vacuum that whipped up a 150-mile-an-hour wind burning at 800 Celsius. The death toll was 37,000 people. (By comparison, the atom bomb in Nagasaki killed 40,000 on day one.)

Chief of the Air Staff Charles Portal had calculated that bombing civilians could kill 900,000 in 18 months, seriously injure a million more, destroy six million homes, and “de-house” 25 million, creating a humanitarian crisis that, he believed, would speed up the war.

This thinking was not trumpeted from the rooftops. But in November 1941 the Commander-in-Chief of Bomber Command said he had been intentionally bombing civilians for a year. “I mention this because, for a long time, the Government, for excellent reasons, has preferred the world to think that we still held some scruples and attacked only what the humanitarians are pleased to call Military Targets. I can assure you, gentlemen, that we tolerate no scruples.”

The debate over this strategy of targeting civilians is still hotly contentious and emotional, in Britain and abroad. There is no doubting the bravery, sacrifice, and suffering of the young men who flew the extraordinarily dangerous missions: 55,573 out of Bomber Command’s 125,000 flyers never came home. The airmen even nicknamed their Commander-in-Chief “Butcher” Harris, highlighting his scant regard for their survival.

Supporters of Britain’s “area bombing” (targeting civilians instead of military or industrial sites) maintain that it was a vital part of the war. Churchill wrote that he wanted “absolutely devastating, exterminating attacks by very heavy bombers from this country upon the Nazi homeland”. In another letter he called it “terror bombing”. His aim was to demoralise the Germans to catalyse regime change. Research suggests that the soaring homelessness levels and family break ups did indeed depress civilian morale, but there is no evidence it helped anyone prise Hitler’s cold hand off the wheel.

Others maintain that it was ghastly, but Hitler started it so needed to be answered in a language he understood. Unfortunately, records show that the first intentional “area bombing” of civilians in the Second World War took place at Monchengladbach on 11 May 1940 at Churchill’s orders (the day after he dramatically became prime minister), and four months before the Luftwaffe began its Blitz of British cities.

Not everyone was convinced by city bombing. Numerous military and church leaders voiced strong opposition. Freemason Dyson, now one of Britain’s most eminent physicists, worked at Bomber Command from 1943-5. He said it eroded his moral beliefs until he had no moral position at all. He wanted to write about it, but then found the American novelist Kurt Vonnegut had said everything he wanted to say.

Like Gregg, Vonnegut had been a prisoner in Dresden that night. He claimed that only one person in the world derived any benefit from the slaughterhouse — him, because he wrote a famous book about it which pays him two or three dollars for every person killed.

Germany’s bombing of British cities was equally abhorrent. Germany dropped 35,000 tons on Britain over eight months in 1940-1 killing an estimated 39,000. (In total, the UK and US dropped around 1.9 million tons on Germany over 7 years.)

Bombing German cities clearly did have an impact on the war. The question, though, is how much. The post-war US Bombing Survey estimated that the effect of all allied city bombing probably depleted the German economy by no more than 2.7 per cent.

Allowing for differences of opinion on the efficacy or necessity of “area bombing” in the days when the war’s outcome remained uncertain (arguably until Stalingrad in February 1943), the key question on today’s anniversary remains whether the bombing of Dresden in February 1945 was militarily necessary — because by then the war was definitely over. Hitler was already in his bunker playing out his final absurd fantasies. The British and Americans were at the German border after winning D-Day the previous summer, while the Russians under Zhukov and Konev were well inside eastern Germany and racing pell-mell to Berlin.

Dresden was a civilian town without military significance. It had no material role of any sort to play in the closing months of the war. So, what strategic purpose did burning its men, women, old people, and children serve? Churchill himself later wrote that “the destruction of Dresden remains a serious query against the conduct of Allied bombing”.

Seventy years on, fewer people ask precisely which military objective justified the hell unleashed on Dresden. If there was no good strategic reason for it, then not even the passage of time can make it right, and the questions it poses remain as difficult as ever in a world in which civilians have continued to suffer unspeakably in the wars of their autocratic leaders.