Robert Fisk: Bombs make no moral distinctions where they fall

To Mannheim for its annual film festival and I am gripped by Armadillo, a documentary on a Danish NATO unit in Afghanistan, real bullets whizzing past one of the bravest directors of photography in the world, real soldiers falling wounded, one with a Wilfred Owen pallour of death on his face.

But he survives. Others don't. After storming a Taliban position, the Danes find at least three Afghans, apparently still alive. There is a crack of gunfire and they are dead. "We eliminated them in the most humane way possible," one of the Danes says afterwards, right there on the soundtrack.

I am stunned. The words "war crimes" are in my mind. Then I stumble out into the cold afternoon to walk back to my hotel past the back of the 19th-century Kunstalle and there are shrapnel gashes up the red stone walls, deep wounds in the brickwork of the school next door, a slash in the basement window casing. Was this from the British fire raid of 1940, or the first attempt to raze the city on 16 April 1943, or the American raids of 1944? Well protected with underground bunkers, only 1,700 Germans were killed here, a mere 0.6 per cent of its residents. War crimes?

I find a small bookshop in a bleak, freezing street – the 1950s block architecture proves how much of this city much we destroyed – and I ask for a history of Mannheim in the Second World War. My request is received without emotion by a stooped, middle-aged man who brings me two beautifully produced volumes – "the last we have," he says – and I buy them at once: Mannheim under the Dictator and Mannheim in the Second World War, both produced with photographs and documents from state and city archives. I flip open the second book and there is a nurse bending over a carbonised corpse. Three women lie on their backs, all in coats; obviously caught in the open, perhaps during the great raid of 5-6 September 1943, in which 414 residents were killed. Two of them are elderly, one of them a much younger woman with high cheekbones and closed eyes.

Opposite is a page of death notices in the Neue Mannheimer Zeitung – like all papers in Germany, controlled by the Nazi party – printed a few days after the raid. Heinz Laubenstein was five when he was killed, his sister Ruth was 17, their father and mother, Hans and Bienchen, were 36 and 35. Katherina Witwe was 68, Berta Werle was 29. Anna Schindler was 56, her husband Anton eight years older. A victim of the 1944 bombing, a gaunt woman – her face like a skull – staring wildly, helped by a civil defence worker and a man who may be her husband clutching a ragged child, stands amid rubble. The Nazis liked to emphasise their own civilian suffering; and they gloated over their enemies. An amateur photograph shows two captured Allied fliers hiding their faces from the camera, trailed by two grinning schoolboys on bicycles; another shows a smouldering British Hereford bomber that has crashed plumb into the middle of a street across the Rhine in Ludwigshafen.

I prowl through the pages, searching for the faces of wartime Mannheim. I try to guess whether they are Nazi party members or not before reading the captions and – amazingly – I always get it right. The smug, fat, dark – yes, sinister – faces all turn out to be Nazis. The gentler, more intelligent faces are always those of old Social Democrats, of Jews and Communists. But I almost make one mistake. There is a photo of a young schoolboy in the 1920s, Gustav Adolf Scheel, again with bicycle, a pleasant but unsmiling face, harmless enough,dressed in a double-breasted jacket. Then I flip the page, add 20 years, and sitting in front of me is a mean-faced bully of a man in uniform, eagle and death's head on his cap, high leather boots. Pleasant young Gustav has turned into an SS officer in the Alpenland.

Hour after hour I read these books, and this has a strange effect. There is Hitler arriving at the small Mannheim airport that I passed in the tram yesterday. There is Goebbels in the opera house near the water tower where I have coffee every morning. There is the very same tower draped with a mighty Swastika. And there is a photograph of a dignified Jewish businessman confronting a bespectacled Nazi officer, whole family pictures of doomed Jews, sitting round a table, many of the pictures horribly torn and crushed. The Mannheim history books duck nothing. There is a whole chapter on the Holocaust.

There was even a baby resistance in Mannheim. A Gestapo photograph shows the tiled wall of a public lavatory on which someone has scribbled in a plaster join the words: "Hitler Shuft" – "Hitler is a gangster." And there is one, most moving letter from a Mannheim communist, whose childish roneo-published anti-Nazi tracts earned him a death sentence. Thirty-six year old Jakob Faulhaber was allowed a last letter, hitherto untranslated into English, to his wife Emma at their home at No 34 Hubenstrasse. "My loved ones," he writes on 14 September 1942, "now the decision has been taken, and everything we tried to do has failed. Our appeal has been denied and all our hope is at an end. When I think back on my life, it was a little too short – but it was worth it to have lived such a life. You all know that I always lived for my ideals and I'm a strong enough person to die for them ... Last greetings to all who were close to me in my life. What is it to live?" Jakob Faulhaber was hanged next day in Stuttgart.

Were some of the other Mannheimers "just like us", decent men and women who risked life for freedom, who responded to daily bombing in a way remarkably similar to that of Londoners in the 1940-41 Blitz, who wrote "Business as usual" on their shops? There is a snapshot of massive ruins in Mannheim and a hoarding which says: "...and despite all! Mannheim is a still a living city." But it was put there by the Nazis. The people, the Volk – and you can see them walking to work between smashed tenements – did not erect posters; they wrote their names and addresses on their ruined homes so that when their husbands and brothers and lovers came back from the front – if they did come back – they could find their families. Some of these men were war criminals in a most deliberate way, decimating entire peoples. But they also killed wounded soldiers, just as we did in Normandy. And in Afghanistan. And we knew, of course, when we bombed Mannheim, that we would kill a Heinz, a Ruth, a Berta and an Anna amid the destruction of the city's factories.

War crimes? Of course, we can all make moral assessments. Eliminated in the most humane way... The Taliban are not as bad as the Nazis. But they are much worse than the Danes. Isn't that right? What is this life?