Among the most compelling of the exhibits at the Auschwitz Museum are the aerial photographs of the Auschwitz, Birkenau and Monowitz Camps taken by Allied reconnaissance in 1944 and early 1945. The images show the camps during some of their busiest – and bloodiest – periods of operation. If sufficiently magnified, it is possible to see groups of people walking from the trains to the crematoria and gas chambers. We can count the openings in the ceilings of the gas chambers of Crematoria II and III through which pellets of Zyklon-B were introduced. Visitors often leave, encouraged by their guides, with the sense that the world knew what was happening and remained silent.

In fact, the truth is more complex. The images were taken using film cameras set to take constant exposures over many miles. The “target” of the surveillance was the chemical factory at Monowitz: built by prisoners in the adjoining Auschwitz III camp, the factory was built by the chemical combine IG Farben to produce synthetic rubber. At the confluence of the Vistula and Sola rivers, and located in a coal-mining region, the site was tailor-made for such a plant. The availability of cheap labour – the SS charged a fee to use prisoner labour – meant that the project could be completed relatively quickly and on a short budget. Although the Bunawerke factory never produced any Buna (synthetic rubber) it was a strategic target. In fact, it was bombed four times: twice in August 1944, once in September, and once in December.

The bombing of Monowitz is one of the most contentious episodes in the history of WW2. Why, critics ask, could the Auschwitz and Birkenau camps not have been bombed as well? In fact, stray bombs from one of the raids did fall on Birkenau, as recorded by survivor testimonies. A conference was organised at the United States Holocaust Memorial Museum in Washington in the late 1990s, with a volume of proceedings published in 2000. A short summary of a complex debate breaks down as follows:

Firstly, knowledge of Auschwitz was both plentiful and of questionable accuracy. Reports by escaped prisoners such as Rudolf Vrba and Alfred Wetzler made clear that mass murder was being carried out. But rumours of death by electrocution or burning were not accurate, and their estimates of numbers were (understandably) excessive. To prisoners caught up in hell, the constant stream of arrivals and the smoking chimneys must have made it impossible to say for certain more than that a very large number of people were being killed. Even perpetrators were unsure of the numbers. At Nuremberg, Rudolf Hoess, the commandant of Auschwitz, estimated that the dead in the camp totalled around 3,000,000. Research conducted in Poland in the early 1990s, however, demonstrated conclusively that approximately 1,500,000 people were deported to the camp, and of those around 1,100,000 were killed. But in 1944, at the closing stages of the war, the facts were unclear and resources at a premium. Auschwitz was at the very edge of operational range, and required a dangerous mission back and forth across Germany.

Secondly, there is the question of technological capability. The key idea here is Circular Error Probable: the likelihood of a given bomb hitting within a reasonable range of its target. Accustomed to footage of munitions that can virtually turn corners to match traffic lights, we forget that in 1944 a bomb was simply explosives set to blow up when it completed its vertical drop. To hit the crematoria, or the railway lines, or any other target, was difficult. The controversial Allied strategy of bombing German cities was employed because the technology made precision difficult unless flying by day – which increased the risk to aircrew. And this is before any thought is given to the likely cost in prisoner lives of any full-scale raids on the camp. Survivors may say that they would have welcomed it – but I am glad they are here to tell the story, rather than blown to smithereens by Allied bombs.

Thirdly, the intellectual framework did not exist to really comprehend what was in the images, even if someone had looked. It had not, as Primo Levi wrote, yet “been introduced irrevocably into the world of things that exist”. There was plentiful information about the Holocaust in both the popular press and the corridors of power, but it was not acted upon in the most basic way. It was not accepted as fact that the German intention was to exterminate the Jews of Europe. Assertions that it was, in the minds of decision-makers, belonged in newspaper headlines and lurid magazine articles, not the formulation of policy. A significant measure of antisemitism also contributed. Surely, some argued, this was just Jewish imagination at work, a persecution complex caught up in the war? In August 1942, Gerhart Riegner of the World Jewish Congress sent a telegram to Sidney Silverman MP, the WJC representative in London:

The ensuing five-day correspondence among officials acknowledged “numerous reports of large scale massacres of Jews” but focused on attempting to verify Riegner’s identity (“Eastern Dept. have no knowledge of Mr Riegner”) and ended with the following remark:

I do not see how we can hold up this message much longer, although I fear it may provoke embarrassing repercussions. Naturally we have no information bearing on this story.

Later in 1942, the activist Rev. James Parkes despaired that “The continued silence of the government in relation to the massacres is evidence of the strength in places of power of reactionary forces – from whom we have nothing to hope.”

But how then can we explain the pictures? Surely these images show that we knew exactly what was happening? There it is, in black and white, neatly labelled.

In fact, this is misleading. As I said above, the photographs were taken on huge rolls of film, covering many miles. The images of Auschwitz and Birkenau were at the end of these reels, after the “target” images of Monowitz. During the war, they were overlooked because analysts were not detailed to look. The images we are familiar with were only produced in the 1970s, when two CIA analysts named Dino A. Brugioni and Robert G. Poirier examined the images and conducted a retrospective analysis, uncovering many of the details that strike the visitor or viewer today. As they said in their report:

In a variety of ways therefore, both technical and historical, not only were the images not looked at until the 1970s, they could not have been looked at earlier. The report also served another purpose than historical reconstruction. The pointed reference to the CIA’s photo-reconnaissance capability was meant to be understood most directly in Moscow: the clear message being that Russian military installations could be spotted, analysed and potentially destroyed.

Why is this important today? A BuzzFeed article prompted these reflections: an article about the treatment of the Uighurs in China. BuzzFeed used commercial technology to identify 268 sites, and was able to confirm that 92 of these are detention centres using documents, eyewitness testimony and academic research. Authorities in the region termed the claims of persecution as “a groundless lie”: “the issue concerning Xinjiang is by no means about human rights, religion or ethnicity, but about combating violent terrorism and separatism”. Some of these sites are sufficient to hold 10,000 people. The testimonies of those who have emerged from the camps to tell the tale are horrendous.

This month, an open letter was sent to the government by more than 70 faith leaders, calling on the UK government “to investigate these crimes, hold those responsible to account and establish a path towards the restoration of human dignity.” The letter invoked the Holocaust, once more demanding that “Never Again” finally – this time – have some meaning.

In 1945, Primo Levi wrote that his liberators were oppressed by the evidence of the crime, “the shame the Germans did not know, that the just man feels at another man’s crime.” But this crime in many ways had only just been introduced into the “world of things that exist”. The legal measures of the late 1940s, the Genocide Convention and Universal Declaration of Human Rights, were landmarks, acknowledging for the first time that rights are human and transnational, that mass death is wrong, and that leaders cannot hide behind the state to evade responsibility. James Fawcett, one of the British contributors to these laws (and grandfather of our current Prime Minister), said in 1961 that their purpose was to ensure that “Sharpeville, Angola, Tibet, are all matters of international concern, though they happen within the jurisdiction of a particular state.” That these lessons were learnt while mired in the hypocrisy and crime of Empire does not detract from the imaginative, moral, ethical and philosophical leap they were.

But that leap was made for us. Now we know. Now, it is other words from Primo Levi that we must remember, before we once more say “Never Again”: “It happened, therefore it can happen again.” It is happening again: once more, as Levi wrote, the lords of death are waiting by the trains. It is our job to try and stop them leaving the station. Knowing is not enough: now we must act.