On 1 September 1939, as the massed German divisions began the invasion of Poland, one of the places that would be quickly overrun was a small and unprepossessing town on a railroad junction close to the Vistula river.

Named Oświęcim, within 10 months it would host the beginnings of the camp the world would know by its infamous German rechristening: Auschwitz.

Today, on the 80th anniversary of the outbreak of the Second World War, it still prompts one of the most shaming questions of the war: why allied leaders, Winston Churchill and Franklin D Roosevelt most prominent among them, failed to prevent the mass slaughter of Europe’s Jews?

A pivotal moment in the development of international law and humanitarianism as a principle of diplomacy, the Holocaust, and the allies’ lacklustre response to it, resonates today not simply for the facts of the worst war crime of the last century, but because of the lessons apparently unlearned, the questions unanswered.

Why, from Pol Pot’s killing fields to Srebrenica, Rwanda, Syria and the current persecution of China’s Uighars, has the international community struggled repeatedly to construct a timely and effective response?

If this inaction is difficult to contemplate today, it’s because even during the years preceding the war, the Nazi persecution of Germany’s Jews was both heavily prefigured and remarked on by British and US leaders, not least Roosevelt himself. Hitler’s attitude towards the Jews had been explicit in Mein Kampf, a threat whose urgency was confirmed by the Nuremberg laws of 1935, excluding German Jews from Reich citizenship and prohibiting them from marriage or sexual relations with persons of “German or related blood”.

Facebook Twitter Pinterest Adolf Hitler and Nazi youth leader Baldur von Schirach inspect Hitler youth at the Nuremberg rally in 1934. Photograph: Ullstein bild Dtl./Getty Images

By 1938, Germany’s Jews were being pushed out of ownership of businesses and property and out of key professions. Indeed, such was the alarm over their treatment, in the first instance, that Britain’s prime minister, Neville Chamberlain, within days of the Kristallnacht pogrom by SA troopers also in 1938, approved the Kindertransport to rescue unaccompanied German Jewish children.

While a popular idea has taken root that somehow the discovery of death camps at their liberation was a surprise to an unsuspecting world, that was far from true.

Even as the massacres of Jews by Nazi mobile killing squads began in summer 1941, in immediate wake of the surprise German attack on the Soviet Union, the first intimations of the Holocaust were being reported back to London and Washington by British military intelligence, by the Polish government in exile and by journalists and diplomats. On 14 August, 1941 Churchill made public fears of what was happening in areas captured by the Germans, referring in a radio broadcast to a “crime without a name” to describe the slaughter of “scores of thousands of civilians”.

So why was the issue so hard for the allies to grasp?

Over the years, historians, both critical and friendly, have offered competing explanations that have run the gamut from proposing that allied war leaders struggled to understand the scale of the crime unfolding to claims that Churchill and Roosevelt were simply indifferent. Others have pointed to the practical difficulties of intervention in the midst of total war, including the range of allied bombers, the diversion of resources required, and the view in allied governments that defeating the Axis powers was the best way to secure the safety of Europe’s threatened civilians, including its Jews.

By the end of 1942, however, there was little doubt what was happening. On 13 December 1942, the celebrated US journalist Ed Murrow gave Churchill’s “crime without a name” a clear framing in a radio broadcast.

“Millions of human beings, most of them Jews, are being gathered up with ruthless efficiency and murdered,” he said. “The phrase ‘concentration camps’ is obsolete… it is now possible only to speak of extermination camps.”

And as the author Dan Plesch – who has spent recent years studying forgotten and suppressed

archives relating to the allied response to the Holocaust – points out, allied leaders were aware of the vast scale of the crime the Nazis were engaged in.

In a joint statement which would be read to a silent House of Commons four days after Murrow’s broadcast by foreign secretary, Anthony Eden, allied governments revealed the ongoing Nazi effort to systematically transport and “exterminate” Europe’s Jews, naming Poland as the “principal slaughterhouse” and threatening to hold German leaders to account.

The catch, however, was articulated almost in Eden’s next breath as he answered MPs’ demands for something to be done. “It would clearly be the desire of the United Nations to do everything they could to provide wherever possible an asylum for these people,” he answered. “But the House will understand that there are immense geographical and other difficulties in the matter.”

The reality is that even when confronted with hard evidence of the unfolding genocide, key institutions in Washington and London, including the state department and the Foreign Office, did not see intervention as a key priority, either through the antisemitism of key officials, because they didn’t care, or saw other military and diplomatic issues as taking precedence.

On the British side, Viscount Cranborne, one of Churchill’s ministers, felt able to tell his fellow lords he did not believe that Jews should be considered a special case and that the British empire was already too full of refugees to offer a safe haven.

Facebook Twitter Pinterest Jewish women and children get off trains at their arrival in Auschwitz extermination camp. Photograph: AFP/Getty Images

Cranborne was not alone. Even as late as August 1944, senior Foreign Office officials were still urging scepticism over stories of atrocities that came from “Jewish sources”.

On the US side, a similar story played out amid a toxic atmosphere towards immigration, not least European Jewish immigration, which began in the 1930s and that informed Roosevelt’s public caution on the issue, not least when confronted by a Congress hostile to his stance on the threat from Nazi Germany. And like the Foreign Office, the US state department was regarded as a stumbling block to those seeking to help Jews; it was accused in January 1944 by US treasury secretary Henry Morgenthau of sabotaging the most modest relief efforts.

Yet even if air raids on the camps or railways, discussed by the Churchill’s war cabinet, were deemed impractical, the most baffling moment of inaction remains the Bermuda conference in April 1943, called following public pressure in the US to discuss the rescue or relief of Europe’s Jews including those still attempting to flee from from Europe. It ended with no change in stringent US quotas for Jewish immigration or Britain’s ban on Jewish migration to Palestine, which could have saved at least some lives. In the end it was too little and too late; a conference designed not for action but to soothe public concerns.

Eight decades on, as the world marks the anniversary of the outbreak of war, it is still necessary to recall not only the allies’ ultimate victory but their greatest failure when confronted with a vast catastrophe foretold.

Inside today: a 32-page magazine to mark the 80th anniversary of the outbreak of war.