Seventy-seven years ago, Soviet troops marching into Nazi-occupied Poland encountered a spectacle to dwarf all other wartime horrors. "A ghastly sight arose before our eyes," wrote one observer: "skeletons clad in skin, with vacant gazes... There were pyramids on the grounds of the camp. Some were made up of accumulated clothing, others of pots, and others still of human jaws."

The soldiers had come upon the remnants of the Auschwitz concentration campA large prison for people held without legal justification, such as political prisoners or persecuted minorities. The first concentration camps were built by British colonial rulers in South Africa, but the term is most associated with Nazi camps, some of which were used to execute Jewish and other inmates as well as to imprison them., where a million Jews had been murdered over the course of the war. It was the moment when the world first began to confront the scale of a genocideThe annihilation of a people, either through killing of its members, or through the suppression of its culture. in which six million were killed in a systematic effort to exterminate European Jews.

This Thursday marks the anniversary of the liberation of Auschwitz: HolocaustThe murder of six million Jewish people in Europe by Nazi Germany. Members of other minority groups were also killed. Memorial Day. Yet the commitment to remembrance is under threat from a rise in online disinformationFalse information intended to mislead, including propaganda, which all too often encourages the spread of misinformation, fake news spread by mistake. It is contrasted with misinformation, which may be spread accidentally. questioning the basic facts of the Holocaust.

Last week the United Nations responded by passing an Israeli motion to "reject and condemn denial of the Holocaust as a historical event". All 193 members backed the motion with the exception of Iran.

There is overwhelming evidence of the Holocaust from many sources: personal testimonies, population statistics, official documents, physical remains, photographs, films. Even the perpetrators themselves admitted their crimes. The death toll of six million, for instance, is supported by claims made by Adolf Eichmann, an architect of the Final SolutionThe Nazi plan to murder all Jewish people in Europe. It was developed at the Wannsee Conference, which took place 80 years ago this month..

Some of the seeds of Holocaust denial lie in Nazi efforts to conceal their atrocities. They used language, such as "special treatment" for killing and "resettlement to the East" for forced transportation to concentration camps. They invited international observers to view conditions in a ghetto, where they falsely staged a thriving community life. They burned human remains and destroyed documentation.

But Holocaust denial is also entangled with the same attitudes that motivated the genocide itself. "It is anti-semitism, racism and prejudice parading as rational discourse," says the historian Deborah LipstadtA scholar who won a court case against British historian David Irving after accusing him of Holocaust denial. Irving ultimately spent time in prison in both Austria and the UK.. Many deniers claim that the Holocaust was invented or exaggerated by Jewish people in order to establish their own state and gain influenceHave an effect on something else, and especially persuade them to act in your favour. over world affairs.

Anti-semitism linked to Holocaust denial can have fatal effects. Just this month, a man who believed that Jews "control the world" attacked a synagogueThe Jewish place of worship. in Texas, holding a rabbiA Jewish religious leader. and three members of his congregation hostage.

Some countries have gone as far as to make Holocaust denial a crime. "We should not wait until it comes to deeds," said German politician Brigitte Zypries, pushing for a Europe-wide ban.

But not everybody who opposes anti-semitism supports such a law. Critics say that a ban on questioning historical narratives threatens free speech and allows those found guilty to pose as victims. "We need not more repressive laws," Indian jurist Soli Surabjee says, "but more free speech to combat bigotry and to promote tolerance".

Yes: Those who deny historical atrocities lay the groundwork for their repetition. Allowing people to question the Holocaust legitimises anti-semitism, with tragic and violent results.

No: Free speech includes the freedom to say things that are provocative and even false. Holocaust denial is wrong, but it should be defeated in open debate, not in the courts.

Or... Outright lies and racism have no place in a free and fair society, but nor do bans on questioning official historical narratives. We must be careful to distinguish between legitimate questioning and malicious misinformation.