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 discovered Auschwitz concentration camp. 

This Thursday marks the anniversary of the liberation of Auschwitz: Holocaust Memorial Day. Yet the commitment to remembrance is under threat from a rise in online disinformation questioning the basic facts of the Holocaust.

Last week the United Nations passed a 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 a huge amount of evidence for the Holocaust. There are personal stories, statistics, documents, physical remains, photographs, films. 

Some of the seeds of Holocaust denial lie in Nazi efforts to conceal their atrocities. They used language to cover up the truth of what they were doing. "Special treatment" meant killing. "Resettlement" meant forced transportation to concentration camps. They even invited people to visit a ghetto, where they staged a happy community life. They burned human remains and destroyed documents.

But Holocaust denial is also entangled with the same attitudes that motivated the genocide itself. "It is anti-semitism, racism and prejudice," says the historian Deborah Lipstadt. Many deniers claim that the Holocaust was invented or exaggerated by Jewish people in order to set up their own state and gain influence over world affairs.

Anti-semitism linked to Holocaust denial can have fatal effects. Just this month, a man attacked a synagogue in Texas. 

Some countries have made 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. It allows those found guilty to pose as victims. 

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 neither do bans on questioning official historical narratives. We must be careful to distinguish between legitimate questioning and malicious misinformation.