At the end of the rail track that delivered more than a million Jews and other people to their deaths in the gas chambers of Auschwitz-Birkenau is a forbidding stone memorial above 19 plaques in 19 different languages, all bearing this message:

“Forever let this place be a cry of despair and a warning to humanity where the Nazis murdered about one and a half million men, women and children, mainly Jews, from various countries of Europe.”

I had joined a guided tour of the Nazis’ largest extermination camp, and by this point the horror of the place was crushing.

On either side of me were the ruins of the two largest gas chambers with their attached crematoria, and the pits where they dumped the ashes. Auschwitz-Birkenau is actually two separate camps, and the tour began with several hours in the claustrophobic barracks and dungeons of the original concentration camp. Some of the barrack rooms have been turned into shrine-like exhibits of the belongings the Nazis stole from each family once they had stepped off the trains. Most disturbing of all is a room filled with mounds of human hair, cropped from each victim for use in stuffing mattresses and pillows. Here and there, little girls’ ponytails poke out from the mass.

As I stood in front of the memorial plaques, silent like everyone else, I thought of the arguments about the details of the Holocaust and why so many still doubt what happened here. And I thought that Auschwitz is not only a warning to humanity. It’s also a monument to facts, and the sanctity of the truth.

Far from exaggerating the Nazis’ crimes — as Holocaust deniers have so often alleged — the custodians of Auschwitz, and Jewish historians, have actually reduced their estimate for the number of people murdered here.

Far from exaggerating the Nazis’ crimes — as Holocaust deniers have so often alleged — the custodians of Auschwitz, and Jewish historians, have actually reduced their estimate for the number of people murdered here, in accordance with the evidence they have gathered.

The plaques you see today, inscribed with the figure of “about one and a half million” murdered, haven’t always been there. If I had visited the Auschwitz-Birkenau memorial in the 1980s, I would have seen another set of plaques, saying that “four million” people had died here — describing them simply as the “victims” of Nazi genocide, with no mention of their identities.

That figure of four million was conjured up by Poland’s then-communist rulers. Auschwitz is in southern Poland, and they wanted to emphasize Nazi atrocities against communists, particularly their own people. The Jews were edited out of this narrative, even though it was clear even then that the Birkenau death camp had been set up to exterminate them, as part of Hitler’s “Final Solution.” This attempt to rewrite history has echoes even today, in Russia’s efforts to ignore Nazi atrocities against Jews on its own soil.

There’s no question that Poland suffered grievously under German occupation during World War II: as many as three million Poles may have died. And because the Nazis initially set up Auschwitz — which is in southern Poland — as a concentration camp for Polish political prisoners, it became one of the focal points of communist efforts to memorialize the country’s suffering. But most of the Poles who perished at the hands of the Nazis died elsewhere, not in this death camp.

By 1989, when communism collapsed in Poland, Auschwitz and Jewish historians had already worked out a more accurate estimate, based on the Nazis’ own transport records of who they rounded up and put on the trains from across Europe.

That is where the figure of 1.5 million on today’s plaques comes from, including at least 1.1 million Jews, as well as Poles, Gypsies, Soviet POWs and some 5,000 nationals of other countries. And in the early 1990s, Poland’s first post-communist prime minister agreed to install new plaques at the memorial.

Despite this strict adherence to fact and evidence, the lies keep coming. And as the annual day of Holocaust remembrance approaches — on 27 January, the day Auschwitz was liberated by the Soviet Red Army — anti-Semitic attacks are rising worldwide.

The day before my visit I had been at a conference in Warsaw, listening to warnings about the corrosive effect of the tide of propaganda and division sweeping the world. A journalist from Hungary reported how the label “traitor” was now being slapped on anyone who tried to counter the falsehoods of Prime Minister Viktor Orban. Who can forget that among Orban’s favorite invented enemies is a Holocaust survivor, George Soros. But when the leader of the free world can be so free with the truth, it is contagious. The tide keeps rising.

The story of Auschwitz-Birkenau is also a parable of what happens when lying becomes institutionalized. When the Nazis rounded up Jewish and Gypsy communities across Europe to be transported to the death camps, they told them they were being resettled (they made them pay for the journey). That’s how they kept them calm as they packed them into the cattle-wagons.

That’s why the Auschwitz museum is now so full of pots, kettles and other cooking utensils that Jewish families packed into their suitcases, the basics for a new life.

In the grainy photos on display, you can see the new arrivals look exhausted and bewildered, but not yet terrified. They were starting to realize something was wrong, as SS officers made their selections — one line for slave labour, another for children and those too weak to work, to be led straight to the gas chambers. But no one yet knew their fate.

They realized something was wrong, as SS officers made their selections — one line for slave labour, another for children and those too weak to work, to be led straight to the gas chambers.

But even as Hitler was trying to create a new truth, that the Aryans were a master race, and that the Jews and others barely deserved the term at all, his underlings knew the truth of what they were doing — and it scared them.

As the Red Army closed in, the SS guards tried to carry out a giant cover-up, by dynamiting the gas chambers, burning paper records and destroying the belongings they had harvested from their victims. But they left it too late. When Soviet troops rolled in, troves of evidence remained — including tons of human hair and stacks of paperwork that had been spirited from the flames.

The Nazis were famous for their record-keeping, but they never wrote down the real cause of death. People in Auschwitz died of coronary heart failure, severe gastric infections or pneumonia, never by being gassed, shot or injected with poison.

As we walked back towards the Birkenau gatehouse, now infamous from so many films, one of the people in my group asked our guide why there were so many solitary chimney stacks where prisoners’ barracks had once stood.

Some were destroyed by the SS, but many of the barracks were dismantled in the years after the war for building materials, Zuzanna explained, before anyone had thought of turning the camp into a memorial. And, she added, with the same dispassionate voice she had employed throughout the tour, “the chimneys were just propaganda anyway.”

Propaganda aimed at who, I asked? The Nazis controlled the camp. The prisoners were destined for death. To whom did they need to lie?

They were worried about visits from the Red Cross, she explained. The Nazis always told Red Cross staff that this was a regular prisoner of war camp, never letting them get too far inside. So they needed to make it look as though all the prisoners had heating.

Zuzanna has been doing her job for more than a decade. “Sometimes, I have to take a break,” she admitted. I thanked her for the way she had told the story of this place. She nodded in acknowledgement, then said: “Everyone should come here.”