Pope Benedict XVI sparked a firestorm by lifting the excommunication of a bishop who claims the Holocaust never occurred. How could anyone believe such a thing?

Who would deny the Holocaust?
All kinds of people, it turns out. The issue made headlines a few weeks ago when the pope reinstated renegade British bishop Richard Williamson, who has dismissed the “so-called Holocaust” as “lies, lies, lies.” Williamson has lots of company. Iranian President Mahmoud Ahmadinejad has called the Holocaust a “myth,” while Hezbollah leader Hassan Nasrallah says “Jews invented the legend of the Holocaust.” Prominent American deniers include Hutton Gibson—an ultra-conservative critic of modern Catholicism and the father of actor Mel Gibson—who says there were more Jews in Europe after World War II than before, and Arthur Butz, a Northwestern University engineering professor and author of The Hoax of the Twentieth Century: The Case Against the Presumed Extermination of European Jewry.

What exactly do they believe?
There are many variants of Holocaust denial. But adherents on both the extreme Left and Right reject the historical consensus that Nazi Germany systematically slaughtered some 6 million of Europe’s Jews. Many put the figure at around 200,000, and they argue that rather than having been singled out for a “Final Solution,” Jews were merely one of many groups—homosexuals, communists, the mentally disabled—targeted for persecution. Deniers often admit that there were concentration camps, but insist they weren’t designed for extermination. The vast majority of deaths in the camps, they claim, were due to disease, overwork, and privation caused by the Allies. And they reject being branded as “Holocaust deniers,” preferring the term “Holocaust revisionists.”

When did this ‘revisionism’ begin?
Almost immediately after World War II. The first notable “debunker” was the French fascist Maurice Bardèche, who argued that the gas chambers had been used only to disinfect clothing, and that the Allies had faked vast numbers of photographs, documents, and court records. He was followed by another Frenchman, former Resistance fighter Paul Rassinier, who actually had spent 19 months in two German camps, Buchenwald and Dora. In his 1964 book The Drama of the European Jews, Rassinier argued that no more than 1.5 million Jews were killed, many by communists.

Did anyone believe him?
Yes. After Rassinier published his book, Harry Elmer Barnes, a leftist American historian, touted the work as evidence that the Holocaust was propaganda designed after the fact to justify U.S. entry into the war. Barnes emboldened other deniers to write books of their own, and a cottage industry was born. Barnes also inspired the wealthy anti-Semite Willis Carto, who in 1978 founded a pseudo-scholarly think tank called the Institute for Historical Review. Carto offered $50,000 to anyone who could prove that Jews had been gassed at Auschwitz.

Did anyone accept his challenge?
Yes. Auschwitz survivor Mel Mermelstein presented Carto with reams of documentation. When Carto refused to pay, Mermelstein sued Carto in California Superior Court. Judge Thomas Johnson ultimately ruled that Mermelstein had, in fact, proved his case, and ordered Carto to pay him $50,000, plus $40,000 for personal suffering. “Jews were gassed to death at Auschwitz concentration camp in Poland,” Johnson declared in his milestone 1981 ruling. “It is not reasonably subject to dispute. It is simply a fact.” The deniers, though, have continued their crusade.

How can they, given the evidence?
By trying to plant small seeds of doubt. “There is an assumption by deniers that if they can just find one tiny crack in the Holocaust structure,” wrote Michael Shermer and Alex Grobman in Denying History (2000), “the entire edifice will come tumbling down.” Hence, Fred Leuchter, a self-styled “execution expert,” has claimed that Zyklon-B, which was used to gas concentration camp victims, is insufficiently powerful for the purpose. Author David Irving has mined archives to “prove” that any campaign to persecute Jews was the brainchild of propaganda chief Joseph Goebbels, not Adolf Hitler. “The primary motivation for most deniers is anti-Semitism,” said Walter Reich, former director of the U.S. Holocaust Memorial Museum. “What better way, in short, to make the world safe for anti-Semitism than by denying the Holocaust?”

Is Holocaust denial widespread?
Not in this country. Despite a thriving anti-Semitic subculture on the Internet, recent polls have found that less than 2 percent of Americans seriously doubt the Holocaust happened. But Holocaust denial is common in the Middle East. A 2007 survey by the University of Haifa found that 28 percent of Israeli Arabs did not believe the Holocaust occurred. Ahmed Tibi, an Israeli Arab and deputy speaker of the Knesset, says he considers Holocaust denial “immoral.” But he suggests that many Arabs are sympathetic to it because of their “reservations about the way the Holocaust is used as a political tool” by Israel.

How should deniers be rebutted?
That question is fiercely debated by historians, anti-defamation groups, and survivors. Many believe that to engage the deniers endows them with undeserved credibility. “The last thing you want to do is give them a media platform,” said Sara Bloomfield, director of the U.S. Holocaust Memorial Museum. “They want the history debated; that is their goal.” But others believe that for the sake of history, the discussion should not be shirked. “The way of fighting Holocaust deniers,” says Deborah Lipstadt of Emory University, “is with history and with truth.”

Under penalty of law
Denying the Holocaust is punishable by fine or imprisonment in Israel and 12 European nations, including Germany, Austria, and France. That has not stopped true disbelievers. Last year in Germany, Horst Mahler, a founder of the radical Red Army Faction, was found guilty of incitement for posting documents online that ostensibly debunk the Holocaust; he was sentenced to 10 months in prison. The most famous Holocaust denier to serve time is British writer David Irving. Arrested in Austria in 2005 for having denied the Holocaust 16 years earlier, he served 10 months of a three-year sentence. The case prompted an outcry from civil libertarians. “Criminalizing speech denying the Holocaust not only threatens free speech, it gives power to the vile views it seeks to suppress,” said constitutional scholar Charles Haynes. “Putting people like David Irving in prison only makes them martyrs.”