On Monday, the Iranian foreign ministry held an international conference. There's nothing unusual in that. Foreign ministries hold conferences, mostly dull ones, all the time. But this one was different. For one thing, the International Conference to Review the Global Vision of the Holocaust dealt with history, not current politics. Instead of the usual suspects—deputy ministers and the like—the invitees seem to have included David Duke, a former Ku Klux Klan leader; Georges Thiel, a Frenchman who has called the Holocaust "an enormous lie"; and Fredrick Töben, a German-born Australian whose specialty is the denial of Nazi gas chambers. The guest list was selective: No one with any academic eminence, or indeed any scholarly credentials, was invited. One Palestinian scholar, Khaled Ksab Mahamid, was asked to come but was then barred because he holds an Israeli passport—and also perhaps because he, unlike other guests, believes that the Holocaust really did happen....

Questioning the reality of the Holocaust has long been another means of questioning the legitimacy of the state of Israel, which was indeed created by the United Nations in response to the Holocaust, and which has indeed incorporated Holocaust history into its national identity. If the Shiite Iranians are looking for friends, particularly among Sunni Arabs, Holocaust denial isn't a bad way to find them.

And yet—this week's event has some new elements, too. This is, after all, an international conference, with foreign participants, formal themes ("How did the Zionists collaborate with Hitler?" for example), and a purpose that goes well beyond a mere denunciation of Israel. Because some former Nazi countries have postwar laws prohibiting Holocaust denial, Iran has declared this "an opportunity for thinkers who cannot express their views freely in Europe about the Holocaust." If the West is going to shelter Iranian dissidents, then Iran will shelter David Duke. If the West is going to pretend to support freedom of speech, then so will Iran. Heckled for the first time in many months by demonstrators at a rally yesterday, Ahmadinejad responded by calling the hecklers paid American agents: "Today, the worst type of dictatorship in the world is the American dictatorship, clothed in human rights." The American dictatorship, clothed in human rights spouting falsified history: It's the kind of argument you can hear quite often nowadays, in Iran as well as Russia and Venezuela, not to mention the United States.

All of which is a roundabout way of saying that this particular brand of historical revisionism is no joke, and we shouldn't be tempted to treat it that way. Yes, we think we know this story already; we think we've institutionalized this memory; we think this particular European horror has been put to rest, and it is time to move on. I've sometimes thought that myself. There is so much other history to learn, after all. The 20th century was not lacking in tragedy.

And yet—the near-destruction of the European Jews in a very brief span of time by a sophisticated European nation using the best technology available was, it seems, an event that requires constant re-explanation, not least because it really did shape subsequent European and world history in untold ways.