Israeli historian Benny Morris is the author, most recently, of "1948: A History of the First Arab-Israeli War."

Most people in the Arab world, according to opinion polls, believe that the Holocaust never happened, that it's a Jewish invention and trick to win the world's sympathy and support. President Mahmoud Ahmadinejad of Iran is similarly minded; he has said so countless times.

In the West, speaking of the Holocaust, most leaders and commentators concede that it did, indeed, occur. But, privately and sometimes publicly, some tell the Israelis: "Get over it." They mean that the murder of 6 million Jews during World War II should not dominate, or perhaps even strongly influence, Israel's policies today.

But is this reasonable or even moral? Should Israel set aside the memory and reality of what happened to its people, and conduct its life as a nation as if nothing happened?

The fact is that Israel's leaders, reflecting Israeli public opinion, take very seriously Iran's oft-repeated threat to create a second Holocaust, to wipe the Jewish state — "the Zionist entity" or "Zionist regime," as the Iranians call it — off the map. They take equally seriously Iran's nuclear program, which the international community, after years of denial or at least skepticism, now accepts is geared to the production of nuclear weaponry. Israelis, at least those who don't bury their heads in the sand, believe that if the Iranians get nuclear weapons they will, in the end, use them — or at a minimum, cannot be relied on not to use them — and that Israel's very existence is at stake....