Obama’s Legacy and the Iranian Bomb

The gravest threat faced by the world today is a nuclear-armed Iran. Of all the nations capable of producing nuclear weapons, Iran is the only one that might use them to attack an enemy.

There are several ways in which Iran could use nuclear weapons. The first is by dropping an atomic bomb on Israel, as its leaders have repeatedly threatened to do. Hashemi Rafsanjani, a former president of Iran, boasted in 2004 that an Iranian attack would kill as many as five million Jews. Mr. Rafsanjani estimated that even if Israel retaliated with its own nuclear bombs, Iran would probably lose about 15 million people, which he said would be a small "sacrifice" of the billion Muslims in the world.

The second way in which Iran could use nuclear weapons would be to hand them off to its surrogates, Hezbollah or Hamas. A third way would be for a terrorist group, such as al Qaeda, to get its hands on Iranian nuclear material. It could do so with the consent of Iran or by working with rogue elements within the Iranian regime.

Finally, Iran could use its nuclear weapons without ever detonating a bomb. By constantly threatening Israel with nuclear annihilation, it could engender so much fear among Israelis as to incite mass immigration, a brain drain, or a significant decline in people moving to Israel.

These are the specific ways in which Iran could use nuclear weapons, primarily against the Jewish state. But there are other ways in which a nuclear-armed Iran would endanger the world. First, it would cause an arms race in which every nation in the Middle East would seek to obtain nuclear weapons.

Second, it would almost certainly provoke Israel into engaging in either a pre-emptive or retaliatory attack, thus inflaming the entire region or inciting further attacks against Israel by Hezbollah and Hamas.

Third, it would provide Iran with a nuclear umbrella under which it could accelerate its efforts at regional hegemony. Had Iraq operated under a nuclear umbrella when it invaded Kuwait in 1990, Saddam Hussein's forces would still be in Kuwait.

Fourth, it would embolden the most radical elements in the Middle East to continue their war of words and deeds against the United States and its allies.

And finally, it would inevitably unleash the law of unintended consequences: Simply put, nobody knows the extent of the harm a nuclear-armed Iran could produce.

In these respects, allowing Iran to obtain nuclear weapons is somewhat analogous to the decision by the victors of World War I to allow Nazi Germany to rearm during the 1930s. Even the Nazis were surprised at this complacency. Joseph Goebbels expected the French and British to prevent the Nazis from rebuilding Germany's war machine.

In 1940, Goebbels told a group of German journalists that if he had been the French premier when Hitler came to power he would have said, "The new Reich Chancellor is the man who wrote Mein Kampf, which says this and that. This man cannot be tolerated in our vicinity. Either he disappears or we march!"

But, Goebbels continued, "they didn't do it. They left us alone and let us slip through the risky zone, and we were able to sail around all dangerous reefs. And when we were done, and well armed, better than they, then they started the war!"

Most people today are not aware that British Prime Minister Neville Chamberlain helped restore Great Britain's financial stability during the Great Depression and also passed legislation to extend unemployment benefits, pay pensions to retired workers and otherwise help those hit hard by the slumping economy. But history does remember his failure to confront Hitler. That is Chamberlain's enduring legacy.

So too will Iran's construction of nuclear weapons, if it manages to do so in the next few years, become President Barack Obama's enduring legacy. Regardless of his passage of health-care reform and regardless of whether he restores jobs and helps the economy recover, Mr. Obama will be remembered for allowing Iran to obtain nuclear weapons. History will not treat kindly any leader who allows so much power to be accumulated by the world's first suicide nation—a nation whose leaders have not only expressed but, during the Iran-Iraq war, demonstrated a willingness to sacrifice millions of their own people to an apocalyptic mission of destruction.

If Iran were to become a nuclear power, there would be plenty of blame to go around. A National Intelligence Report, issued on President George W. Bush's watch, distorted the truth by suggestion that Iran had ended its quest for nuclear weapons. It also withheld the fact that U.S. intelligence had discovered a nuclear facility near Qum, Iran, that could be used only for the production of nuclear weapons. Chamberlain, too, was not entirely to blame for Hitler's initial triumphs. He became prime minister after his predecessors allowed Germany to rearm. Nevertheless, it is Chamberlain who has come to symbolize the failure to prevent Hitler's ascendancy. So too will Mr. Obama come to symbolize the failure of the West if Iran acquires nuclear weapons on his watch.

Mr. Dershowitz is a law professor at Harvard. His latest book is "The Case for Moral Clarity" (Camera, 2009).