In response to President Mahmoud Ahmadinejad's landslide victory, Vice President Biden expressed "doubts" about the validity of Iran's election.

Of course the Iranian election was fixed. But here's the real problem: The ayatollahs who fix Iran's elections decided that the best public face of the regime is a radical Twelver.

Our media and political leaders regularly inform us about Ahmadinejad's Holocaust denial, his hot pursuit of nuclear weapons, and his desire to destroy Israel. But they seldom mention that he's a radical Twelver.

Ninety percent of Iranian Shiite Muslims are Twelvers, which means they believe in 12 divinely ordained successors to Muhammad known as imams, beginning with the son-in-law of the prophet, Ali. The last and current imam is Muhammad al-Mahdi. He's ruled the Shiites since the year 874.

The Mahdi took charge upon the death of his father, the 11th imam, who, like the 10 before him, is thought to have been killed at the direction of the Sunni caliph. The Mahdi was 5 years old at the time. For his own protection, he is thought to have been sent into hiding, or "occultation."

Twelvers believe the Mahdi will emerge from hiding - with Jesus - when the world has fallen into chaos and civil strife, and that he will establish Shia Islam across the globe. Ahmadinejad has said, "Our revolution's main mission is to pave the way for the reappearance of the 12th imam, the Mahdi."

Ahmadinejad speaks of the return of the 12th imam in many of his speeches. To the consternation of the ayatollahs, he has even said the Mahdi has communicated with him personally. And he has spent heavily on road and civic improvements along the route the Mahdi is expected to travel when he returns - from the Iranian city where the imam is to reappear, Qom, to the capital, Tehran.

So what makes Ahmadinejad a radical Twelver? First, the vast majority of Shiites believe that only Allah knows when the Mahdi will return. But since his election four years ago, Ahmadinejad has been predicting the imminent return of the Mahdi. Even more ominous, Ahmadinejad believes that it is within his power to call the Mahdi back from his occultation.

How? It's believed that the Mahdi will return at a time of great chaos and war. This sheds some light on Ahmadinejad's pursuit of nuclear weapons and his talk of wiping out Israel: They threaten to create the kind of chaos and war amid which the Mahdi is expected to return.

The U.S. State Department believes all this is irrelevant, since Grand Ayatollah Ali Khamenei, Iran's supreme leader, is not a radical Twelver. That may be true, but here's a question: Why would the grand ayatollah select a president who is?

In the face of all this, President Obama has conceded Iran's right to develop nuclear power and has proposed talks with the regime without preconditions. Ahmadinejad, meanwhile, has consistently said there is nothing to discuss; the nuclear issue is off the table, and he has no desire to talk to the evil empire or its president.

Obama's policy of engagement has been rebuffed. Moreover, the regime sees it as a sign of weakness, and dissenters see it as an act of betrayal.

So what should the United States do?

There is widespread speculation that Israeli Prime Minister Benjamin Netanyahu is preparing for a military strike on Iran's underground nuclear facilities. Israel would no doubt seek our help with such a mission, given its difficulty. There would be international and regional consequences for both countries, and no guarantee of success.

Economic sanctions are having some impact on the Iranian economy and the popularity of the regime. But sanctions alone will not do the trick.

That leaves a revolution within Iran as the last best hope.

Last week's election sparked genuine outbursts of opposition. Tragically, President Obama has served up high-flown platitudes about fair elections instead of being squarely behind a budding revolution that could eliminate the most dangerous regime on the planet.

Let's hope Obama seizes this moment in history. If not, he will be left with two choices: working with Netanyahu to defang an emboldened Ahmadinejad-led Iran, or, some day, explaining to a world in chaos and war why he did not.