What a world with an Iranian nuclear weapon would look like:

    "I would sleep happier if there were no Iranian bomb," writes former Times of London editor Simon Jenkins. "But a swamp of hypocrisy separates me from overly protesting it." Iran, he adds, "is a proud country that sits between nuclear Pakistan and India to its east, a nuclear Russia to its north and a nuclear Israel to its West. . . . How can we say such a country has 'no right' to nuclear defense?" In other words, what's the big deal?

    Well, the deal is the combination of the world's most destructive weapons in the hands of clerical radicals who might use them. And even short of using them, Tehran's rulers could use the leverage of the bomb to dominate the Middle East and limit America's ability to defend itself and fight terrorism.

    The most immediate threat in the region would be to Israel, an ally that only this week President Bush said we would defend against Iran. President Mahmoud Ahmadinejad has publicly mused that the Jewish state should be "wiped off the map," and former President Ali Akbar Hashemi Rafsanjani has said that "the use of a nuclear bomb in Israel will leave nothing on the ground whereas it will only damage the world of Islam." Why should we assume they don't mean this?

    All the more so because Iran's current leaders seem possessed of an apocalyptic Islamist vision that wouldn't mind an episode of pan-global martyrdom. "We must prepare ourselves to rule the world and the only way to do that is to put forth views on the basis of the Expectation of the Return" of the Mahdi (Shiite Messiah), says Mr. Ahmadinejad.

    The Iranian President is sometimes dismissed as a figurehead, especially on nuclear questions, but he wouldn't have his job without the consent of the ruling clerical council for whom he also serves as a mouthpiece. His fanaticism suggests a mindset that isn't vulnerable to the normal calculations of deterrence that governed during the Cold War. The complacent tell us not to worry because no state would dare use a nuke because that would only guarantee its own destruction. But what if you're a cleric who likes that trade-off?

    A bomb would also give Iran far more leverage to press its influence abroad since it will believe it is immune to retaliation. In Lebanon, the Shiite terrorist group Hezbollah sits on Israel's northern border with 7,000 medium-range missiles, deterred only by Israel's conventional arms superiority. But the military balance changes once Hezbollah's patron becomes invulnerable.

    A nuclear Iran could also wield a predominating influence in OPEC. It could disrupt maritime traffic in the Persian Gulf and force the U.S. Navy out of its narrow, shallow waters. It could menace Europe, and eventually the U.S. homeland, as its ballistic missile capabilities develop. It could arm Palestinian terrorists with sophisticated weapons, turning Gaza into a risk not just for Israel but the entire Mediterranean basin.

    It would be in a position to extend its influence into the Caspian region and neighboring Afghanistan. It could meddle in the affairs of traditional rivals such as Saudi Arabia (which, like Iraq, has a sizeable Shiite population in its eastern provinces); the Saudis--as well as the Egyptians and the Turks--might respond by seeking nuclear weapons of their own.

    It's also important to consider the effect that a nuclear Iran would have on the potential for a democratic Iran. Its nuclear project is often portrayed as a matter of national prestige, the implication being that any strike against it would rally the regime's domestic opponents to its side. What Iranian dissidents tell us is closer to the opposite. A nuclear Iran would enhance the mullahs' sense of invulnerability and facilitate domestic repression.