What are we going to do about Iran? When Hillary Clinton surreally promised to obliterate the Islamic Republic if the mullahs nuked Israel, she at least recognized that a nuclear-armed clerical regime is a serious menace, and that successful diplomacy with Tehran without the threat of force is fantasy. How to handle Iran may well be the decisive foreign-policy question of the 2008 presidential campaign--especially if Tehran continues to exploit the vacuum left by the collapse of the Bush administration's Iran policy and the general listlessness of the U.S. presence in the Middle East outside of Iraq.

Tehran is on a roll. Its development of a nuclear weapon progresses. The European Union's attempts to cajole the mullahs to abandon uranium enrichment--the most demanding part of developing the bomb--has become ever-more plaintive; the Europeans promise incentives more than they threaten sanctions. Anxiety in Tehran about the possibility of an American military strike against the regime's nuclear facilities--produced by the president's and vice president's "saber-rattling" and helpfully amplified by French president Nicolas Sarkozy and his foreign minister, Bernard Kouchner--almost vanished in December with the publication of the National Intelligence Estimate, which incongruously asserted that Iran had stopped its quest for a bomb in 2003.

Running with the gift from Langley, Iran's president Mahmoud Ahmadinejad outflanked the more cautious and polished crowd led by Ali Akbar Hashemi Rafsanjani, Iran's second most powerful mullah. Rafsanjani, his sidekick Hassan Rohani, a former nuclear negotiator, and Ali Larijani, an intelligent, titanium-tough former Revolutionary Guards commander who, as Rohani's successor, played well with European diplomats, all appeared worried that Ahmadinejad's aggressiveness might actually provoke George W. Bush to attack another member of the axis of evil. After the NIE's release, all three men gave reluctant concession speeches, emphasizing Iran's victory over the West more than the success of Ahmadinejad's unflinching approach. In a triumphalist mood, Ali Khamenei, Iran's supreme leader who has consistently backed Ahmadinejad, let loose a broadside against the United States in January 2008, referring to America as "Satan incarnate" and calling on Muslims worldwide to emulate the Islamic republic and not Westernized Muslims, who lead to national weakness and perdition.

Spurred by its nuclear success against the Europeans and Americans, the clerical regime is causing trouble on the West Bank and in Gaza, Lebanon, Syria, the Persian Gulf, and perhaps most of all, Iraq. Israel may soon be embroiled in an ugly war with Hamas, the Palestinian Islamic fundamentalist movement supported spiritually and militarily by Tehran. This could turn into a two-front confrontation, as Hezbollah, revolutionary Iran's most faithful offspring, is demonstrating its willingness to use force to become the dominant player in Lebanon. Rearmed massively by Tehran since the 2006 summer war against Israel, Hezbollah could again let the missiles fly against northern Israel, while Hamas attacks from Gaza.

Make no mistake about it, Iran is gaming for this kind of confrontation, which will be difficult and costly for Israel. Tehran loved the outpouring of Arab warmth for Hezbollah's leader Hassan Nasrallah in 2006, when even secular Arab moderates started openly to rethink whether they had to live with a Jewish state. Europeans and Americans, especially those of a "realist" persuasion who imagine that Iran's Islamic mission civilisatrice has played out, just don't appreciate how much the clerics still enjoy the adulation of anti-American Arabs, especially those who have dropped pan-Arabism and embraced "Islamic values."

The Iranian ruling elite, from the mild-mannered reform-minded Mohammad Khatami to the die-hard spiritual soldiers of the 1980-88 Iran-Iraq war, like Ahmadinejad and Tehran's more bon-vivant mayor, Mohammad Baqir Qalibaf, has a vision of the Middle East that is free of America's power and the devilishly seductive pull of its culture. The Hamas-Hezbollah axis, if it holds, is a dream come true for Tehran: At last, Sunni and Shiite militants are working together to bleed Israel, America's colony and the anchor, as they see it, of American imperialism throughout the region. Hamas and Hezbollah allow Iran's rulers--and it is impossible to overstate the extent to which Rafsanjani and Khamenei hate Israel--to be frontline combatants against the Jewish state without incurring (so far) frontline risks of devastating retaliation.

The only real brake on Iranian complicity with Hezbollah and Hamas has been the fear that their aggression against Israel, if seen by Americans and Israelis as Tehran-directed, could increase the odds of a U.S. or Israeli military strike on Iran's nuclear facilities before the regime enriches sufficient uranium for a nuclear arsenal.

This nuclear concern is probably behind the North Korean-Syrian nuclear cooperation, which took a possibly lethal hit when the Israeli air force in September 2007 destroyed a breeder reactor under construction at Dayr az-Zawr in eastern Syria. Although it is possible that cash-strapped Syria on its own undertook to develop nuclear weapons, it is more likely that Iran supported this enterprise as a back-up to its own atom-bomb program. Israel's preemptive strike is a setback for Tehran, but its echo inside Iran appears to be limited, since neither the Israelis nor the Americans used it rhetorically to show what could happen to the mullahs' nuclear project.

Most important, Iran has pushed hard in Iraq, giving aid and military training to militant Shiites, whose targets have included Sunnis, Americans, and other Shiites. The mullahs and their Revolutionary Guards corps have become a small expeditionary force in Iraq and have clearly shown that they aren't peace-loving Persian uncles trying to bring stability and prosperity to their Shiite Arab nephews.

President Bush's surge caught the Iranians off-guard and turned what had been a winning situation for Iran in Iraq--multiple Shiite parties dependent upon Iranian aid and good will in a savage battle against Sunni insurgents and al Qaeda--into a potentially huge defeat for Tehran. Barring a strike by President Bush against Iran's nuclear sites before January 2009, Iraq is the only arena where the administration is capable of moving effectively against Tehran.

The Iranians have seriously overplayed their hand along the Tigris and Euphrates. In their love of the Hezbollah model, they have helped to build up Moktada al-Sadr, the scion of Iraq's most revered clerical family, who became a Shiite street hero for his defense of the Shia against Sunni insurgents and al Qaeda. Sadr's followers include the only Shiites willing and able to kill Americans--another hugely attractive factor to the leadership in Tehran, since wounding America in Iraq is as indispensable to the ruling elite's sense of purpose as raining Katyushas down on Israelis.

Yet Sadr's men are a hypercharged mix of Arabism and Islamism; as a rule, they are not terribly fond of Persians. They were inevitably going to clash with the followers of Abd al-Aziz al-Hakim, the leader of the Supreme Islamic Iraqi Council (SIIC), a group founded in Tehran and which has maintained deep ties to many in Iran's religious establishment. The Sadrs and Hakims dislike each other. In the streets of Qom, Iran's most prestigious seat of clerical education, the representatives of the Sadr and Hakim families often throw shoes at each other. (Among clerics that is very bad.) Tehran should have known that it couldn't back both the Sadrists and the SIIC.

Although conscious of the fleeting loyalty of Iraqi Shiites who once took refuge in Iran from the wrath of Saddam Hussein and are now blessed with ever-larger Iraqi oil revenues, Tehran probably didn't anticipate how quickly Shiite sentiment in Iraq could change. The Iranians didn't see the rapid rise of the Iranian-born Grand Ayatollah Ali Sistani, who has become the most popular ayatollah in Iran as well as the most powerful cleric in Iraq. Iranian and Iraqi clerical ties are old, complicated, intensely personal, and often quite affectionate--all of which now plays powerfully against the Iranian ruling elite's cynical politics in Mesopotamia.

It is a very good bet that Sistani and other prominent Iraqi clerics have remonstrated vociferously with their Iranian interlocutors in Qom against Iranian-fed violence among Iraqi Shiites. We can see the Iranian side of this in former president Mohammad Khatami's accusing Khamenei virtually by name of spilling Shiite blood in Iraq and turning Iran's Islamic revolutionary message into a call for violence and upheaval beyond its borders. Khatami's recent speech at Gilan University is an astonishing sermon from a man not known for boldness.

In the time remaining to it, the Bush administration should do all it can to reinforce this Shiite dissent and outrage. The surge aside, it is the most effective vehicle for checking Iran in Iraq and stabilizing Iraqi politics. The U.S. government should broadcast as loudly as possible any and all information showing Tehran's complicity in the death of Iraqi Shiites. If the United States can again arrest members of the Revolutionary Guards Corps inside Iraq, it should do so, interrogate them rigorously, and make the information public. The tide may have turned for good against Al Qaeda in Mesopotamia, with potentially huge ramifications for hearts and minds throughout the Sunni Arab world. The clerics in Tehran could be dealt out of the inner circles of Iraqi Shia politics. With continued progress in Iraq, the next administration would be in a position to turn its full attention to thwarting Iran elsewhere in the region--and to preventing the mullahs from acquiring nuclear weapons.

--Reuel Marc Gerecht, for the Editors