WASHINGTON DC., January 31, 2006

Jimmy Carter presented Iran with 52 hostages. George Bush has done a lot better, sending 130,000 Americans across the ocean as guarantees of his administration's good behavior toward the Islamic Republic. Last week, Tehran reminded us of its ability to make life unpleasant for US forces in Iraq by hosting Moqtada al Sadr for a high profile visit, in the course of which he obligingly pledged that his militia, the Mahdi army, would retaliate for any American attack on Iran. His spokesman quoted him as telling his hosts "If any Islamic state, especially the Islamic Republic of Iran, is attacked, the Mahdi Army would fight inside and outside Iraq."

This warning should be taken seriously. The Jaish al Mahdi, al Sadr's militia, has emerged as a formidable force since its formation in 2003. Fifteen months ago, in November 2004, when it was less well trained and equipped than today, this army held off a determined assault by US Marines for three weeks in Najaf.

But Iranian interest and influence in Iran are by no means confined to the radical Shi'ite cleric and his fighters. SCIRI, the principal party in the dominant Shi'ite coalition that triumphed in the Iraqi elections, was after all originally founded and fostered in Iran. Its first leader was Ayatollah Mohammed Shahroodi, presently head of the Iranian judiciary. SCIRI's military arm, the Badr Army, fought on the Iranian side in the Iran-Iraq war, and was long regarded as the direct instrument of Iranian intelligence. Elsewhere, Iranian intelligence can look to such assets as Abu Mehdi al-Mohandis--"the engineer"--resident in Najaf with mentoring responsibilities for Sadr's militia there.

In the north, in and around the Kurdish enclave, credible sources attest that Iranian intelligence has been providing some measure of support to Sunni insurgents, including the militant Islamic Sunni group Ansar al Islam. Indeed, the dozen or so senior Iranian Revolutionary Guard (IRGC) commanders killed in a plane crash two weeks ago, possibly including Mohammed Sulaimani, the key Guards official involved in Iraqi affairs, were on their way to Oroumieh in north west Iran, the main base for Iranian operations in northern Iraq.

It may seem counter-intuitive for the Shi'ite Iranians to be supporting groups with a militantly anti-Shi'ite agenda, but this same regime sheltered the Afghan fundamentalist Sunni leader Gulbeddin Hekmatyar for many years, despite deep seated mutual antipathy.

Furthermore, power in Iran is diffused. Iraq is a huge prize, and control of this asset, so obligingly proffered to Iran by George Bush when he toppled Saddam Hussein, is inevitably a matter for contention among powerful factions inside the regime. Revolutionary Guards commanders may have a different agenda from that of the "Etalaat"--intelligence services, or the office of Supreme Leader Khamanei, let alone that of the elected President Amahdinejad. Among other imperatives, these various fiefdoms have financial interests at stake in Iraq. Many of the IRGC commanders, for example, are "Moawedun," meaning they are of Iranian descent but born in Iraq, who have property interests in Iraq.

Following the US invasion, the most influential voice in Iranian policy toward Iraq was that of President Hashemi Rafsanjani, who opted for limited cooperation with the occupiers. Despite alarmist rumors circulating in Baghdad that "One million Iranians had infiltrated into Iraq with fake Iraqi ID cards," most of the Iranians on view were pacific pilgrims thronging the shrine cities of Najaf and Karbala. The consensus in Tehran appeared to be that Iraq should be maintained in what officials called "managed chaos;" both to keep the country weak and discourage a prolonged US occupation while avoiding the wholesale disintegration of Iraq into anarchy.

However, the defeat of Rafsanjani by Mahmoud Ahmadinejad in the Presidential election and the steadily escalating confrontation with the US over Iran's nuclear program have changed the rules of the game. Ahmadinejad is close to some of the more radical IRGC leaders, and shows little desire to defer to American sensitivities. His outspoken defiance of the west over the nuclear issue, not to mention his remarks about Israel, have only bolstered his political position at home, while his ability to play the Iraq card should certainly give Washington pause. As a close aide to one of the leaders of SCIRI, which is generally considered less violently radical than Moqtada Sadr's group, told me recently "If America attacks Iran, then all bets are off." With such a deterrent at hand, who needs a nuclear weapon?

Andrew Cockburn is the co-author, with Patrick Cockburn, of Out of the Ashes: the Resurrection of Saddam Hussein.