Iran War: Iran's new hostages

"God may smile on us, but I don't think so." - anonymous Pentagon adviser quoted by Seymour Hersh April 2006 on Bush administration plans to pressure Iran militarily

Gareth Porter reminds of one of the major risks an attack on Iran would create: US troops in Iraq are Tehran's 'hostages' Asia Times Online/Inter Press Service 09/22/06.  He writes:

For many months, the administration of US George W Bush has been complaining that Iranian meddling in Iraq is a threat to the country's stability and to US troops. The irony of this publicity campaign over Tehran's alleged bid to undermine the occupation is that Iran may well be the main factor holding up a showdown between militant Shi'ites and US forces.

The underlying reality in Iraq, which the Bush administration does not appear to grasp fully, is that the United States is now dependent on the sufferance of Iran and its Iraqi Shi'ite political-military allies to continue the occupation.  (my emphasis)

                          Muqtada al-Sadr

Porter argues that the main threat to the US occupation right now comes from Muqtada al-Sadr and his Mahdi Army:

But the Bush administration and the military in Iraq still appear to believe that there is some way to contain Muqtada's power. They have not yet accepted that Muqtada has both the intention and the capability to bring down the US occupation.

Yet Muqtada has made no secret of his intentions. In an interview with the Washington Post published on August 11, his top deputy, Mustafa Yaqoubi, said, "If we leave the decision to [the Americans], they will not leave. They'll stay. To get the occupiers to leave, they need [to make] some sacrifice."

As he reminds us, the Iraqi Shi'a bear a long-term grudge against the US.  In their view, Old Man Bush encouraged them to revolt right after the Gulf War of 1991, and then left them in the lurch.

I thinkthey're basically right about that.  Robert Fisk discusses that incident at some length in The Great War for Civilisation (2005).  That was one of the main underlying reasons that the reception of the American forces in 2003 was less enthusiastic than expected.  The Shi'a were our natural allies in that situation.  But they didn't trust us because of 1991.  Porter writes:

If Muqtada and his followers are already preparing for a showdown with the US occupation forces, the only factor that appears to be restraining the Mehdi Army now is Iran.  After all, Tehran's interest lies not in forcing an immediate withdrawal of US forces, but in keeping them in Iraq as virtual hostages.  The potential threat to US forces in Iraq in retaliation for an attack on Iran is probably Tehran's most effective deterrent to such an attack. ...

Only Iran's ability to persuade Muqtada to hold off on his effort to end the occupation can prevent a violent confrontation between Shi'ite militants and the occupation forces.  But Bush's advisers may still not understand how fundamentally the power equation in Iraq has shifted.  (my emphasis)