By Ken Pollack in the New York Times:
So Tehran can influence the behavior of the Shiite groups, but it probably would have a hard time forcing them to do things they do not want to do — like disbanding their militias, accepting a national reconciliation agreement, participating in an equitable oil-sharing scheme or accepting any of the other major changes that the Bush administration is seeking. If Iran were to threaten to end its support for these groups, they would most likely tell Tehran to get lost. What’s more, the Iranians seem to understand this, having so far proven reluctant to try to force any of the Shiite groups to radically change course.