The backgrounder makes no assumptions and is fairly thorough. Here is the link: http://www.cfr.org/publication/12521/
CFR also has an interview with Ken Pollack of the Brookings Instiution.
On the relationship between Iraq and Iran:
KP: ...We’ve seen the Iranians do some things that are unhelpful to the security of Iraq. But we’ve also seen the Iranians do some things that are actually very helpful to the security of Iraq. In particular, whenever they’ve had the opportunity, they have encouraged their allies inside Iraq to go along with reconstruction, to participate and not to fight the United States. And that’s extremely important. The Bush administration, on the other hand, seems to be regarding the Iranians as the source of many, if not all, of Iraq’s problems today. To me, it is dangerously reminiscent of how they talked about the Syrians in 2004 and 2005, when they ridiculously exaggerated Syria’s role in the Sunni insurgency. I’m afraid they are starting to do the same thing with Iran and the Shiite insurgency.
CFR: Why do you think they’re doing that?
KP: Just as in 2004 and 2005, when it was much easier to blame the problems of the Sunni insurgency on Syria, it’s much easier today to blame the problems of Shiite ethnic cleansing on the Iranians. The truth is that Iraq is a mess. It is in a state of low-level civil war. And all of these groups are largely self-motivated. But it’s much easier to blame it on the Iranians and use the traditional methods of state power—saber rattling, deploying carriers to the Gulf—as a way to try to deal with the problem.
I also do think there are some members of the Bush administration who have always favored war with Iran. They wanted to use military force against Iran back in 2004 and 2005 to deal with Iran’s nuclear program. They were basically shut down at that time. The administration decided to follow a diplomatic approach instead. And I fear the same group of people never gave up that aspiration, and they are now finding a back door to war with Iran.