The new fad in American thinking on Iraq is the Darwin Principle, which basically says "three ethnic groups just can't get along, let's get rid of one so there are just two." The method to accomplish this is to look the other way while Shi'ite militias cleanse Baghdad, southern Iraq, and basically anywhere that isn't al-Anbar of Sunnis (many Sunnis have been leaving Shi'ite areas already).
So, to take a trip down memory lane, our reasons for invading/occupying/not withdrawing from Iraq:
Saddam has WMD (Rick Santorum still thinks he did)
Saddam can not be contained or deterred (he was)
Saddam perpetrated crimes against humanity (back in the early 1990s, before he was contained)
Spreading democracy (going great!)
Counter Iran's influence in the region (Opposite has happened)
Protect oil fields (Failed)
Prevent an Iraqi civil war by balancing against Shi'ites to protect Sunnis (Iraqi civil war happening anyway)
And now finally, the Darwin principle, i.e., Facilitate and then end the Iraqi civil war as fast as possible by bandwagoning with Shi'ites.
Reasons this Darwin bandwagoning idea is bad:
1) All our allies in the region (Saudi Arabia, Jordan, Turkey, Egypt) are Sunni Muslim and will not like it if we aid the slaughter of their allies in Iraq. Saudi Arabia has even threatened to intervene themselves (while the author of that op-ed, Nawaf Obaid, has since been fired by the Saudi government, that does not necessarily mean his proposal for aiding Iraqi Sunnis has been discounted by the Saudis since there is currently a big shake-up in the Saudi foreign policy community and honestly nobody has any idea what's going on).
2) It is immoral (aiding ethnic cleansing, duh).
3) It's immorality would seriously damage US credibility in the world, which is basically the only reason hawks can cite to keep American troops in Iraq (other than shouting "What if we lose!?!?").
4) It would also reduce American credibility because it would be a complete 180 reversal of current strategy (take, hold, build, or whatever the idiotic marketing slogan is). By its very nature as a 180 reversal it would be ineffective because it would damage perceptions of American reliability.
5) Sunni/Shi'ite identities aren't wholly exclusionary, bringing up the question "how Sunni must you be in order to be deported/killed/etc?"
6) It would aid Sunni Islamist terrorists in their goal of radicalizing as many Sunni Muslims as possible; depending on media coverage, it could radicalize more than the invasion and original occupation itself.
7) It assumes the American military has some semblance of control of the current situation (i.e., would doing something be that big a change from the status quo of doing nothing, in light of our capabilities or lack thereof?)
Sigh. I would have thought this was too horrible of an idea to ever see the light of day, but I keep thinking that and I keep being proved wrong...