The Sunni Coalition is splintering. That coalition is/was a group of tribal alliances who were fighting against what they saw as unwanted foreign influences. No, not the United States. Their hate was directed Al-Qaeda and other “foreign fighters” because, really, not very many people like the idea of setting up a neo-Taliban complete with Sharia laws in their country (It's the Muslim version of NIMBY, I guess. But it's important to remember that the jihadist are really, really extreme and unpopular.). Which was widely seen as a really good thing for the US and Iraq, as a whole.
So what screwed it up? The US trying to get their sticky fingers all over it.
As usual, our presence in the country and its politics winds up making things worse than they would be otherwise. Because while the average Iraqi might loathe Al Qaeda, the United States runs a close second. Our occupation inflames the people trying to get us out of the country and demoralizes the ones trying to oppose them. As Mr. Yglessias says, the only sensible move here is to step away and let things play themselves out.