"The Sunni turn against al-Qaeda had very little to do with American diplomacy or military efforts, and far more to do with local power struggles and preparations for the widely-expected coming war with the Shia. The origins of this shift in Sunni politics date back to last year's attempt by al-Qaeda in Iraq to impose its hegemony over the Sunni insurgency and to establish physical and political control in a variety of locales.
Al-Qaeda's attacks on Iraqi Shia had always been controversial among the insurgency's factions, many of which preferred to keep a tight focus on attacking American forces and Iraqi government personnel. Al-Qaeda made many enemies with its grandiose rhetoric, attacks on local political figures, attempts to enforce Islamic morality, and decisions to muscle in on tribal smuggling routes. When it declared the "Islamic State of Iraq" as an umbrella for the insurgent groups, the major "nationalist" factions which make up the overwhelming majority of the insurgency decided they had seen enough. The Islamic Army of Iraq released the first public denunciation, other factions followed suit, and nasty fighting (both verbal and military) ensued. The root of the conflict was a struggle for power within the Sunni community -- not attitudes towards the United States or even the central Iraqi government. The turn against al-Qaeda did not mean abandoning the insurgency, even if some of the groups are willing to use American support for their current tactical needs." (Thanks Datta)