Yglesias makes the important, if obvious, point:

The news that the Islamic Army of Iraq (one of the main Sunni insurgent groups) fought a battle against al-Qaeda for control of a Baghdad neighborhood would, in a decent world, put to a rest the idea that we’re fighting some consolidated “jihadist” menace in Iraq. We’re fighting a whole bunch of people. Many of those people are fighting other people who we’re also fighting.

Also in that decent world Yglesias mentions would be a policy that recognised this divergence between different warring factions and which sought to exploit the divisions among them to American advantage.