The capture of Fallujah came amid an explosion of violence across the western desert province of Anbar in which local tribes, Iraqi security forces and al-Qaeda-affiliated militants have been fighting one another for days in a confusingly chaotic three-way war.
A few thoughts:
(1) The U.S. 'surge' of 2006-07 in Iraq depended on making allies of the local Sunni tribes in the west in the fight against al-Qaeda. Those groups still oppose each other, but what was more-or-less a two-way conflict then has now become a three-cornered one, as the tribes are apparently no longer willing to make any kind of common cause with the Iraqi government. This point leads to:
(2) Had Maliki's government made more of an effort to reach out to Sunnis and bring them into positions of responsibility/authority, the disaffection of the Sunni tribes in Anbar province might have been less and there might not have been the demonstrations against the Maliki government that led to the Iraqi security forces' response and thence to the current situation that the linked article describes. That's a lot of "mights," but it seems hard to avoid the inference that Maliki's shortsightedness, foolishness, fearfulness or a combination thereof have contributed to the current mess.
(3) It might be tempting to argue (as McCain and others no doubt will) that had the Obama admin adopted a more interventionist position on Syria, the al-Qaeda forces currently operating across the Iraq-Syria border would not have had the opportunity to reconstitute themselves in the way they have over the past year or so. But this assumes, first, that a U.S. intervention in Syria would have been able to alter the dynamics of the Syrian civil war fairly quickly and easily, and second, it assumes that if Assad had been removed from power, 'moderate' rebel forces in Syria would have been strong enough both to hold the reins of the state and to keep at bay al-Qaeda and/or the Nusra front and the other anti-Assad Islamist elements. Both these assumptions seem questionable (if not simply wrong).
Added later: Liz Sly (WaPo) has another piece on various Syrian rebel groups fighting against ISIS in northern Syria. (I will put in the link later.)