Some of whom paid with their lives. Let's not fall into a new kind of racist condescension: If the locals do something that agrees with what we want (anyway), it can only be because we either tell them, weak and simple-minded as they are, to do it or bribe them to do it. Otherwise the pathetic poor souls just muddle along or do the right thing and fight us. We see this self-serving attitude in Iraq, in Ukraine, and in more than a few other places. At best those who work or agree with our domestic political and class enemies show false-consciousness, but usually they're craven, weak and pathetic, or just grasping and working against the good.
So in Anbar, factor #1 was local disgust with the Islamists. This was both the heavy-handed tactics that the "saviors" had done, that many were from out of area, and that they were challenging the local traditions and power structures. The local resistance to the Islamists came first, but the Islamists were mopping up the floor with the stumps of their severed heads.
Factor #2 was the American forces who acted under the shelter of the tribal leaders. This stopped the mopping up bit and encouraged otherwise weak tribes and clans who feared opposing the Islamists to join in. Certain martyrdom for the faith is fine, for some zealots at least; certain martyrdom that leads neither to power nor to honor is a non-starter. There is no dishonor in yielding to an enemy that you can't possibly defeat. You sometimes just side with the stronger power to preserve what you can.
Factor #3 was money, for several reasons. To leave out any reason knowingly is to implicitly assert that a partial truth is a whole truth, and in many ways that's worse than a lie because it's believable and non-falsifiable. US money paid some otherwise indifferent young men to fight for the Awakening and not for the Islamists (civic-minded souls that they were, their honor and loyalties often went to the highest bidder), it bribed some tribal leaders who were on the fence and in some cases found working with the Islamists to be an okay revenue source. However it also paid those who honestly did oppose the Islamists to fight--both because they needed the money and wouldn't have the time to fight without it, but also because to fight and kill somebody by accident would let loose a blood feud that could be ended with diyyah. Except that diyyah meant money. Finally, the money and cooperation and even the token subservience the US troops who were working with local leaders showed helped enhance the reputation and honor of the tribes that were fighting and made cooperation with the Americans or resisting the Islamists no longer betrayal and dishonorable.
Notice that #3 isn't a completely independent reason: In some ways it was, but it also leveraged #2 and #1.
So currently the "troops" have a very specific explanation. The Sunnis have been humiliated; it was part of the reason for the fighting in the '00s. They were in charge; they were properly the rulers of Iraq; they were disgraced by being tossed out of office, told that their icon was evil, and even told they weren't a majority of the population. Their revenue and Sunni privilege weren't just revoked, instead revenue went to Shi'ites and there was Shi'ite privilege. "Humiliation" is the right word for what they felt. Reclaiming honor and prestige--not to mention privilege--is important.
Most of those fighting the IS in the Arab areas are Shi'ites. Most of the Arab areas with fighting are Sunni. (The Kurds have their own turf to reclaim.) To side against Sunnis with Shi'ites is to own your humiliation. Moreover, the Shi'ites are Iranian allies. Now, if you think Americans have a bad rep, try being Persian. Simple enough, so far.
The explicit purpose of the Am advisors in Anbar is to train Sunni soldiers to fight. Now, this may be a losing proposition. But as before, and as has been repeatedly pointed out in the defense of Sunnism and Muslims, most Sunni Muslims do not support the IS and even oppose it: Individual tribes may support it, but that doesn't mean each clan or tribal member does, and not all tribes support it because they agree with it. See "factor #2" above. The goal is to turn that passive opposition into active opposition. Then it would be Sunni fighting Sunni. Now, this creates problems for all sorts of people.
For the Islamists, it's fitna, but then again they're often pompous takfiri. They'd have no trouble declaring the Sunni fighters kafir. But more importantly the battle is no longer Shi'ite v Sunni, no longer is humiliation the dominant factor. Instead, it can be billed as trying to reclaim honor, and the force necessary for this wouldn't have to be all that large. (Think of it as a class struggle for $, if that helps, even though it's not. Americans think everything boils down to money or power, which are closely linked. In many cultures, and in pre-Enlightenment Europe and in some subgroups and contexts even after that public honor was important and personal shame hadn't replaced it as a cultural determinant.)
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