Following the US withdrawal Sunnis in Iraq took to protest. Multiple Iraq resistance factions formed socio-political wings and temporarily "disarmed". Three such groups were Jaish al Naqshabandia and they're protest wing, Intidadat Ahrar al Iraq, Ansar al Islam and Islamic Amy in Iraq, which formed Sunni Popular Movement. Additionally many tribal leaders in Iraq formed the Herak Movement as a means protesting the Iraqi gov. All of these aforementioned groups-political and military- were all hostile to ISIS. They were fighting ISIS in Iraq before the FSA or Ahrar ash-Sham or SRF or Jabhat al Nusra became things. These gys were all branded as apostates by Zarqawi himself(and Abu Omar). Anyways protesting was going no where and slowly became more violent. Still no insurgent acts were taking place from them in particular. Massacres took place in many sit-ins. One such was in March 2013 hosted by Jaish al Naqshabandia. The brutal nature of ISF caused many to fight back with violent means but this was quickly quelled. The Iraqi gov afterwards claimed it was ISIS doing the fighting. It wasn't. At this moment the Free Iraqi Army formed but they were largely irrelevant until 2013 of Dec but I'm getting ahead of myself. So anger is slowly building. This would cycle on and off until Dec 2013. On one of the largest sit-in protest became ugly and Maliki called in the Army. Protest camps were torm apart and people maimed en masse. At this point Iraqi Sunnis finally said "Fuck this" and officially took up arms. The Tribal Revolutionary Council was formed and Sunnis reached the point of know return (in their minds at the time).
Enter the beginning of the Anbar Conflict.
ISIS after being rooted out of of Iraq due to the formation of the Sahwa and "the surge" were silent. Notable attacks happened infrequently or rather less frequently. The US pulled out soon after. Off topic but around this time was when Salil al Sawarim 1 was released but since ISI were overall not as active as they were during the Civil War the MSM overlooked it. Syria happens as we al know and ISI do their thing there and network and rebuild strength and followers that they lost due to infighting among Iraqi rebel groups and massacres of Sunni civs. So ISIS creeps back into Iraq with Sunni with arms wide open in Decmber. They were still not the only group. And they with the help of others took over Fallujah and you know he rest.
source: I'm an Arab, not some random white dude in America fantasizing about Assad dick. I actually speak Arabic and come in contact with actual Arabs in actual Arab countries. My father actually owns property in multiple Arab countries(Palestine, Lebanon and Jordan). So I know a thing or two about the Arab world.