So all wars should turn into an insurgency? It's pretty clear that you guys haven't been to the Middle East. An official military investigation launched every time one of us fired a bullet to ensure that no war crimes were committed (to include if we fired warning shots, as those were against the published Rules of Engagement). Sure, we got in some firefights, and yeah, my guys and I did some raids to kill/capture bad guys. But we spent a lot more time training the host-nation forces, doing things to build rapport (setting up clinics, schools, and playgrounds), and trying to help. Get that Hollywood stuff out of your heads because it simply isn't true. It's not like we were looking to kill innocent people for sport or something. That would be absolutely insane, and no reasonable person would even want to do that.
It's not that simple. There are plenty of really bad Shias, such as the supporters of al-Sadr. The Sunnis get a lot of the attention though, as AQI, ISIS, and al-Nusra are all Sunni organizations.
This reminds me of a tailor that worked on one of the Forward Operating Bases I was on. That tailor's name was John, and he was originally from Syria. His family had moved to Turkey when he was young. He was an absolutely great dude, and I hope he's well today, wherever he is. Anyways, when I was asking him where he stayed, he point to one of the housing units on the base. I asked him if he ever left the base outside into the local population. He said, "No! Of course not! If I stayed out there for a day, someone would find out that I've been friends with the Americans! They'd cut my head off!" So stuff like that matters to people. It's not like every single Iraqi is indifferent, as long as their needs are met. And it's not like they just hated Americans. There were plenty of great Iraqis who were good people, some of which even fought alongside us against the militants. It's a pretty awesome experience when you and your Iraqi counterparts are shooting in the same direction, and when one of your dudes needs help, an Iraqi bravely exposes himself to enemy fire to come to the aid of an American (and vice versa). But unfortunately, these do not represent a majority of the people. I wish things were different.