After having destroyed Saddam's army and dispossessing the Sunnis in favor of the Shi'ites, after Abu Ghraib and it's indelible pictures, after the total destruction of Fallujah, in short after a victory achieved with the utmost brutality, contempt and humiliation of Iraq and Iraqis, the Entity was in charge.
Then the "insurgents" appeared. They put improvised explosive devices along the roads so, with a phone call, they could destroy patrols of the Entity. They made car bombs so that every vehicle approaching a check-point might spell doom. They donned suicide vests to blow themselves and any nearby Entity soldiers up. Entity soldiers couldn't go into the streets. Every move they made could be their last. The enemy was everywhere and nowhere.
These people would rather die then be ruled by these idiotic mechanized barbarians. Everything seemed peaceful, but at any moment, out of nowhere, they could be blown to pieces. That kind of thing wears on you. Their patrols, pointless bouts of Russian roulette, ended up as parked "search and avoid" missions. Life went on without the clanking monsters. Entity bases were like Kaposi sarcoma in AIDS patients.