"When US troops are looking for "insurgents", they kick down the doors of houses. In neighborhoods, towns and villages. Where Iraqis live. When an insurgent goes out, he or she can go to the store, work, training, whatever. By themselves. And then go home.

When US troops sleep, they sleep in encampments, fortified areas, where Iraqis don't live. It's not safe for them in the communities. When they go out it's in convoys, patrols, armored vehicles, in groups. And then retreat back to their camps.

Now tell me who is popular, and who isn't."

Iraqis know and can recognize US troops. US troops don't kill innocent citizens. If 50 US come to an area, the Iraqi can relax. If 50 unknown thugs come to area, Iraqis are fearful.
Your problem is you seem to be imagining that all these insurgents are kind of like part time thug- rather than some of them soldiers without uniforms working for various local factions or foriegn govts. In addition in your view seems to be is the large number of street gangs all over this world are what you would call popular. For instance a group of people selling drugs in a residential area are viewed by the local population as something they like and if only they didn't have such a thing as a police force, then everything would be so much better.
In addition you imagine that local gangs are friendly to gangs from other areas.
What you fail to understand is that US forces are not going going to areas which are quiet and peaceful. The Kurd area is mostly quiet and peaceful- so no US soliders are required to patrol that area- and there lots of other areas like this.

Instead they going to areas in which there is unrest and/or areas in which gangs are controlling an area instead Iraqi police controlling the area.

"Migawd, Tom, it's elementary, simple and if that can't be understood that CAN explain how we got into this mess.

I repeat. A successful insurgency, and this one is successfull, is only possible if there is popular support. That's doctrine, everyone's doctrine, even the US military. It's common sense. "

It's also true in regards gangs that sell drugs or The Mob or organized crime. Without popular support all crime would vanish. But this should not be understood to mean that most people want crime, nor even movies that glamorize crime and large numbers of the public actually pay money to watch mean they like criminals [though no doubt they could have silly romantic notions regarding them].