It's easy if you don't like 'em.
Yes, I realize it's a 'duh' thing, but hear me out.
For instance, say I'm somewhere in South America. Say I'm up in the Andes, in Colombia, me and a squad of Jarheads or Green Berets. Say we're there for a week, watching this little dirt airfield that has a buncha out buildings on the property surrounded by acres and acres of fields. Say I have intelligence telling me this place is a harvesting, processing, and transport location for some big Colombian coke dealer. Fine.
I don't like these guys. I don't like the guys fueling and loading the R4D that's sitting there, I don't like the guys working in the buildings, and I don't like the guys working in the fields in this valley between the peaks. They're drug dealers. They're the enemy.
Now just say I happen to make radio contact with a flight of Navy F-14s or Marine A-6s who just happen to be doing some combat air patrol drills just offshore in the Pacific and ask for their 'assistance'.
The lead pilot is going to ask me for coordinates. Now, am I going to give him a vector that will bring him in over the airfield and destroy just that or am I going to bring him in at the far end of the valley and let him bomb everybody in the valley, field workers, mechanics, and hangers on?
Of course, I'm gonna kill everybody. Why? Because I have no attachment to them. They're the enemy, whether or not they make any money from the drug trade, whether or not they even knew exactly who they were working for. They're the reason I'm sitting in this bug-infested, snake-infested hidey hole for a week. I don't care because our actions are part of the 'War on Drugs', and these people are working for the enemy. So long and good night, let the fireworks begin, take it up with your god when you get there.
Why do I relate this little story? The Vietnam guys know. It's the reason we didn't win there and the reason we won't win in Iraq. It's the reason we ain't winning the War on Drugs.
The grunts hate the Iraqis.
After the 3rd and 4th deployments, with no light at the end of the tunnel except the oncoming train of more deployments, the grunts have had it. They're angry at their situation and they're gonna blame the people whom they see to be keeping them there. The Iraqi people.
They see the Iraqi police, or army, or whatever you want to call them, not showing up for missions, running out on missions. They see the insurgency, gunmen who come out of nowhere to harass them and then melt into the crowd. They are beginning, thanks to the insurgency, to look at every Iraqi as a possible enemy. They are beginning to dislike the Iraqis.
So now I ask. You're a grunt in Iraq on your 3rd deployment and you gotta clear a house. There's a buncha folks in that house, some enemy some not, maybe. Are ya gonna go in on full automatic?
It's an easy decision to make if you don't like 'em.