I don't think ordinary people can read Sydney Freedberg's excellent cover story in the new National Journal but the teaser text explains the basic dilemma well:
Sometimes U.S. troops kill Iraqis in self-defense. Sometimes they kill them for other reasons. And sometimes it's hard to tell the difference.
The crux of the matter is that soldiers in ambiguous situations understandably tend to err on the side of their own personal safety and that of their fellow soldiers. Likewise, officers faced with ambiguous situations tend to err on the side of giving the soldiers under their command the benefit of the doubt. And courts-martial, likewise, err on the side of taking a favorable view of American soldiers.
All of which is fine. Unless you happen to be an Iraqi. Which is precisely why people tend not to enjoy being under foreign military occupation.
The reality of the matter is that to succeed, our troops would need to behave the way police officers do. But they're not cops, they're soldiers. And there's a good reason that soldiers act the way soldiers do. There's no way that it would be politically feasible -- or even appropriate -- for the US military to start treating Iraqi lives as more important than American lives. But that would be the only way to actually pull off what they've been asked to pull off. It's an impossible situation, and not one we should be putting people in.