From the NPR article:
Lt. Col. John Nagl wrote a book about fighting insurgents called Learning to
Eat Soup with a Knife.
He remembers working closely with an Iraqi police chief who provided
valuable intelligence. Then, he learned that the man he had trusted was supporting the enemy -- "providing weapons, ammunition, body armor to the insurgents in Fallujah who were then fighting the Marines. And against some of my soldiers."
Nagl said he found himself "faced with a horrible dilemma."
"What do I do to this police chief who has clearly risked his life to help
us? Every time I think about it, I wonder if I did the right thing. But ultimately what I decided to do was -- nothing. My assessment was that for Ishmael to stay alive this is the minimum he had to do -- this is the minimum tax he had to pay to the insurgents."
Part One of the NPR series: Training the Trainers at Fort Riley.