The war in Afghanistan was in its twelfth summer. The Taliban launched a sophisticated suicide attack on the Presidential Palace, filling morning rush hour in the country’s most secure location with gunfire and carnage. Meanwhile, the Doha peace process, was creating more furor than rapprochement, and relations between the Afghan government and the US were deteriorating into acts of intimate, unpredictable violence.

Assigned by TIME to investigate incidents in which Afghans turned on their coalition partners, McDonell zeroed in on a single bloody night in August when an Afghan National Policeman murdered three of the United States’ most elite Special Forces operators. In the aftermath authorities detained the wrong man — whom the media implicated but never cleared — and the real attacker escaped. He had been living next door to the Marines he killed for over a month.

“Insider attacks” like this one — “green on blues” — had, in their opacity and violence, come to define the war. In 2012, green-on-blue incidents claimed the lives of 64 personnel serving in the international coalition of forces occupying Afghanistan. As General John Allen, former supreme commander of the coalition, put it, they are “the signature attack.”

McDonell unraveled a particularly complicated one. Tracking the players through Kabul and Helmand he reconstructed their lives, and the process which led to the fatal betrayal. In the tradition of Sebastian Junger’s War and Jim Frederick’s Black Hearts, Green on Blue is reportage that opens up the conflict through individual men who would never escape it.