It's the unresolved moral confusion that lingers. One of the marines called me the other day and said, "I did all of these things my soul recoils from, but going to war was the best thing I've ever done." Killing, being able to murder—it's empowering. Maybe that marine who broke down over the little girl cried because shooting his M16 at a car full of civilians is the best thing he's ever done.

I want to show the world as it is, not as it ought to be. It hit me while writing the book how monstrous it was to feel so little remorse. Sometimes I wanted to grab the radio, call in an airstrike, and level an entire fucking village just to stop one sniper from shooting at me. I realized in war that zombie movies are sort of true: There's civilian life and then the zombies come out and everyone is eating each other's flesh—that's what we do to each other. The military guys used to say, "We went to war and America went to the mall," and they're right: You have no idea what humans are capable of doing when you live on the peaceful side of things.

I was supposed to embed with the well-known journalist Hampton Sides. I still had a chip on my shoulder from being "the Hustler guy," I was worried he'd file stories before me so I told him horror stories about my grandfather being gassed in World War I. I'd say stuff like, "Do your kids know you're here?" "Do you have insurance?" "What will happen if you die?" The night before we embedded he pulled me aside and said, "I feel a lot of anxiety." "Hampton," I said, "you have an obligation to your family," and it worked. I got rid of him.

My photographer bailed last minute too. The Leica M6 I brought had a broken light meter so I got some crappy digital camera in Bahrain. I also found a bulletproof vest on eBay for $900. It was Vietnam-era and I had to duct-tape it together across my body. Later I found out that sweat and time degrade Kevlar's efficacy so, basically, I brought with me a worthless piece of shit.

Reporters always ask, "How do you get people to talk?" The answer is time: Get in a car and take a long road trip with your subject. It's weird how people just open up in cars. I was lucky—I got into a Humvee with four marines and drove with them miles and miles across Iraq.

At one point Gen. Matthis said, "Evan, you're the only reporter left." By then I had amassed several thousand pages of notes, 50 rolls of films, and 48 hours of audio. The thought of leaving these men, the intensity of war, made me depressed. I remember Sgt. Antonio Espera said, "Are you serious, dog? The shit we've been through, the people we are—a fucking idiot could write this down and it would be a great book."