Junger’s central question is this: What happens when a few dozen young men have nearly every conceivable civilizing influence—even by the standards of a war zone—stripped away from them, and are then exposed to more than a year of frequent combat against an evasive and intractable enemy? How do emotions like fear, courage, and love change, intensify, and distort within such an inferno? Junger discovers that, for the men fighting in the Korengal Valley, war is something formless and intermolecular, an airborne toxin as euphoric as it is lethal.

I have never felt more admiration for a group of American soldiers than I did after finishing this book. I have also never been more terrified by a group of American soldiers. Most members of the Second Platoon want to fight and kill; some even claim to live for firefights. A number of them have infidel tattooed across their chests. When they hear one of their scouts describe a wounded insurgent crawling along a mountain path toward his own blown-off leg, they lustily cheer. Almost all are, by their own admission, terrible garrison solders: undisciplined, lippy, and disdainful of any authority unscarred by combat. Jacked up on testosterone, fuel-injected by adrenaline, emotionally shaped by sexual deprivation, and under mortal pressure, they joke about raping mothers and sisters and even one another. (One soldier fond of “smoochy come-ons” is finally asked outright if he would have sex with another man. Sure, he says. “It would be gay not to.”) The men of the Second Platoon show their admiration and respect by subjecting the object of that respect to a savage group beating. Shockingly, not even the Second Platoon’s commanding officers are exempt from this ritual, which is widely practiced within another American subculture: street gangs.

Following the release of the notorious WikiLeaks video that shows the American pilot of an Apache helicopter firing into a crowd of Iraqis who do not appear to pose any immediate threat, many were appalled by the pilot’s lighthearted running commentary. Nobody expected the pilot to blurt out a meditative essay on the morality of warfare. Still, the glee with which he and his colleagues responded to men (and, as it turned out, children) being torn to shreds by bullets seemed almost inhuman.

Americans like to imagine that we understand the soldier. We know, of course, that we cannot comprehend his experience, but we feel sure that the soldier, at the end of the day, is probably not terribly unlike ourselves—which is to say, inclined to do the right thing, whatever that may be. Much encourages us in this assumption. Since 2001, no one has enjoyed higher iconic standing within our popular culture than the American soldier. Our politicians speak of him as though he were a holy knight of democracy. We clap for him on airplanes. We give him our seats on buses and trains and subways. We wear our yellow ribbons. We thank him for his service.

What this obeisance tends to obscure is that a soldier is someone who has been trained to kill without hesitation. Most soldiers, of course, never get the chance, but infantry units are different. Any man who fights within an unusually imperiled platoon must possess a willingness to kill that would probably strike most civilians as psychopathic.

For his part, Junger admits to being troubled when he hears the Second Platoon laugh at the plight of the wounded insurgent crawling toward his own severed leg: “I couldn’t stop thinking about that cheer; in some ways it was more troubling than all the killing that was going on. Stripped of all politics, the fact of the matter was that the man had died alone on a mountainside trying to find his leg.”

When Junger finally talks to a member of the Second Platoon about it, he is told, “The cheering comes from knowing that that’s someone we’ll never have to fight again.” I do not find that very convincing, and neither, I think, does Junger. As he writes:

Combat was a game that the United States had asked Second Platoon to become very good at, and once they had, the United States had put them on a hilltop without women, hot food, running water, communication with the outside world, or any kind of entertainment for over a year. Not that the men were complaining, but that sort of thing has consequences. Society can give its young men almost any job and they’ll figure out how to do it. They’ll suffer for it and die for it and watch their friends die for it, but in the end, it will get done. That only means that society should be careful about what it asks for.

Tom Bissell is the author of five books, including his most recent, Extra Lives: Why Video Games Matter.