War, too, for all of its brutality and ugliness, satisfies some of our deepest evolutionary yearnings for connectedness. Platoons are like tribes. They give soldiers a chance to demonstrate their valor and loyalty, to work cooperatively, to show utter selflessness. Is it any wonder that so many of them say they miss the action when they come home?  As a former anthropology major, Mr. Junger takes a special interest in tribal life. He notes that a striking number of American colonists ran off to join Native American societies, but the reverse was almost never true. He describes the structure and values of hunter-gatherer groups, including the ones that lasted well into the 20th century, like the !Kung in the Kalahari.  Unfortunately, these parts of the book are also the dullest and most problematic. There’s a numbingly familiar quality to much of the social science research he cites. It is not exactly news that nations with large income disparities are less happy than those without them, or that group cooperation increases levels of oxytocin, the bonding hormone.  Though Mr. Junger cautions against romanticizing tribal cultures, he sometimes does exactly that, and in ways that can be annoying.  He notes, for example, that American mothers in the 1970s had a level of skin-to-skin contact with their babies that traditional societies would consider criminally low. Fair enough. I wonder, though, if he realizes that in saying this he’s crashing open the gate for every attachment-parenting demagogue out there? And that parents who actually have to go to work for a living — and therefore can’t have their babies pinned to their chests all day long for three years straight — will read these words and start to smack their heads against their desks?  But his arguments about how “detribalized” we’ve become are undeniably strong. It’s not just that our personal loyalties have shrunk to a universe the size of a teacup. (Our immediate families, maybe a few friends.) It’s that we have so little regard for what’s collectively ours. We litter. We fudge on our taxes. Medical providers defraud Medicare; bankers perform sleights of hand with the markets and destroy the commonweal.  But Mr. Junger’s most powerful — and surprising — argument is the one he makes about the military’s epidemic of post-traumatic stress disorder, which in many cases he suspects may not be PTSD at all. Why, if you think about it, would roughly 50 percent of our Iraq and Afghanistan veterans apply for permanent PTSD disability when only 10 percent of them saw combat? “The problem doesn’t seem to be trauma on the battlefield,” he concludes, “so much as re-entry into society.” And, he suggests, this problem might deserve its own diagnostic term.  If the United States were more hospitable to veterans’ needs, its returning soldiers wouldn’t be foundering. It is our failure to adapt to veterans that’s the problem, not the other way around.  According to the work of three anthropologists who have looked closely at this issue, Mr. Junger writes, there are three critical factors to soldiers’ successful reintegration: First, they must be rejoining a society with reasonably equal, and decent, economic opportunities. Second, they must be rejoining a society that has an intuitive understanding of what soldiers have gone through, rather than perceiving them as victims. And third, “veterans need to feel that they’re just as necessary and productive back in society as they were on the battlefield.”  Which means that of all the countries on earth for a soldier to come home to, the United States is among the very worst.  Our veterans re-enter an unstable working class. They are awkwardly thanked by strangers for their service — which, as Mr. Junger ruefully observes, only highlights the schism between the few who have served and the great many who have not. And instead of jobs, they are offered lifelong disability.  Soldiers go from a world in which they’re united, interconnected and indispensable to one in which they’re isolated, without purpose, and bombarded with images of politicians and civilians screaming at one another on TV.  “How,” Mr. Junger asks, “do you make veterans feel that they are returning to a cohesive society that was worth fighting for in the first place?”  It’s an urgent question. Anyone want to weigh in on it? Hillary? Donald? Is this thing on?  A version of this review appears in print on May 19, 2016, on page C1 of the New York edition with the headline: Disbanded Brothers Back in a Divided Nation. Today's Paper|Subscribe
“Tribe” examines the benefits of small societies, citing Native American cultures and military outposts, among others, as examples.