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Sebastian Junger was in a New York City subway car during rush hour one day in late 2010 when he panicked. The author ("The Perfect Storm,") and filmmaker ("Restrepo") had recently returned from Afghanistan, where he had spent two months in harsh combat conditions to write about an important tribal leader.

His subway anxiety was later diagnosed as post-traumatic stress disorder (PTSD). Further research revealed that U.S. troops were experiencing it at a much greater rate than ever before, a subject on which he wrote an extensive, 7,000-word article for Vanity Fair magazine.

Junger is an excellent journalist, and that ironically is the biggest weakness in "Tribe," his new, slender book that includes only 138 pages of text. (The rest is acknowledgments and an extensive list of source notes.) All the salient and important points he makes can already be found in the magazine piece.

Still, where you read what Junger has written is less important than that you read it. The substance of the article and the book is significant well beyond soldiers returning from war.

Only 20 percent of people diagnosed with PTSD end up with chronic or long-term trauma.

In fact, he notes, if you wind up with the disorder, "that is in great part a function of experiences … before going to war." If, for example, you suffered the "death of a loved one or … weren't held enough as a child you are seven times more likely to develop the anxiety disorders that contribute to PTSD."

The severity of the problem seems to depend on what happens after the troops return home. Many soldiers miss the war — not the fighting, but the fierce tribalism, the strong communalism they experienced.

But they return to "a country that regularly tears itself apart along every possible ethnic and demographic boundary," focusing on differences rather than our shared humanity.

Junger suggests that we miss the fraternity that thousands of years of tribal life have programmed us for. Instead, we live at a time where a sense of entitlement has replaced a sense of village.

We are capable of joining together. We did after 9/11, but that feeling soon dissipated. We are a strong nation. The only one who can destroy us, he notes, is, well, us, "which means that the ultimate terrorist strategy would be to just leave us alone."

"Tribe" is an important wake-up call. Let's hope we don't sleep through the alarm.

Curt Schleier is a critic in New Jersey.

Tribe
By: Sebastian Junger.
Publisher: Twelve, 169 pages, $22