Filmmaker and author Sebastian Junger poses for a portrait in Washington on May 20. Junger is the director of the new film “Korengal.” (Photo by Ricky Carioti/The Washington Post)

It was a simple thing, a thoughtful 15-minute explanation of why military veterans have a hard time moving on psychologically after engaging in combat. The man who provided it had never served a day in uniform, but his description of the brotherhood formed between troops in combat rocketed across the Internet, garnering more than a million views since it was posted last month.

“Brotherhood is different from friendship,” the journalist, Sebastian Junger, said in the video. “Friendship happens in society, and obviously the more you’d be willing to do for them. Brotherhood has nothing to do with how you feel about the other person. It’s a mutual agreement in a group that you will put the welfare of the group, you will put the safety of everyone in the group, above your own. In effect, you’re saying, ‘I love these other people more than I love myself.’ ”

Perhaps that understanding explains why Junger’s work has been so successful. As a journalist, he has spent years of his life in conflict zones, most notably embedded in 2007 and 2008 in Afghanistan with the U.S. Army’s Battle Company, 2nd Battalion, 503rd Infantry Regiment, 173rd Airborne Brigade Combat Team. Those experiences in the infamously violent Korengal Valley served as the basis for his best-selling 2010 book, “War,” and a critically acclaimed documentary, “Restrepo.” But he isn’t stopping there.

Junger is in the midst of working on not one, but two more projects that examine the veteran experience. The first is “Korengal,” a follow-up documentary that draws on the same combat deployment as “Restrepo,” assessing the psychological impact that engaging in heavy combat has had on the men of Battle Company. The second is “The Last Patrol,” a documentary that was shot in the United States. It will follow Junger, “Restrepo” soldier Brendan O’Byrne and two other men along railroad tracks on the East Coast as they live outdoors and discuss the heavy impact war has had on them.

“It was sort of high-speed vagrancy,” Junger said of “The Last Patrol,” which is expected to air on HBO in the fall around Veterans Day. “It was illegal, obviously, so we had to be avoidant of the police and we had to keep a very low profile because we were sleeping outside. We had to stay on the outskirts of towns and under bridges. We bought food as we went, bathed in rivers and cooked on fires and had a long conversation about war and why it’s hard to give up.”

“Korengal” forces viewers to consider that, too. The documentary reached a small number of theaters this month, following a successful online Kickstarter project in April and May in which fans of “Restrepo” pledged some $75,000 within weeks. They have since pledged another $42,000, ensuring the film will be shown in even more cities across the United States.

Junger says he isn’t surprised by the support “Korengal” has received after the success of “Restrepo,” but he is grateful for it. The combat footage in it was all shot in 2007 and 2008 and the interviews were recorded in 2008. Nevertheless, he wanted to circle back to his time in Korengal because it appeared to resonate so well with viewers, especially combat veterans, he said.

“I’ve met people who tell me, ‘I watch ‘Restrepo’ every week,’ ” Junger said in a phone interview. “There’s a pretty hardcore fan group there.”

The result is a thoughtful documentary that exposes viewers to both angry combat footage and, afterward, the quiet reflections of soldiers depicted in it. At times, it can feel like an extension of “Restrepo” that doesn’t break much new ground. But it also has some powerful moments, especially when the soldiers reflect on the struggles of conscience they are forced to confront.

” ‘You did what you had to do’ — I hate that comment,” said O’Byrne, who left the Army as a sergeant and has overcome struggles with alcohol since. “I didn’t have to do any of it. But I did. ‘You did what you had to do.’ Is that what God’s gonna say? I don’t think so.”

O’Byrne and Junger have grown closer since the time in Korengal, and speak almost daily, Junger said. They’ve already filmed “The Last Patrol.” For that project, they were joined by another journalist and another U.S. combat veteran on the trip, Junger said. A lot of veterans will relate to it, Junger says, because of the way in which they’re forced to pass through society almost invisibly during the film.

“It’s about encountering our society that we live in, in the most raw possible away,” Junger said. “Railroad lines go straight through the middle of everything. It goes right through ghettos, and right through wealthy suburbs, and right through industry and farms. It skewers everything, and you really see America from the inside out. And, we’re marginal. We’re breaking the law every step of the way.”

Junger has said repeatedly that he is done covering combat. He made the decision was made within an hour of learning that his partner on “Restrepo,” photographer Tim Hetherington, had been killed on April 20, 2011, he said. If some consider him a spokesman for veterans and troops because of his experiences, he appreciates it, and considers it important to remain nonpartisan when talking about war, focusing instead on the troops. Still, he won’t miss the close calls and near-death experiences.

“I miss the work,” he said. “I miss the experience. But I’m glad I’m not doing it anymore.”