Ultimately, war is merely the continuation of politics by other means. But intimately, it’s all about sociology and biology.

And that’s the aspect of war which engaged Sebastian Junger, in writing his book War, and making the documentary Restrepo — and also most of the overflow crowd which attended his address to the Hebrew Center Summer Institute on Thursday night.

Mr. Junger’s goal was not to explain the forces which drive the politicians who declare wars, but to explain the forces which drive those who fight them.

What motivates young men — for his subjects are all young men — to battle in godforsaken places for reasons they don’t really understand, to risk death and maiming?

Love of country, a patriot might answer. But Mr. Junger’s long experience — he has covered wars since the early 1990s — indicates something a whole lot more primal than that.

Indeed, one need not even go to war to get an inkling of it. He illustrated his point first with a scene he witnessed long before he ever went off to Afghanistan. It was in Pamplona, Spain, in 1986, on the eve of the running of the bulls, something he called “another idiotic male idea.”

He had fallen in with a pair of young Spanish men, who were “cross-eyed drunk,” one of whom was wearing, for reasons unknown, a cheap plastic bike helmet.

They got into an argument with three tough young French Moroccan men who alleged the helmet had been stolen from them. As Mr. Junger translated for both parties, a fight seemed imminent. There was a tug of war over the bike helmet.

But both sides paused when the helmet began to rip. Then one Spaniard transferred his hold on the artifact to Mr. Junger, demanding he defend the helmet while he went to the bar.

“And he goes to the bar and pays for the biggest and cheapest of jug red wine that they had. Then he filled the helmet with the wine.”

The man offered the biggest Moroccan the first mouthful of wine, then the helmet was passed around so all might drink.

“About an hour later, I looked over, and the biking helmet is completely forgotten under a table, and they’re all there with the jug of wine, they’re so drunk that they can’t stand up and they’re hanging on to each other, trying to sing some song simultaneously in Spanish and French,” he said.

It was not the stupid conflict which struck him, but the bonding.

And his subsequent experience in Afghanistan had underlined the point. Why do so many soldiers come back from war feeling lost and empty? Why do so many sign up for tour after tour? Why is the first priority of so many of the wounded to get back to their units?

It is not that they like the conflict.

“I think they miss the part of that energy that gets them to affiliate with each other. Everyone likes to be connected to other people and that happens in a very, very profound way in war,” Mr. Junger said.

To better understand it he became embedded with troops, the 173rd Airborne Brigade, through their tour in 2007-2008, in the Korengal Valley in Afghanistan, then perhaps the most dangerous place on earth.

“For a while one-fifth of all combat in all Afghanistan was happening within those six miles. They went through something like 400 firefights in their deployment,” he said.

The first day he was at the outpost named Restrepo, after the platoon medic who died there, the outpost was attacked four times.

Between times, there was no TV, Internet, running water, bathroom, privacy or women. The soldiers worked in body armor in 100-degree heat and wore the same clothes until they rotted off their bodies.

And yet, when the tour was over, and the chance came to get out of the Army, only one guy chose to get out.

“His first year back home after the deployment was harder than the deployment,” said Mr. Junger.

Mr. Junger invited him to a party, where the soldier was asked by another guest whether there was anything he missed.

“Without any irony at all he said, ‘Ma’am, I miss almost all of it.’

“I realized his answer had to be explained,” said Mr. Junger.

The author can explain some of it in terms of personal experience; he narrowly missed being shot, he was in a Humvee which was blown up.

That incident brought on a series of intense emotions: initial calm, then fear, then a “black depression” that came that night.

Strongest of all was the “unbelievable high of not dying ... a completely intoxicating thing.”

But it was only a “limited truth” to suggest that it was just the excitement which bound young men to war. Anyone could get that adrenalin rush without going to war, by driving fast or jumping out of planes or robbing banks.

“The profound thing these guys miss is a brotherhood. That’s the drug, once you’re exposed to it,” he said.

It was different from friendship, in that it had nothing to do with one soldier’s personal feelings for another. There were members of the platoon who hated one another, he said.

Rather, it was an understanding that one would risk his life, lose his life, to save the group.

He noted the humans evolved in small interdependent groups. He cited sociological evidence that certain optimum group sizes promoted varying degrees of emotional affiliations.

“Those are nine or 10 people, 30, 40 or 50 people, and 150 people. That is – to a man – a squad, a platoon and a company.

“One guy said to me ‘I’d jump on a hand grenade for anyone in the [second] platoon, maybe some of the guys in first platoon or the third platoon, but definitely all the guys in the second.”

As to why young men, rather than young women or older people, are more inclined to make such affiliations, he offered a theory of social status.

“A 19-year-old guy [is] really at the bottom of the food chain, socially. You don’t know what society expects of you, your identity is really unclear,” said Mr. Junger.

“But you put that guy in a platoon in combat and all those problems are solved. It doesn’t matter if you’re ugly, it doesn’t matter if you’re rich, it doesn’t matter if your dad’s in prison, it doesn’t matter if you’re gay or straight. None of it matters except that you are a good soldier, and that’s not by the definition of some rule book; that’s by the definition of your peers.

“That you are prepared to sacrifice yourself for everyone else — that’s the definition of a good soldier,” he said.

“In the context of the platoon, you are completely self-defining. For a 19-year-old, that is a blessing. What a relief to have control over how people perceive you.”

Mr. Junger finished up with an acknowledgment of his collaborator in Afghanistan, Tim Hetherington, who was killed earlier this year in Libya.

Not only was it “shockingly painful” to lose someone so close for the first time, but it brought home his last lesson about the brotherhood of war.

It came in an e-mail from a Viet Nam veteran in Texas, who began by saying he was worried his note would sound harsh.

Mr. Junger paraphrased: “You guys came really close, with your books and your movie, to understanding what war is about. But you didn’t get all the way.

“The core truth about war isn’t that you might get killed. It is that you absolutely, for sure, will lose your brothers.”

“Now you know everything you need to know about it.”

And now that he knows, said Mr. Junger, he will never go off to war again.