Early on in “Atonement,” there’s a battle so mired in the helter-skelter chaos of combat violence that it puts us right in the fog of war. Near the start of the conflict in Iraq in 2003, we’ve spent a few scenes getting to know the Khachaturians, a boisterous Iraqi family that’s jammed three generations of itself into a single home in Baghdad, all to stay out of harm’s way. But then a bomb blast intrudes, and the family members jump into cars and head across town, to where they think it will be safe. There’s no reason to think otherwise; there are lots of Iraqis out on the street, going about their business.

But as the cars approach an urban center, there’s gunfire in the distance — a squad of U.S. Marines is up on a roof, trying to pick off enemy combatants — and as bullets tear into the car, the grandmother, Mariam (Hiam Abbass), waves her white handkerchief out the window, to signal that they’re civilians who have come in peace. But it’s too late; her husband and two adult sons get out of the car, and before a moment can flash by they’re lying dead in the street.

In war coverage, a phrase it’s easy to grow numb to is “civilian casualties.” (It sounds horrific, but it also sounds abstract.) “Atonement” rubs our noses in the shock and terror and individuality of civilian casualties — in their moral calamity. And in this case, the calamity extends to the soldiers who fired the bullets. They know, all too well, what they did. Yet it was an accident, so they’re also in denial. The Second Lieutenant, Lou D’Alessandro (Boyd Holbrook), wonders what civilians were even doing there, as if it was their job to know the shifting sands of war-zone geography. Talk about blaming the victims.

The film then leaps ahead 10 years, when Lou, having undergone eight deployments, is living in San Diego, holding down jobs as a nightclub bouncer and construction worker. He’s trying to get into law school, but it doesn’t help his case that he received a dishonorable discharge (he was on black-market drugs for his physical pain, all because the VA health plan wouldn’t cover it). You might say that “Atonement” is a story of PTSD. Lou, or at least the Lou we see, does not have nightmare flashbacks to the carnage of combat. But something else is eating away at him — a post-traumatic guilt disorder, a sensation of unresolved anxiety at what he did during the war, leading the squad that killed those civilians. That’s the torment he can’t run away from.

“Atonement” is based on a 2012 New Yorker article by Dexter Filkins, who ended up bringing an Iraq War soldier together with a family whose members the soldier’s squad had killed. In the movie, the Filkins character — fictionalized as Michael Reid — is portrayed by Kenneth Branagh, who when he plays Americans always seems to be overstating his understated American accent. As Reid, he exudes diligent decency and not much more; the film’s plot, once it moves stateside, is a little schematic. Yet “Atonement,” as written and directed by Reed Van Dyk (it’s his first feature), builds to a climactic sequence of stirring nuance and power, in which Lou, having read Reid’s article about the Khachaturians and gotten in touch with him, visits the home of Mariam and her family.

Is he there to apologize? To ask for forgiveness? Have the Khachaturians agreed to the visit to give all that to him, or because there’s something they need as well? The stirring spirit — I’m tempted to call it the emotional intelligence — of “Atonement” is that the movie understands, full well, that we in the audience already know how inadequate any of those gestures would be. Lou apologizing can’t bring anyone back. And if it relieves him to unburden himself, if it makes him feel better…why would that be the Khachaturians’ responsibility? It wouldn’t be; but maybe it would be part of their journey.

At first, the film embraces the ethical awkwardness of the situation. Yet it also pushes the idea that, as Lou’s girlfriend Anna (Gratiela Brancus) says during a support-group meeting, “When you pick up a gun and shoot, the bullet moves both ways.” That’s a fresh idea for a war movie to articulate. Boyd Holbrook shows us the unspoken pain a soldier can live with. He gives a fearless and moving performance that does justice to Lou’s deep-down conflict, his need to cast off the weight of something he didn’t mean to do — yet he did it, so what does that mean? And then, as Hiam Abbass takes center stage as the proud and rather stubborn Mariam, who was too changed by the tragedy to offer easy answers, we see what forgiveness really is: the toughest form of grace. Abbass, in this role, exudes a prickly and lyrical authority worthy of Vanessa Redgrave. “Atonement” comes to a place that, in a lesser film, might appear sentimental but in this one is bracingly real. You can feel the movie burning away the fog of war.