Emotionally raw, unnervingly tense and impeccably acted across the board, Brothers is a return to form for director Jim Sheridan.

Sidetracked in 2005 by 50 Cent’s middling Get Rich or Die Tryin’, he’s back on familiar footing of families fractured by wartime violence (The Boxer, In the Name of the Father).

Only in Brothers, the milieu is Midwestern, not Irish. And with Daniel Day-Lewis off song-and-dancing in Nine, Tobey Maguire steps into the lead for Sheridan as Capt. Sam Cahill — a dyed-in-the-wool U.S. Marine captured by the Taliban in Afghanistan who, after being presumed dead, is found and brought home.

Descending to darker depths than he’s ever dared, the still-boyish 34-year-old Maguire offers a turn worthy of his first Academy Award nomination.

When captured, Sam asserts that holding onto the memory of family is a weakness — a connection to be perverted and used for torture. Maguire’s jaw seems squarer, his eyes more electrified and — as his features harden with resolve to survive and a beard covers his face — he resembles Robert De Niro circa The Deer Hunter.

This is before those features slacken altogether — after Sam is faced with an impossible paradox and his love for his family both saves him and condemns him. When he snaps — all snarls, snot and spit — Sheridan and Maguire discomfortingly place you in Sam’s percussive, throbbing psyche.

It’s a sensation breathlessly revisited in the film’s climax, which follows Sam’s unexpected return to his Minnesota family — high-school sweetheart and now-wife Grace (Natalie Portman), two young daughters (Bailee Madison and Taylor Geare) and his alcoholic former-Marine father, Hank (Sam Shepard).

More recently, Sam’s ne’er-do-well brother, Tommy (Jake Gyllenhaal), has returned home — a paroled ex-con and untenable screw-up in Hank’s eyes. (Shepard and Gyllenhaal sell their mutual resentment with furtive, incisive insults.)

Believing Sam to be dead, Tommy takes on the responsibility of a protective uncle and brother-in-law out of respect for a brother to whom he’s never measured up but whom he unquestionably loves.

Brothers has a nice rhythm for how home improvement projects are undertaken, laughter rises again and life goes on (even as Thomas Newman’s atypically awful score sounds more akin to a CMT video than a tough drama).

Over time, Grace’s cold shoulder warms to Tommy — culminating in a late-night kiss that avoids obvious emotional approaches and payoffs. Watch how Gyllenhaal and Portman react in this scene — she pushing through it with uncertainty and loneliness, his unexpected reaction of true shame and a relapse to his old ways.

This is Maguire’s movie, but Gyllenhaal and Portman also deliver some of their best work yet. Convincing as a doting mother of two and abandoned wife, Portman is torn between joy at the bountiful life before her and grief for the love she’s lost.

Gyllenhaal’s forceful moments arrive with a plot development that could turn melodramatic and maudlin but, instead, pulls a taut garrote around the Cahill family.

When Sam returns home and observes Tommy and Grace acting “like a pair of love-struck teenagers,” he presumes they’ve slept together. Confronted, Tommy dodges the question and Grace owns up to the kiss.

With physical nuance, Gyllenhaal shows Tommy is torn up by his inability to say that he didn’t want to do more. Emotionally coveting is worse than a one-off consummation to him.

As Sam’s belief in the infidelity persists, his post-traumatic stress disorder rises under the assumption of heroism and the absence of psychological support. (Counseling is mentioned in a throwaway capacity — the first of several overlooked details that could have strengthened what is already a sturdy film.)

Sam’s paternal relationship also deteriorates. Playing Isabelle, Sam and Grace’s older daughter, Madison gives 2009’s best child-actor performance. It’s psychologically unnerving at a dinner table when she contorts her behavior to match Sam’s disaffected demeanor in a desperate, futile attempt to curry affection.

Eventually, Sam’s outbursts grow worse and what could become melodramatic instead grows more bracing. Brothers isn’t out to demonize war, merely to chronicle a small chapter in the life of some good people whom it has demonized.

Whether that damnation is temporary or perpetual is anyone’s guess. After all that transpires in Brothers the capacity to forgive — both one’s self and one’s family — isn’t something so easily resolved, and its ultimate ambiguity is appropriate.