At the height of the Civil War, Newton Knight (Matthew McConaughey), a poor farmer from Mississippi, leads a group of small farmers and local slaves in an armed rebellion against the Confederacy in Jones County. (  / STX Films)

Matthew McConaughey seethes with righteous fury in “Free State of Jones,” a tale based on an intriguing, little-known chapter of Civil War history that, despite impeccable intentions, becomes mired within a poorly structured, dubiously focused movie by Gary Ross.

It’s easy to see why McConaughey embraced the chance to play Newton Knight, the scrappy iconoclast at the story’s center. A native of Jones County, Miss., Knight deserted the Confederate army in 1862, enraged both by conscription policies that favored the wealthy sons of slave-owners and by the marauding bands of rebel soldiers who routinely commandeered his fellow yeoman farmers’ livestock and supplies. Over the next several years, he led a growing band of impoverished deserters and freed slaves who fought against the South from within, finally establishing the country-within-a-country of the film’s title.

Wearing what looks like a set of yellowed prosthetic teeth, his face hidden behind stringy hair and a scraggly beard, McConaughey delivers a solemn, sharp-eyed portrayal of Knight, whose character and contradictions, one senses, can’t possibly be contained even in a nearly 2 1/2 hour movie. After returning home to his wife, Serena (Keri Russell), and their young son, he eventually takes up with an enslaved house servant named Rachel (Gugu Mbatha-Raw). With her, he starts another family, setting up his swampland community and delivering sermons on self-sufficiency, altruism and the immorality of slavery. Meanwhile, he becomes a mythic figure in Mississippi, where he and his guerilla fighters will in time gain control of three counties.

Matthew McConaughey as Newton Knight. (Murray Close/STX Entertainment)

McConaughey (Knight) and Gugu Mbatha-Raw (Rachel) star in “Free State of Jones.” (Courtesy of STX Entertainment)

Knight is a captivating figure, and Ross makes sure that his ideas — borne of his primitive Baptist upbringing, but of a piece with emerging Marxist theories — resonate with unmistakable prescience. “Free State of Jones” traces Knight’s crusade all the way into Reconstruction, when attempts by African Americans to claim political and social space were met with push-back and terroristic violence that feel depressingly familiar at a time when some of us still need reminding that black lives matter.

But it’s just the sprawling, multifaceted nature of Knight’s story that gets the better of Ross, for whom “Free State of Jones” has been a passion project for the past decade. Understandably trying to cover as much incident and ideological discourse as he can — including some ill-advised flashes forward to a 1940s court case involving Knight’s descendants — he winds up with an earnest but inert and unwieldy historical drama that, in its commendable attempt to rescue a forgotten figure from obscurity, too often looks and feels like “12 Years a Woke Dude.”

A former slave named Moses (Mahershala Ali), left, and Knight (McConaughey) form an alliance in “Free State of Jones.” (Murray Close/STX Entertainment)

In interviews, Ross has insisted that he didn’t want “Free State of Jones” to become another white savior movie, but that’s precisely what it is, especially during scenes when the murderous injustice of slavery is refracted through Knight’s frustrated tears. Again, this is where Ross, who wrote the script from a story by Leonard Hartman, might have made an enormous difference with just a slight tweak, reframing the narrative to focus more on Knight’s alliance with a former slave named Moses (a composite character, played in a breakout performance by Mahershala Ali), or his relationships with Serena and Rachel, which according to the historical record were vexingly complex, and grew more so as time wore on.

Instead, Ross seems intent on presenting Knight as a charismatic visionary, cut from the same zealous cloth as John Brown — an appealing image for a star vehicle, but one that seems simplistic and self-congratulatory in a cinematic era defined by “12 Years a Slave” and “Selma” on the one hand, and Nate Parker’s upcoming “The Birth of a Nation” on the other. Attractively photographed by Benoit Delhomme, “Free State of Jones” possesses verdant, atmospheric beauty and some occasionally striking iconography, especially when Knight is rallying the local womenfolk to defend their homes with long rifles and level-eyed aim. And the political critiques embedded within its epochal moment — when economic, racial and sexual oppression converged so violently — make for fruitful reflection, if not always pulse-quickening viewing. As gratifying as it is to see forgotten history brought to light, it’s disappointing, too: There’s an epic story to be told within “Free State of Jones,” but this white-knight tale isn’t it.

R. At area theaters. Contains brutal battle scenes and disturbing graphic images. 139 minutes.