Good intentions become a fatal flaw in “Free State of Jones,” Gary Ross’ epic-length biography of a fascinating figure in Southern history. Matthew McConaughey plays Newton Knight, a Confederate Army deserter who becomes the charismatic leader of a secessionist territory based on racial equality.

“Free State of Jones” is an authentic-looking film with strong work from a fine cast, but it makes the fatal mistake of trying to broaden its scope to encompass nearly 90 years of race relations in the United States.

Knight is a somewhat murky figure who has been painted as both hero and opportunist, but “Free State of Jones” — written by Ross — chooses to hang a halo over the man.

Barely a moment goes by when Knight is not rescuing a wounded soldier, freeing a chained slave or outwitting white bullies. If we buy this line, it’s thanks largely to McConaughey, a natural in the role of a lean, flinty, principled Southern man.

Any film about a ragtag army, from “Braveheart” to Ross’ own “The Hunger Games” is bound to have its rousing moments, and “Free State of Jones” has its share. The movie’s violence can be strong and even a bit overdone, with strangulations, castrations and imploded faces. Still, the movie conveys genuine outrage at the injustices and terrors of the era. That makes the love between Knight and Rachel (Gugu Mbatha-Raw), a house slave, seem less like an obligatory romance and more like a triumph.

One of the film’s most harrowing sequences involves the freedman Moses (an Oscar-worthy Mahershala Ali) as he attempts to register blacks to vote. Therein, however, lies a problem. “Free State of Jones” continues past the Civil War through emancipation, reconstruction and all the way up to the late 1940s, when a light-skinned descendant of Knight (Brian Lee Franklin) runs afoul of entrenched racial laws.

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That storyline, told in flash-forwards, wrecks the rhythm of the movie and takes us out of the more compelling Civil War drama.

“Free State of Jones” wants to show that Knight’s enemy — namely, racism — hasn’t yet been vanquished. That’s a valid point to make, but the result is an overlong and meandering movie.