This image released by Paramount Pictures shows Micheal Ward as Marco, left, and Stephen Odubola as Timmy in a scene from "Blue Story." (Nick Wall/Paramount Pictures via AP)

War is futile. War is a waste. War lays ruin to generations of promising young people.

It’s the message of so many war films, but even more when the war in question is not between nations but between urban gangs — youths killing each other over petty grievances and turf battles that mean nothing to anyone else. Such films generally do not conclude there’s anything useful about gang violence.

Seen in that light, “Blue Story,” a chronicle of youth gangs in south London, is hardly revolutionary. What distinguishes this debut feature from Andrew Onwubolu, aka Rapman, is firstly its storytelling structure, making welcome use of the writer-director’s rap talents to serve as a Greek chorus. And secondly its cast, with several vital performances of note, especially from heartbreakingly vulnerable newcomer Stephen Odubola.

This doesn’t make “Blue Story” an easy experience. You’ll be ground down, over 91 minutes, not only by the brutality of these street wars, or even their futility, but by the depressing sameness of it all. This is is likely part of Rapman’s aim. But be warned: there may be moments when, faced with another pointless foray by hooded youngsters with guns and knives into the menacing streets, you’ll be tempted to give up.

“Blue Story,” which stems from a short YouTube trilogy Rapman did in 2014, is not a tale of two cities but of two boroughs — or postal codes, as the director has put it — and one key friendship. The rapport between school mates Timmy (Odubola) and Marco (Micheal Ward, brooding and charismatic) is so convincingly rendered that it’s a true shock when, later, we see how easily such a bond can dissolve.

We briefly meet Timmy as a young boy, being dressed up in a crisp shirt and tie to go to school in Peckham — a different school than all his friends, he complains, but a better school, says his no-nonsense mother, who works two jobs to keep him fed.

The young Timmy strikes an instant friendship with Marco. Flash forward to the present, with the two as teenagers, just as rambunctious and football-loving and happy in each other’s presence, though Timmy is clearly the more academically motivated. There’s a welcome lightness to these early scenes, and you might even find yourself chuckling (hold onto those moments for dear life).

There’s a sweetness to Timmy, who is notable not only for his loyalty to friends, but his evolved attitude toward women. When he meets Leah (an affecting Karla Simone-Spence), he tells his sex-minded mates: “I don’t want sex. I just want her.” The boys double over with dubious laughter.

Some of the most engaging scenes involve Leah, whom Timmy woos with no more than his shy smile, an offer to binge-watch “Game of Thrones,” and some Doritos. At one point he compliments her singing and tells her she could be a star. “How many singers do you know from Peckham?”she asks, skeptically. He reminds her that John Boyega — of “Star Wars” no less — is from Peckham. It’s not just a humorous throwaway line. He’s telling her that location doesn’t need to define her.

But when tragedy strikes months later, it is indeed location that determines loyalties, and fates. Timmy and Marco will be drawn into a turf hatred that seems likely to engulf them both. Being a bystander isn’t an option.

Following an event that bisects the film — no spoilers here — we’ll see the change most plainly on Timmy’s face. It’s a striking transformation that Odubola makes, from bouyant and hopeful to brittle and cynical. In Marco, we’ve already seen this quality flickering. For Timmy, it’s something new.

Aside from these compelling young actors, the success here is the fresh and original voice of Rapman, who has said “Blue Story” is largely based on his own life; though he wasn’t part of a gang, he witnessed gang rivalries as a London youngster.

“I hope these young’uns wake up and they start seeing the light,” goes one of his raps for the film. “I ain’t tryna justify but imma show you what these young boys are fighting for.” Depressingly, though, what we learn is they’re fighting to defend something as trivial as what postal code they wound up in.

“It’s all because the council housed them on different ends,” Rapman says elsewhere, poetically and sadly. “This is real, not pretend.”

“Blue Story,” a Paramount Pictures release that is now available to rent or purchase on digital platforms, has been rated R by the Motion Picture Association of America “for violence and language throughout, drug use and some sexuality.” Running time: 91 minutes. Three stars out of four.