When Warner Bros. greenlit Zack Snyder’s return to the troubled “Justice League,” vastly expanding and reediting the film into a new edition that debuts March 18 on HBO Max, one of Snyder’s first calls was to composer Tom Holkenborg.

The Dutch-born, L.A.-based composer (known in the pop world as Junkie XL) had written some 40 minutes of music for the original “Justice League” back in 2016, before Snyder departed the project and was replaced by Joss Whedon (who in turn brought in Danny Elfman as composer).

Holkenborg — who had collaborated with Hans Zimmer on Snyder’s two previous DC Universe movies, “Man of Steel” and “Batman v. Superman: Dawn of Justice” — looked at the earlier music and asked Snyder: “Would you mind if I start over?” The director’s response, per the composer: “‘The shackles are off. Go for it. Do whatever you feel is right.’”

The result is nearly four hours of music for the long-anticipated, now critically praised, “Snyder cut”: what Holkenborg calls “my Mount Everest in film scoring” — his longest-ever score and one that calls upon all his skills from his years as an electronic-music producer, his 1980s and ’90s work with world music, and his love of classical orchestration.

“This score is extra-personal and extra-intense for me,” says Holkenborg. “The fans made this happen,” he adds, referring to internet demand for Snyder’s original after the much-maligned box office disappointment of Whedon’s 2017 “Justice League.” Holkenborg never saw that one, out of loyalty to Snyder, with whom he has now done six projects including “300: Rise of an Empire” and the forthcoming “Army of the Dead.”

Working alone in his studio for eight months during the pandemic, the composer says, “I had time to really look at my strengths. I wanted to create a score that was going to show my full spectrum without losing a distinctive throughline: strong thematic statements, orchestrational and electronic colors, an arc of four hours that needed to be properly filled.”

One of Snyder’s only specific requests was for “a national anthem” for the Justice League, and Holkenborg delivers a rousing, heroic theme as the team comes together. He wrote new themes for most of the characters, including an especially evocative one for Wonder Woman featuring wordless vocals by Iranian soprano Delaram Kamareh; a delicate piano and strings treatment for the tormented Cyborg; a huge brass signature for Aquaman; and dark, clashing, atonal sounds with “a crazy choir from hell” for supervillains Steppenwolf and Darkseid. Batman gets a new theme (for “a dark, bad-ass m.f.,” Holkenborg says) while Superman’s theme from “Man of Steel” returns.

While the composer created most of the score in his private studio, using “two small synths, some percussion, one bass, one guitar and a few pedals,” he did require nearly 60 London musicians for the string and brass elements. Warner’s WaterTower Music is releasing the entire four-hour score along with the movie, a rarity in the soundtrack world.

And Holkenborg isn’t done with world-shaking battles: He also wrote the score for “Godzilla vs. Kong,” which HBO Max will unveil March 31.