It is the sequel to X-Men (2000), as well as the second installment in the X-Men film series, and features an ensemble cast including Patrick Stewart, Hugh Jackman, Ian McKellen, Halle Berry, Famke Janssen, James Marsden, Rebecca Romijn-Stamos, Brian Cox, Alan Cumming, Bruce Davison, Shawn Ashmore, Aaron Stanford, Kelly Hu, and Anna Paquin.

Michael Dougherty and Dan Harris were eventually hired to rewrite the work, and changed the characterizations of Beast, Angel, and Lady Deathstrike.

The film's premise was influenced by the Marvel Comics storylines Return to Weapon X and God Loves, Man Kills.

At the White House, brainwashed teleporting mutant Nightcrawler attacks the President of the United States, wounding many agents; he is shot and retreats.

Military scientist Colonel William Stryker approaches the President and receives approval to investigate Xavier's mansion for their ties to mutants in the wake of the recent attack.

Colossus leads the remaining students to safety while Logan, Rogue, Iceman, and Pyro escape, and Stryker's assistant Yuriko Oyama captures Cyclops and Xavier.

An impulsive officer shoots Logan in the head after feeling threatened by his claws, Pyro refuses to surrender and takes matters into his own hands by attacking and fending off the police with his pyrokinesis.

Stryker had also previously used Jason's powers to orchestrate Nightcrawler's attack as a pretense to gain approval to invade Xavier's mansion.

Disguised as Logan, Mystique infiltrates Stryker's base, letting the rest of the mutants in while she and Magneto head to disable Cerebro before the brainwashed Xavier can activate it.

Nightcrawler teleports Storm inside Cerebro, where she creates a snowstorm to break Jason's concentration and free Xavier from his control.

Cameo appearances include Katie Stuart as Kitty Pryde, Bryce Hodgson as Artie, Kea Wong as Jubilation Lee / Jubilee, Steve Bacic as Dr. Hank McCoy, Shauna Kain as Theresa Rourke / Siryn, and Chiara Zanni, who voiced Jubilee in the X-Men: Evolution animated series, as a White House tour guide at the start of the film.

[17] Michael Dougherty and Dan Harris, the film's writers, cameo in scenes of Wolverine's Weapon X flashbacks as surgeons.

[20] Bryan and producer Tom DeSanto envisioned X2 as the film series' equivalent to the Star Wars franchise's The Empire Strikes Back (1980), in that the characters are "all split apart, and then dissected, and revelations occur that are significant... the romance comes to fruition and a lot of things happen".

[23] Producer Avi Arad announced a planned November 2002 theatrical release date,[24] while David Hayter and Zak Penn were hired to write separate scripts.

[27] Penn was partially hired when he convinced Singer to not adapt "The Dark Phoenix Saga" storyline for the film, feeling that the franchise's universe should be established much more before "going cosmic".

Instead, in what he feels was his major contribution to the project, Penn based the film's outline on Chris Claremont's graphic novel X-Men: God Loves, Man Kills (1982) before leaving to work on another movie.

[28] Michael Dougherty and Dan Harris were hired to rewrite Hayter and Penn's script in February 2002,[29] turning down the opportunity to write Urban Legends: Bloody Mary (2005).

[17] A reference to Dougherty's and Harris's efforts to include Angel remains in the form of an X-ray on display in one of Stryker's labs.

[32] In Hayter's script, the role eventually filled by Lady Deathstrike was Anne Reynolds, a character who appears in God Loves, Man Kills as Stryker's personal assistant/assassin.

"[16] Producer Lauren Shuler Donner had hoped to start filming in March 2002,[25] but production did not begin until June 17, 2002 in Vancouver and ended by November.

[36] Cinematographer Newton Thomas Sigel and two stunt drivers nearly died when filming the scene in which Pyro has a dispute with police officers.

[43] For scenes involving Stryker's Alkali Base, Vancouver Film Studios, the largest sound stage in North America, was reserved.

[15] Visual effects supervisor Mike Fink was not satisfied with his work on the previous film, despite the fact it nearly received an Academy Award nomination.

[48] The first cut of X2 was rated R by the Motion Picture Association of America, due to violent shots with Logan when Stryker's army storms the X-Mansion.

[49] The film premiered in London on April 24, 2003, and then had the widest release ever, opening on May 2, 2003, in 93 markets, on 7,316 screens overseas and in 3,741 theaters in the United States and Canada.

[53] It surpassed Star Wars: Episode II – Attack of the Clones for having the highest opening weekend for a 20th Century Fox film.

[55] X2, The Matrix Reloaded, Finding Nemo, Bruce Almighty and Pirates of the Caribbean: The Curse of the Black Pearl all became the first five films to cross the $200-million mark at the box office in one summer season.

The website's consensus states: "Tightly scripted, solidly acted, and impressively ambitious, X2: X-Men United is bigger and better than its predecessor—and a benchmark for comic sequels in general.

[62] Peter Travers of Rolling Stone wrote that Hugh Jackman heavily improved his performance, concluding "X2 is a summer firecracker.

In addition, Bryan Singer (Direction), Dan Harris and Michael Dougherty (Writing), and John Ottman (Music) all received nominations.