Which prequel to the X-Men trilogy was released in 2011?

The 2011 prequel to the original X-Men trilogy is "X-Men: First Class," commonly shortened to "First Class." The film explores the early friendship between Charles Xavier and Erik Lehnsherr, who later become Professor X and Magneto.

Directed by Matthew Vaughn, the movie is set primarily in 1962 during the Cuban Missile Crisis. Its story shows Xavier and Magneto assembling an early team of mutants while confronting Sebastian Shaw and the Hellfire Club, whose plans threaten to trigger a nuclear conflict. James McAvoy played the young Xavier, while Michael Fassbender portrayed the young Erik.

Although it functions as a prequel, "First Class" also works as a soft reboot. Its continuity does not always fit perfectly with the earlier trilogy, and the film takes creative liberties with the characters’ histories. That is why it is better described as a prequel and partial reset rather than a completely seamless origin story.

The film premiered in May 2011 and received a wide U.S. release on June 3. It became the starting point for a prequel sequence that continued with "X-Men: Days of Future Past," "X-Men: Apocalypse," and "Dark Phoenix."