"Civil War" is a 2006–07 Marvel Comics crossover storyline consisting of a seven-issue limited series of the same name written by Mark Millar and penciled by Steve McNiven and various tie-in books. The storyline builds upon events in previous Marvel storylines, particularly "Avengers Disassembled", "House of M", and "Decimation". The series' tagline is "Whose Side Are You On?"
The plot begins when the U.S. government passes a Superhero Registration Act, ostensibly to have super-powered individuals act under official regulation, somewhat akin to law enforcement. Superheroes who oppose the act, led by Captain America, find themselves in conflict with its supporters, led by Iron Man. Spider-Man is caught in the middle, while the X-Men take a neutral stance. The superheroes who support the law, including Mister Fantastic and Ms. Marvel, become increasingly authoritarian. Civil War explores the conflict between freedom and security against a backdrop of real-life events and discussions, such as the U.S. government's increased surveillance of its citizens.
The series polarized critics but it was a commercial success. A sequel, Civil War II, debuted in June 2016. The 2016 film Captain America: Civil War in the Marvel Cinematic Universe loosely adapted the storyline.