Civil War follows the implementation and consequences of the Superhuman Registration Act, a legislative bill which required the mandatory registration of any person based in the United States with super powers. The act arose due to public pressure for accountability following a series of superhuman-related events causing significant damage and death within the Marvel universe, such as an attack on Manhattan in reprisal for Nick Fury's "Secret War", and the Hulk's rampage in Las Vegas which resulted in the death of 26 people. When the mutant population was drastically reduced in the aftermath of M-Day, itself caused by a mutant, anti-mutant hysteria caused by extremist groups caused a majority of the remaining mutants, known as the 198, to relocate to the Xavier Institute, and raised public support for the proposed act.
Public sentiment toward superheroes plummeted after the New Warriors, a group of young superheroes and the focus of a reality TV show, botched an attempt to apprehend a group of supervillains in a quest for better ratings. In the resulting fight the villain Nitro used his explosive powers to destroy several city blocks, including an elementary school at the epicenter, resulting in the death of over 600 civilians, 60 of whom were children, with just Speedball of the Warriors and Nitro himself surviving. Although many high-profile superheroes assisted in the relief and rescue effort, there were a number of isolated revenge attacks, and support for registration rose.
The prospect of registration divided the superhuman community down the middle, with Tony Stark, the superhero Iron Man who had previously tried to halt the act, becoming the pro-registration figurehead, and Captain America leading the anti-registration group. Iron Man, with Mr. Fantastic and Henry Pym, argued that the changing political landscape meant that resisting the law was pointless, and that it is reasonable for heroes to have proper training and oversight, whereas Captain America, alongside Luke Cage and Falconargued that heroes required secrecy in order to protect aspects of their 'normal' life, such as spouses and children, and to allow them to act in whatever means necessary against threats which the ordinary emergency services couldn't cope with. Although nominally a U.N. agency, S.H.I.E.L.D. assumed the brunt of enforcing the act under acting directorMaria Hill.
