After a terrible accident involving the super hero team New Warriors results in the death of hundreds of school children, the long debated Super Hero Registration Act is finally made law.
Heroes must register their identities with the government and be trained as proper law enforcement officers. Failure to register is a crime punishable by incarceration.
Heroes have divided into two camps - those allied with Iron Man, who supports the law, and those who follow Captain America, who has moved underground to fight back against a government that has turned its back on him.
Iron Man 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 argued 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.