Why are the superheroes fighting each other in marvel Ultimate Alliance 2

Its called the "Civil War"
They're fighting each other because the government signs a bill that makes all superheroes register to be law-enforcement officers.Its passed because Nitro blows himself up in Stamford,CT,which kills lots of people.Some agree with the act,and some don't,and some don't want any part in it at all.So the ones that agree are called Pro-Registration and their leader is Ironman.The ones that don't are Anti-Registration and their Leader is Captain America.Hope this answered your question.