We all know that Marvel is releasing Captain America: Civil War this May. Why are Captain America and Iron Man fighting?
In the comics they fight becasue of the Superhero Registration Act. The Superhero Registration Act is to have superpowered individuals act under official regulation, somewhat akin to law enforcement. However, superheroes opposed to the act, led by Captain America, find themselves in conflict with those supporting the act, led by Iron Man, with Spider-Man caught in the middle of the fight. But, why are they fighting in the movie?
Reason 1- Superhero Registration Act
Tony thinks that all Heroes should be identified and known. That means that the public has the right to know there real name and all about them. The anti-registration group is lead by Captain America. Captain thinks 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 are needed, against threats which the ordinary emergency services couldn't cope with.
Reason 2- Bucky
Bucky is a huge deal in this movie. The army is after Bucky becasue he was a assassin. He killed many people but, the army and Tony think that was not right. Captain had said "He’s my friend," and Tony replies, "So was I," not absolutely tear you up? What, are you made of stone? Are you dead inside? Captain team is trying to defend bucky and decline the Superhero Registration Act. While Iron Man team is trying to bring down Bucky and approve the Superhero Registration Act.