Captain America: Civil War differs from a lot of superhero movies in that it’s a direct adaptation of a single comic book storyline. Sure, Christopher Nolan’s Batman movies took plot points from The Dark Knight Returns and The Long Halloween, and The Winter Soldier loosely retold the characters origin tale, but generally most comic book movies just pick various characters and elements and mash them all together. Civil War however is the rare instance where it’s based almost entirely on Mark Millar and Steve McNiven’s 2007 limited series of the same name.
It’s not entirely faithful by any means—it’s not like it’s a scene for scene adaptation or anything—but basically sticks to the plot of the original comic. Superheroes are causing too much collateral damage, and the powers that be want them to be registered and controlled by law. Iron Man is down with it, and instrumental in bringing the legislation through. Captain America however doesn’t think governments should have that much power, and their independence is important. Various heroes take different sides, and they fight about it.
That’s the basis of both the comic and the movie, but they play out pretty differently. Here’s the major points of divergence.