"I'm sorry Tony. You know I wouldn't do this if I had any other choice, but he's my friend." "So was I."
Right there, in that moment between Steve Rogers and Tony Stark, you get to the crux of the film. Yes, it might be the Sokovia Accords that start it, but this is going to be far more personal than some government act or bill. By this stage tensions are already going to be pretty high between the pair, but this is what's really going to tip things over the (bleeding) edge. Steve Rogers has a choice to make, and he chooses Bucky over Tony, with a bruised Tony looking and sounding rather upset by it. This decision is one that will have major ramifications, fully giving us Rogers v Stark, making it about more than a difference of opinion or ideologies, but a very personal story involving Cap, Iron Man, and Bucky, leading us to that epic 2-on-1 fight. Speaking of which...