His Pursuit of Bucky Will Trigger The Battle

As already touched upon in this article, it will be revealed that Bucky Barnes AKA the Winter Soldier killed Tony Stark’s parents, when he was under the control of HYDRA, and Tony Stark will be extremely pissed about that. Stark loved his mother, though he was never particularly close to his father during his life, but in Iron Man 2 he received a heartfelt posthumous message from him that belatedly made him feel closer to him.

He’ll want revenge for their deaths and, in spite of the fact that it really wasn’t Bucky’s fault (he was mind-controlled, after all), Stark will want him to pay. The fight seen in the trailer in which Captain America and Bucky Barnes are fighting Iron Man will be the culmination of an initial one-on-one born out of Stark’s hunt for the latter. Cap will likely save Bucky’s life, just prior to Stark landing what would have been the fatal blow.

This fight will set the tone for the entire movie.