Lainey Gossip reports that Captain America and his new Avengers team will fight Crossbones in Africa, which leads to the deaths of some civilians. This is reportedly “the first big action set piece of the movie…that kicks off the Civil War.” This could be the international incident in the synopsis that helps spark the war.
As far as the Winter Soldier goes, the site reports he is the “touchstone of the conflict.” Furthermore, Iron Man “may be using the government registration” to try to track down the Winter Soldier, who murdered his parents. Black Panther is also apparently hunting the Winter Soldier, while Crossbones wants him back under HYDRA control.
The site also added this piece of information: “Civil War is a critical moment for Sebastian Stan and Bucky Barnes—he has to get everyone on his side by the end of this movie, because his role in the Marvel Cinematic Universe is about to get a lot bigger.”