When production started on Captain America: Civil War back in May, Marvel released the official synopsis for the movie:
“another international incident involving the Avengers results in collateral damage, which leads to a system of accountability and a governing body to determine when to enlist the services of the team.”
We know now that this “governing body” is known as the Sokovia Accords, named for the Eastern European city that was destroyed in Avengers: Age of Ultron, which was also mentioned in the end credits scene of Ant-Man, when Captain America and Falcon find Bucky in the post credits scene and Rogers mentions that “The Accords” won’t allow Iron Man to give them any assistance.
The above mentioned incident reportedly involves Captain America and Scarlet Witch, who team up against Crossbones (Frank Grillo), who is wearing a “suicide vest” packed with explosives.
“In order to protect the civilians on the ground, Captain America orders Scarlet Witch to levitate Crossbones above ground. She does and unfortunately is unable to contain the massive explosion that sets off near a populated building causing death and destruction.”
Looks like this is how the Civil War starts.