DAMASCUS, Syria — As rebels advanced toward the Syrian capital of Damascus on Dec. 7, the staff in the hilltop Presidential Palace prepared for a speech they hoped would lead to a peaceful end to the 13-year civil war. Aides to President Bashar al-Assad were brainstorming messaging ideas. A film crew had set up cameras and lights nearby. Syria’s state-run television station was ready to broadcast the finished product: an address by Mr. al-Assad…