After just a few days of rapid advances, the rebels appeared to control Syria’s capital. Rebels declared that they would work with a top official in President Bashar al-Assad’s government. Mr. al-Assad’s location was unclear. Rebel fighters stormed into Syria’s capital, Damascus, on Sunday, taking them to the brink of a stunning victory as evidence mounted that Syrian forces and their leader, President Bashar al-Assad, had fled. In just two weeks, a bitter stalemate that had held for years in Syria has been snapped.