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 announcing a plan to share power with members of the political opposition, according to three people who were involved in the preparation.

Working from the palace, Mr. al-Assad, who had wielded fear and force to maintain his authoritarian rule over Syria for more than two decades, had betrayed no sense of alarm to his staff, according to a palace insider whose office was near the president’s.

The capital’s defenses had been bolstered, Mr. al-Assad’s aides were told, including by the powerful 4th Armored Division of the Syrian Army, led by the president’s brother Maher al-Assad, the insider said.

They had all been deceived.

After dusk, the president slipped out of the capital, flying covertly to a Russian military base in northern Syria and then on a Russian jet to Moscow, according to six Middle Eastern government and security officials.

Maher al-Assad fled separately that evening with other senior military officers across the desert to Iraq, according to two Iraqi officials. His current location remains unknown.