Russian and Syrian Regime Forces Strike Rebel-Held Territory as Assad Faces Biggest Threat in Years

BEIRUT—Syria’s President Bashar al-Assad is facing the most serious challenge to his grip on power in nearly a decade after rebels captured most of the city of Aleppo in a swift offensive that crumpled his Russian- and Iranian-backed forces and has the potential to alter the balance of power in the Middle East.

Russian and Syrian government planes launched airstrikes in rebel-held territory Sunday in an attempt to disrupt the rapid advances seen in recent days, including the seizure of Aleppo, a strategic prize that, before the war, was Syria’s largest city and a commercial hub for the broader region.