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.

Syrian rebels have launched a surprise offensive against Syrian President Bashar al-Assad’s regime, a significant escalation in Syria’s civil war, which is raising concerns about a potential wider regional conflict in the Middle East.

Assad, the Syrian dictator, to retake Aleppo after part of the city fell to insurgents in 2012. Less than four days is how long it took him to lose it. On November 27th rebels launched a surprise offensive in north-west Syria.