The 13-year civil war in Syria has roared back into prominence with a surprise rebel offensive on Aleppo, one of Syria’s largest cities and an ancient business hub. The push is among the rebels’ strongest in years in a war whose destabilizing effects have rippled far beyond the country’s borders.

It was the first opposition attack on Aleppo since 2016, when a brutal air campaign by Russian warplanes helped Syrian President Bashar Assad retake the northwestern city. Intervention by Russia, Iran and Iranian-allied Hezbollah and other groups has allowed Assad to remain in power, within the 70% of Syria under his control.