What Rebels’ Offensive in Syria Means for a Stalled Civil War

Syrian opposition fighters have made their most significant advance in years against government forces, heating up a civil war that had long been at a stalemate.

The new rebel push began on Wednesday in Aleppo Province in northwestern Syria. By Saturday, antigovernment rebels had captured most of the major city of Aleppo, according to the fighters and a British-based monitoring group, the Syrian Observatory for Human Rights, taking control of much of it within hours and encountering little resistance from Syrian government forces.

The offensive aims to stop attacks by government forces and their Iran-backed militia allies, a rebel commander said, and is the most serious challenge to President Bashar al-Assad’s government in years.

Three days of fierce clashes have killed more than 150 combatants from both sides, according to the Observatory. The group gathers information from a network of anti-government activists and others across Syria, and its numbers could not be independently verified.

In addition to those deaths, more than a dozen civilians have been killed by Syrian and Russian airstrikes, according to the White Helmets, a rescue group based in opposition areas. Russia and Iran have for years helped Mr. al-Assad’s autocratic regime stave off the rebels.