BEIRUT - Rebel forces opposing President Bashar al-Assad have launched their biggest offensive in years this week, controlling a majority of Syria’s second city of Aleppo according to a monitor.

Government forces offered little resistance, the war monitor said, and the army admitted that rebels had entered “large parts” of the city.

Why have the Syrian rebels and their allies from Turkish-backed factions decided to attack after years of relative calm, and what is at stake?

Why now?

On Nov 27, Hayat Tahrir al-Sham (HTS), a militant alliance led by Al-Qaeda’s former Syria branch, and allied factions attacked government-held areas of the northern province of Aleppo and the northwestern Idlib region.

The Britain-based Syrian Observatory for Human Rights war monitor said the rebels had seized dozens of towns and villages in the north and “took control of most of” Aleppo.

The violence has killed at least 311 people, mostly combatants on both sides, but also including at least 28 civilians, said the Observatory, which relies on a network of sources inside Syria.

Ms Dareen Khalifa, a researcher at the International Crisis Group think-tank, said the rebels had prepared for months for this offensive.

“They’ve framed it as a defensive move against regime escalation,” Ms Khalifa said, as Syrian government and Russian strikes on the area intensified leading up to the attack.

But HTS and their allies are “also...