The swift rebel offensive is a huge embarrassment for Syrian President Bashar al Assad and raises questions about his forces’ preparedness.
Attacks have now returned to Aleppo for the first time since 2016, when President Assad and his allies Russia, Iran and regional Shia militias retook it.
Syria has experienced more than 13 years of civil war. Here are some of the key events since the conflict began.
2011: Anti-regime sentiment spreads across Syria after protesters are shot dead by government forces. Conflict breaks out between rebels and pro-Assad troops.
2012: Government forces relentlessly bombard rebel territory, particularly the besieged city of Homs.
2013: The Islamic State (IS) is founded and begins to grow in strength and territory. A chemical weapon attack in Ghouta, Damascus, killed hundreds. The UN found that sarin gas was used, but did not assign blame to the Assad regime or the rebels.
2014: IS declares a “caliphate” covering much of Syria and continues to fight both the Assad regime and the rebels. Foreign countries begin bombing IS strongholds.
2015: IS fighters seized the historic city of Palmyra, destroying monuments across the city, a UNESCO Heritage Site.
Russia launches its first airstrikes in Syria. Moscow said it was targeting IS, but the West accused it of also attacking rebels to support Damascus. This proved to be a turning point in the conflict.
2016: Syrian troops, backed by Iran and Russia, recapture Aleppo – a significant blow to the rebels.
2017: IS is driven from Raqqa, the capital of the “caliphate”, by the US-backed Syrian Democratic Forces – widely seen as a symbol of the terror group’s decline.
2018: At least 70 people died and hundreds were injured in an alleged chemical attack on Douma, eastern Ghouta. The US state department called it “one of the worst chemical attacks in Syrian history”. The US, UK and France launched airstrikes against Assad targets in response.
2019: Turkey launches an offensive into northeastern Syria.
2021: Daraa in southern Syria sees intense fighting after the region boycotted the presidential election. A ceasefire is agreed in September.
2022: IS militants attack a prison in northeast Syria, during which 500 people are killed and some fighters escape.
2023: A devastating earthquake strikes Turkey and Syria, killing tens of thousands of people.