The rebels who have swept through Syria are led by Islamist alliance Hayat Tahrir al-Sham, or HTS, along with an umbrella group of Turkish-backed Syrian militias called the Syrian National Army.
Both have been entrenched in the north-west. They launched the shock offensive on 27 November with gunmen capturing Aleppo, Syria’s largest city, and the central city of Hama, the fourth largest.
The founder of HTS, Abu Muhammad al-Jolani, was once a participant in the Iraqi insurgency against the US as a member of the group that eventually became Islamic State.
In its former incarnation as Jabhat al-Nusra or the al-Nusra Front, HTS later declared allegiance to al-Qaida. It eventually publicly broke those ties in 2016 and rebranded as Hayat Tahrir al-Sham, or Organization for the Liberation of the Levant.
HTS is now the most powerful rebel faction in Syria.
It is designated as a terrorist group by the US and there are serious human rights concerns in the area it controls, including executions for those accused of affiliation with rival groups and over allegations of blasphemy and adultery.
HTS and the Syrian National Army have been allies at times and rivals at others, and their aims might diverge.
The Turkish-backed militias also have an interest in creating a buffer zone near the Turkish border to keep away Kurdish militants at odds with Ankara. Turkey has been a main backer of the fighters seeking to overthrow Assad but more recently has urged reconciliation, and Turkish officials have strongly rejected claims of any involvement in the current offensive.
