HTS was set up under a different name, Jabhat al-Nusra, in 2011 as a direct affiliate of al-Qaeda.
The leader of the self-styled Islamic State (IS) group, Abu Bakr al-Baghdadi, was also involved in its formation.
It was regarded as one of the most effective and deadly of the groups ranged against President Assad.
But its jihadist ideology appeared to be its driving force rather than revolutionary zeal – and it was seen at the time as at odds with the main rebel coalition under the banner of Free Syria.
And in 2016, the group’s leader, Abu Mohammed al-Jawlani, publicly broke ranks with al-Qaeda, dissolved Jabhat al-Nusra and set up a new organisation, which took the name Hayat Tahrir al-Sham when it merged with several other similar groups a year later.
For some time now, HTS has established its power base in the north-western province of Idlib where it is the de facto local administration, although its efforts towards legitimacy have been tarnished by alleged human rights abuses.
It has also been involved in some bitter infighting with other groups.
Its ambitions beyond Idlib had become unclear.
Since breaking with al-Qaeda, its goal has been limited to trying to establish fundamentalist Islamic rule in Syria rather than a wider caliphate, as IS tried and failed to do.
It had shown little sign of attempting to reignite the Syrian conflict on a major scale and renew its challenge to Assad’s rule over much of the country – until now.
