In July, an uneasy six-month truce between two hardline Islamist groups came to a sudden and violent end in Idlib city, the capital of Syria's only remaining rebel-held province.
After a week of bloody clashes, Hay'at Tahrir al-Sham (HTS) fighters chased their Ahrar al-Sham rivals out of the city.
The symbolic conquest of the capital was a fitting end to a blitzkrieg which has seen the group sweep through more than 30 other towns and villages in the region, Al Jazeera reports.
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Idlib, once a stronghold of the pro-democracy rebels fighting to replace President Bashar al-Assad, is now the "largest Al Qaeda safe haven since 9/11", according to White House special envoy Brett H. McGurk.
So who are HTS? And are they about to become a game-changing player as the Syrian civil war grinds through its seventh year?
Who are HTS?
HTS is a merger of five separate Islamist factions of the Syrian opposition, formed in January.
The largest of the faction's members is Jabhat Fateh al-Sham, which originated as an outpost of al-Qaeda called the al-Nusra Front.
Other HTS members include Jaysh al-Sunna, itself the result of a merger, and the Nour al-Din al-Zenki Movement, which has now been part of seven coalitions in four years.
This complicated history is typical in the fast-moving world of Syrian rebel politics, where groups frequently rebrand to reflect shifting alliances. New mergers and coalitions appear all the time.
What do they believe?
HTS subscribes to the Salafist school of thought. Often used interchangeably with Wahhabism, Salafism is an ultra-conservative current of Sunni Islam that advocates a strict, literalist interpretation of sacred texts. Its name derives from the Arabic world for "forefathers", reflecting the ideology's emphasis on returning to the "pure" form of Islam practised by the earliest Muslims.
One sub-set of the Salafi doctrine, called salafist-jihadism, believes in using military force to restore the perfect Islamic caliphate established by Muhammad and his immediate descendants.
Salafi-jihadist terror groups like Islamic State and al-Qaeda have given the movement a "disproportionately huge impact", says The Guardian.
How do they differ from other Syrian opposition groups?
In the early stages of the civil war, the Western-backed, pro-democracy Free Syrian Army was the largest rebel faction, but in-fighting steadily decimated its ranks.
Although the FSA aren't entirely out of the running – their flagging fortunes received a boost last year when long-time backer Turkey waded into the conflict – since 2013, Islamist groups have taken over as the dominant anti-Assad force in the conflict
But while the dozens of Islamist rebel factions may share a broadly similar Salafist ideology, they are far from united.
In November 2014, an attempt to form a super-coalition which would have united Islamic State and other major Islamist insurgencies against the government broke down as old grudges and rivalries proved insurmountable, according to the Syrian Observatory for Human Rights.
Dozens of coalitions have been formed since then. Almost all have fallen apart over disputes about everything from territory to religious doctrine.
For instance, the Army of Conquest, an umbrella organisation of 50,000 Islamist fighters from various groups, seized much of Idlib province in spring 2015 but later fractured due to disagreements over the finer points of Islamic law, Middle East Eye reports.
The shrinking size of the rebel territory has only intensified those rivalries, with dozens of factions left to compete for control over the single province still outside government control.
During the HTS's July campaign for control of Idlib, the bloodiest clashes were not with government forces, but with rival Islamist faction Ahrar al-Sham.
Ahrar al-Sham was once allied with HTS's previous incarnation as the al-Nusra Front. It carried out joint operations as late as May 2016, but the ever-turning kaleidoscope of rebel politics has turned them into bitter enemies.
HTS's decision to push its former ally out of Idlib came at a time when Ahrar al-Sham was "trying to market itself as a moderate rebel group", says the Kurdish news agency Rudaw.
What will happen now?
"Hayat Tahrir Al Sham has emerged even more dominant than six months ago," says The National's Hassan Hassan, "and its attempts to dominate are far from over".
Before it can think of expanding its territory, HTS must cement its grip on its recent gains – a delicate task, says Al-Monitor.
An overly-aggressive strategy risks provoking a popular uprising from war-weary civilians in no mood to let their towns become the next Raqqa or Mosul.
Human rights activist Ibrahim al-Idelbi told the news site that there had already been anti-HTS protests in at least two towns in the region.
"They are asking for the dissolution of HTS or for the organisation to direct its weapons against Assad instead of local rebel factions," he said.
Keen to prevent the establishment of a jihadist stronghold on its southern border, Ankara has urged HTS to disband or face the wrath of the US and Russia, according to the pro-government Turkish daily Yeni Safak.
In the meantime, "the jihadists have said they will create a civilian body to govern the province," says the New York Times, "but it remains unclear when that could occur or what such a body would look like".
Idlib's valuable strategic purpose as a "safe" zone for prisoner swaps and population transfers has so far spared it from the kind of bloody ground assault by Assad's forces seen in Aleppo.
However, civilians fear that HTS's emergence as the dominant power might be the final straw that ends Idlib's status as the closest thing Syria's displaced people have to a safe haven.
"They are sending everyone here and we don't know what will happen to them in the end," an engineer told the NY Times.
"We are just going from one tragedy to a bigger tragedy."
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