Al-Nusra Front, which Thursday announced it was no longer part of the al-Qaeda international jihadist network, is one of the most powerful opposition forces in the Syrian civil war.
Its battle-hardened fighters and use of deadly suicide attacks to soften up targets have allowed it to play a key role in rebel victories, notably the expulsion of government forces from almost all of the north-western province of Idlib in early 2015.
But its allegiance to al-Qaeda and its dedication to the international extremist network's ultra-hardline interpretation of Islamic law have also made it a political liability for the opposition.
It is designated as a "terrorist" organization by the United States - which broadly supports the opposition - and by the United Nations.
Russia has used its presence in the ranks of rebel fighters on many fronts across the war-torn country as justification for targeting opposition positions in its air campaign in support of President Bashar al-Assad.
The group's founding members were sent to Syria in 2011 by the leadership of al-Qaeda in Iraq, according to Stanford University's Mapping Militant Organizations project.
Al-Qaeda in Iraq's leader Abu Bakr al-Baghdadi saw an opportunity in Syria's descent into conflict. But at first, al-Nusra was careful not to put off potential supporters by admitting openly to its links with al-Qaeda.
The group first came to prominence in early 2012 when it claimed a number of deadly suicide attacks, mainly on security forces.
It increasingly moved to take part in guerrilla fighting and slowly became one of the most powerful, though not numerically largest, rebel forces.
As hardline Islamists became more prominent among rebels, al-Nusra found it easier to form close working alliances with them - most notably with the Ahrar al-Sham Islamic Movement, which may be the largest single rebel group in Syria today.
There was a turning point in April 2013.
Al-Baghdadi announced that he had originally founded al-Nusra, and declared that he was merging it with his own organization as the Islamic State in Iraq and the Levant (ISIL).
Al-Nusra leader Abu Mohammed al-Jaulani had other ideas, and he struck back by pledging allegiance to al-Qaeda's overall leader, Ayman al-Zawahiri, who gave al-Jaulani his backing.
The group's forces split. ISIL increasingly sought to gain sole control of territory, fighting with other rebel groups and later with al-Jaulani's al-Nusra loyalists.
Daesh ended up in control of most of eastern Syria, with al-Nusra concentrated in opposition-held areas in the more populated centre and north.
It remained on good terms with most other rebels, although in mid-2014 it began to impose its vision of Islamic law in a number of strongholds in the north-west.
In September 2014, the US launched airstrikes against both al-Nusra and Daesh, which had rebranded as "the Islamic State" after a series of victories in neighbouring Iraq.
Al-Nusra hit back by routing two of the largest US-backed moderate rebel forces, forcing them both to disband in the subsequent months.
Al-Jaulani meanwhile began to gain a more public profile, giving interviews to broadcaster Al Jazeera in 2013 and 2015, in which he stressed the group's commitment to the Syrian conflict rather than attacks abroad.
Al-Nusra would not seek to monopolize control if the al-Assad regime fell, he promised.
His message for Syria's religious minorities, such as the Alawite and Druze sects which tend to support al-Assad, was less reassuring.
They would be protected, he said, as long as they renounced their religious errors. In other words, they would have to effectively convert to al-Nusra's form of Sunni Islam.
March 2015 brought more victories for al-Nusra as it formed a new coalition with Ahrar al-Sham and other rebel groups that, over two months, drove regime forces from almost all of north-western Idlib province.
The group remains a key force on many of Syria's most active front lines.
Analysts say that, aware of its strength, but also the danger of its ties to al-Qaeda, other opposition figures have been trying to persuade it to break that link.
But many are doubtful that its announcement on Thursday really changes anything about the group's hardline jihadist nature.
"If rumors are true that JN [al-Nusra] has agreed to break from AQ [al-Qaeda], then opposition is being lured into trap," analyst Charles Lister of the US-based Middle East Institute wrote on Twitter before the news was confirmed.
By Pol O Gradaigh
