The coalition of rebel groups fighting to remove Bashar al-Assad from power has always been a loose one; held together by the hatred of a common enemy.
The rag-tag army is made up of brigades and battalions each with different ideologies and visions for what a future Syria will look like – from radical Islamists fighting a global Jihad to secular nationalists seeking a democratic system.
With such unassailable visions it was perhaps inevitable that these groups would eventually come to blows. That moment appears to have arrived. A tacit agreement to settle their differences only after they had dealt with Assad appears to have fallen apart, and the rebels have now turned their guns on each other.
With the Syrian army holding firm across much of the country and the capital Damascus, the rebels have entered into a premature battle over the form of a post-Assad Syria.
On Wednesday afternoon, in the border city of Azaz, heavy clashes broke out between fighters with the Islamic State of Iraq and as-Sham (ISIS) – an off-shoot of al-Qa'ida – and the Northern Storm Brigades of the Western-backed Free Syrian Army. By nightfall there were several dead on each side and ISIS gained control of all entrances to the city. The outbreak of violence followed weeks of growing tension between ISIS and more moderate elements operating under the nominal command of General Salim Idriss, the head of the Supreme Military Council and leader of the coalition of brigades operating under the Free Syrian Army banner.
Last week, ISIS announced the beginning of a new campaign dubbed "Expunging Filth"– the target of which was not Assad’s army, but the FSA-affiliated Farouk Brigades, a force of around 14,000 fighters.
In north east Syria too, ISIS and its ideological ally Jabhat al-Nusra have been attacking Kurdish areas in a bid to gain control of key oil production. Fighting has also taken place in Deir ez-Zour, where the FSA accuse ISIS of attacking their headquarters.
As the recipient of a healthy flow of weapons and funding from its patrons in Iraq and donors in the Gulf, ISIS has earned a reputation for ferocity on the battlefield. ISIS is known to have attracted the most foreign fighters of all the groups fighting in Syria. Its 7,000- 10,000 share a global outlook, and see the fight in Syria as one part of a greater battle.
They have often taken the lead on the front lines and played a decisive role in key battles, such as the conquest of the Menagh air base north of Aleppo, which had held out for over a year before a concerted campaign led by ISIS that involved the use of suicide bombers.
Using its superior firepower, ISIS has sought to consolidate its power in the north. In some areas, the order and security brought by ISIS has been welcomed – at least temporarily –by civilians who had long complained that FSA fighters were corrupt and unruly.
This consolidation has extended beyond flexing its military might. ISIS and Jabhat al-Nusra have in recent months been engaged in a campaign to win the hearts and minds of the civilians in areas under their control. One particularly memorable example of this was a family fun day of sorts held in Aleppo by the two groups – used to operating from the shadows – complete with an ice cream eating competition. They have also set up Sharia courts and doled out punishments to locals and fighters for breaking their newly introduced laws.
But the increasing dominance of ISIS has also caused resentment. Stories of civilians angry over the growing imposition of al-Qa’ida ideology on what was previously (for all its faults) an inclusive state, are becoming more commonplace.
Last month in the provincial capital of al-Raqqa, residents held protests against the growing influence of ISIS in the city. The abduction and execution of 14-year-old Mohammad Qattam in Aleppo in June, punishment for making an off-hand remark about the Prophet, provoked revulsion among residents in a city already battered by war.
Among the more moderate rebel brigades and activists, that resentment has existed for some time. Last year in Aleppo province, activists spoke with unease of their extremist bedfellows. Many were keen to make a distinction between the "real" Free Syrian Army and people who claimed the mantle. There was discomfort too, of being aligned with a group that was carrying out atrocities such as executions and suicide bombings in civilian areas.
Yesterday, as the fighting erupted into the open, there was a sense that this was expected.
"We knew this would happen from day one," said Abu Admad, an activist with an FSA-affiliated media centre in Aleppo. "From the moment foreign fighters came flooding into the country saying they were fighting for Jihad and for Sharia we knew it would end like this."
The battle lines being drawn in Azaz could have a dramatic impact on the course of the war in Syria. The most immediate effect will be a weakening of the front-lines where rebels are battling against Assad's army, as protagonists on both sides call reinforcements to fight it out in the north.
Although there have been skirmishes among the rebels before, Aron Lund, a researcher and expert on the Syrian opposition, said this round of fighting appear to be more serious.
“The new thing is that you've had an uptick in battles between the Islamic State and others in several areas at the same time, and there's an impression spreading that they're knifing the rebellion in the back,” he said.
The fight will make clearer the divisions within the rebel movement as each group is forced to pick a side. And while it is not instructive to divide Syria’s rebels simply into “moderate” and “extremist”, the alliances that may form in a battle between al-Qa’ida and the more palatable (to the West at least) Free Syrian Army would present a clearer picture where each group stands than exists currently.
The implications stretch beyond the borders of Syria. The United States has been forthcoming in expressing its desire to support moderate elements in Syria, but is has been constrained by fears that any weapons they send might easily fall into the hands of rebel groups who would just as sooner use them against the US as they would against the Syrian army. The outbreak of open conflict between Idriss' Free Syrian Army and ISIS may go some way to allying those fears. If the FSA is being shot at by al-Qa'ida, they are hardly likely to give them the weapons to do it.
Even as the fight was still ongoing on Thursday, the Free Syrian army called for support from the West.
“We are asking for the West’s help,” said FSA spokesman Louay al-Miqdad. “We cannot fight on two fronts. Don’t leave us between Assad, Hezbollah and al-Qa’ida.”
Mr Miqdad said that FSA leaders were holding meetings to decide on a response to the attack on Azaz.
“I don’t think there is a chance of a solution [with ISIS],” he said. “They are fighting for different goals. They are fighting for their own agenda.”
“We are short in resources and we need help, but they shouldn’t forget we have been here for 7,000 years, living together. We will not allow them to hijack our revolution.”