With the international limelight locked on the U.N.'s pursuit of a diplomatic solution for Syria, an al-Qaeda-linked rebel group wrested control over the town of Azaz along the Turkish border on Wednesday with relatively little outside attention.
On Friday, after two days of clashes, the Islamic State in Iraq and the Levant (ISIS), which falls under the umbrella of the greater al-Qaeda network, signed a cease-fire pact with the Western-backed Free Syrian Army. The truce, brokered by a separate Islamist Brigade in the nearby city of Aleppo, called for the release of roughly 100 prisoners and referred the groups' dispute over control of a nearby border crossing into Turkey to an Islamic court.
Though the sometimes-rival rebel groups have agreed to lay down arms for now, analysts say the two-day battle for Azaz points to an internecine trend among the armed opposition.
"This is a fight that's been brewing for some time," says Joshua Landis, director of the Center for Middle East Studies at the University of Oklahoma and a foremost Syria expert.
"Both sides, particularly the FSA, have been trying to pursue a big tent strategy and work together with all the extremists, but that's failing."
Charles Lister at Jane's Terrorism and Insurgency Center says that the majority of armed rebels fall between extremist groups like ISIS and the Western-friendly FSA, but notes that "the Syrian insurgency and its countless component parts are in a state of constant flux."
ISIS, which analysts estimate is composed of 10,000 to 12,000 extremist fighters from Syria, Iraq and elsewhere, has traditionally fought against the Assad regime and alongside the FSA, which is supported and armed by the United States.
While al-Qaeda-linked factions and FSA battalions have only recently begun locking horns on the battlefield as they did in Azaz, assassinations and disputes over who has the right to administer rebel-held territories have plagued Syria's armed opposition for months.
In July, ISIS fighters assassinated two FSA commanders in separate incidents, exposing tensions between the groups. While the incidents riled up some FSA fighters concerned that their allies could not be trusted, FSA leadership calmed the seas.
"ISIS are our brothers who came to help us in a time when other Islamic and Western countries kept silent about the regime's crime," read a statement from the FSA's Aleppo Military Council on July 15.
FSA leadership tolerated extremist transgression because the better-funded ISIS was a valuable ally on the battlefield, and taking on another enemy would distract from the revolution's aim. But recent fighting threatens the tenuous cooperation of the past couple months.
"The FSA have been hoping that they could ride this jihadist tiger to victory over Assad," says Landis. "Now it's biting them."
As its name suggests, ISIS' stated goal is to establish an Islamic state in Syria as a prelude to restoring the Islamic Caliphate that once ruled the region.
Analysts say that at some point moderate rebels will have to confront this radical vision for a theoretical post-Assad Syria.
ISIS group has already taken small steps toward this goal by declaring "emirates" in certain northern towns and villages and dispatching emirs (Arabic for prince) to oversee each one. The emirs do not always take – videos uploaded to YouTube depict protests against extremist presence in places like Raqqa, where ISIS has a headquarters.
But Aymenn Jawad al-Tamimi, a Shillman-Ginsburg Fellow at the D.C.-based Middle East Forum, says the audacity of ISIS should be a warning sign for the FSA.
ISIS' vision for Syria "is in complete contrast to most FSA-banner battalions who think strictly in terms of Syria as a nation-state," he says.
That nation-state vision is spearheaded by the Syrian National Council, the political wing in exile of the Western-backed opposition, which on Friday issued a statement condemning ISIS.
"ISIS no longer fights the Assad regime," read the statement, which also accused ISIS of serving foreign agendas and repeatedly calling "to establish a new state inside Syria violating national sovereignty."
Tipping the balance
The infighting between rebel groups has been exacerbated by U.S. involvement in Syria -- both in the form of weapons provided to moderate rebel groups and the proposed U.S.-led strike.
Arms began to flow at the end of August, but the strike has been temporarily shelved while the U.N. Security Council considers a joint U.S.-Russia plan to confiscate and destroy the Syrian regime's chemical arms stockpile.
"The greater American involvement has necessitated a showdown for these groups," says Landis.
Al-Qaeda fighters in Syria fear that the U.S. strike, which President Barack Obama has said would hit regime targets exclusively, would also target ISIS and other extremist groups, paving the way for an FSA victory.
"They worry that the U.S. sees them as a greater danger than Assad, which is probably true," Landis says. "This underlines their anxiety that the FSA was recruited by the U.S. to fight al-Qaeda."
The power struggle between the FSA and ISIS may be coming to a head. Given the limited and sporadic foreign aid for moderate rebels, better-funded extremist factions are drawing more and more defectors from moderate counterparts.
On Friday, hundreds of rebel fighters pledged their allegiance to ISIS and Jabhat al-Nusra, ISIS' homegrown Syrian partner, activist and Islamist sources said.
"This is a sign the radical groups are still growing in power. This region could fall to the jihadists," said an activist in the northeastern town of Raqqa, who asked not to be identified, to Reuters. "We may see this become a trend."
A hardline Islamist brigade called the Raqqa Revolutionaries, with about 750 fighters, was the largest group to pledge allegiance to al-Qaeda's Syrian front.
Lister of Jane's Terrorism and Insurgency Center says that hardline Islamists like the Raqqa Revolutionaries may tip the balance.
"Much will depend on localized dynamics and how many hardline Islamist groups choose to side with the jihadists," says Lister.