Syria's civil war took an interesting turn over the weekend. Three years after dissident Syrians took up arms to topple President Bashar al-Assad, the rebel groups started fighting one another. Specifically, various moderate and Islamist rebel groups banded together to kick out a group dominated by foreign extremists, the Al Qaeda-aligned Islamic State of Iraq and the Levant (ISIL).

The rebel-on-rebel fighting started on Friday in the northern provinces of Aleppo and Idlib, according to British group the Syrian Observatory for Human Rights, after ISIL killed Dr. Hussein al-Suleiman, the popular leader of rival faction Ahrar al-Sham. The killing ignited long-simmering anger against ISIL, whose ultimate goal is establishing a 7th century–style caliphate in a region encompassing eastern Syria and western Iraq. ISIL is reportedly detested by local populations, since it habitually imprisons critics and imposes its hardline Islamic rules in the areas it controls.

The united rebel groups — including the moderate Syrian Revolutionary Front, Ahrar al-Sham and the allied Islamic Front, and a new Aleppo group called the Mujahedin Army — had a productive weekend, retaking several cities and towns from ISIL. The intra-rebel fighting had spread to the eastern city of Raqqa by Monday. About 100 people have died in the fighting, according to Syrian Observatory for Human Rights estimates.

Nicholas Blanford at The Christian Science Monitor provides some helpful background:

The Free Syrian Army, originally the main armed opposition group, has been overshadowed by the rise of more militant Islamist groups. The drift by rebel factions toward Islamic extremism was partly due to the regime's brutal tactics, but it was also motivated by a desire to boost their appeal to wealthy Arab patrons in the Gulf.

The competition among rebel groups is a Darwinian contest for funding and weapons. Two of the strongest individual factions today are the al Qaeda-affiliated groups Jabhat al-Nusra [Al-Nusra Front] and the Islamic State of Iraq and the Levant. More recently, Saudi Arabia backed the creation of a new rebel coalition, the Islamic Front, grouping together 45,000 to 60,000 fighters from some of the strongest factions. [Christian Science Monitor]

As long as the rebel groups are fighting among themselves, the big winner should be Assad's forces. After all, if the various groups are fighting ISIL, they aren't fighting the Syrian army and its proxy militias.

But so far this seems like a winning fight for Syria's rebels.

First of all, ISIL gives them an enemy they can beat, and that has to provide a psychological boost. The rebels and Assad's forces are stuck in a sort of stalemate, with Assad holding Damascus, Homs, and Hama, as well as the coastal strip in the northwest corner of the country. The rebel groups are holding the northern swath and, at least for now, Syria's second-largest city, Aleppo.

Fighting an al Qaeda–aligned group will also help assure the U.S. and other Assad critics that it might be relatively safe to supply the rebels with more weapons and other aid. Few people expect the upcoming Geneva II peace talks in Montreux, Switzerland, to produce much in the way of an end to the fighting, but it should put any participating rebel groups in direct contact with Western diplomats.

This unified attack on Al Qaeda is "an important opportunity for some self-proclaimed friends of the Syrian people," says Lebanon's Daily Star in an editorial. "Countries in the West and elsewhere have been warning against the rising influence of these militants in Syria for months, perhaps using this as an excuse to avoid getting involved in the crisis."

"Now is their chance to support the rebels who openly declared that their goals have nothing to do with those of religious extremists trying to hijack the uprising against the regime," the Daily Star adds.

At the same time, some reports from Syria suggest that the ISIL isn't doing that badly. Indeed, in some areas, the militants aren't losing so much as defecting to the Syrian-dominated Al-Nusra Front or slinking away to fight other battles. "The Islamic State is pulling out without a fight," activist Firas Ahmad tells Reuters. "Its fighters are taking their weapons and heavy guns."

Some of the fighters appear to be heading toward Aleppo, but others are probably going back to Iraq, where the group originated. Last week, ISIL took over parts of the key Iraqi cities of Ramadi and Fallujah, the first sustained territory grab by the Sunni insurgents in years, certainly since U.S. forces withdrew at the end of 2011. Iraq and some local tribes are fighting back.

This is where ISIL's big battle will take place. As long as ISIL is pouring its resources into Iraq, the winners in the short term should be the Islamic Front, the Free Syrian Army, and the Mujahedin Army. Not only will they have more territory and fewer distractions from a bloody rival faction, but Iraq will be less willing and able to send militias into Syria.