What does rebel infighting mean for Syria conflict?
With something like 400 fighters and civilians reported killed in the first five days of the confrontation between the al-Qaeda affiliated Islamic State in Iraq and the Levant (Isis) and other rebel groups, the casualty rate is rivalling, if not outstripping, the toll inflicted by the wider war between the regime and opposition.
There is already one other war-within-a-war going on in Syria, as the Kurds in the north-east assert their control over areas where they predominate, clashing daily with Islamist and other rebel groups.
Will the vicious fighting with Isis become entrenched as a third ongoing conflict, draining the rebel movement and distracting it from its primary mission of confronting the regime?
The clashes, raging over a wide swathe of territory in at least four provinces - Hama, Idlib, Aleppo and Raqqa - raise many other questions too.
Is it just coincidence that this comes at a moment when Isis fighters are coming under pressure from Iraqi government forces in Anbar province, adjacent to the border with Syria?
Is it part of an internationally-backed campaign to root out the Islamist radicals in both countries, heading off the emergence of a militant caliphate straddling the border?
Is it aimed at clearing the deck in advance of the "Geneva II" peace talks, scheduled to begin on 22 January?
While the Americans, Russians and other outsiders will not be grieving at the sight of the most radical of the radical Islamists taking a hiding, most of the evidence suggests that there is no role for outside instigation in the affair.
"It was a pure, 100% Syrian decision [to attack Isis]," said one opposition official.
"Does it serve the Americans? Perhaps. But that's not why it's happening. Even if the [Western-backed opposition National] Coalition had wanted to, it doesn't have the leverage. There's a disconnect with the main fighting forces on the ground such as the Islamic Front."
Isis began causing friction with other rebel groups - and even more so with civilians - almost as soon as it arrived on the scene around a year ago.
It was soon being accused of being more interested in monopolising control of "liberated" areas and imposing its own harsh version of Islamic rule than in fighting the regime, so much so that there are even unverifiable charges that it is deeply penetrated and manipulated by Syrian state intelligence and Iran's Revolutionary Guards.
"What it's been doing on the ground is exactly what the regime was doing - detaining and killing activists, torturing people, and crushing freedom in general," said an opposition source.
As Isis became ever more assertive, it began impinging increasingly on rival rebel groups, taking over their bases and abducting and sometimes killing fighters and trying to control vital supply routes from Turkey.
The Syrian Revolutionaries' Front, one of the groups fighting Isis, accused it of having abducted and killed at least 400 rebel fighters, and holding more than 2,000 others prisoner.
Mounting tensions came to a head on 31 December, when ISIS handed over the mutilated body of Dr Hussein al-Suleiman, known as Abu Rayyan, a popular and respected commander of the powerful Ahrar al-Sham faction, one of the main components of the Islamic Front. He had been captured by Isis three weeks earlier.
That was the last straw. It set in motion a series of attacks on Isis bases and positions by a variety of combat groups, including the Islamic Front and two other recently-emerged fighting coalitions, the Mujahideen Army and the Syrian Revolutionaries' Front.
The onslaught on Isis by such a disparate collection of umbrella groups had all the hallmarks of a spontaneous outburst of pent-up frustration, rather than a concerted externally-orchestrated campaign.
Conditions and players varied from region to region. In some places, especially Raqqa, the attack was led by groups linked to the al-Nusra Front, itself an affiliate of al-Qaeda.
In other areas, al-Nusra has acted as a mediator between Isis and other factions.
'Setback, not defeat'
The fighting has yet to run its course, but it seems clear that Isis has lost a lot of ground in most areas, and is under pressure in its main stronghold, Raqqa, in the north-east, the only provincial capital to have fallen out of government control.
Some analysts attribute its rapid collapse in many areas to the fact that ISIS was spread thin and its numbers were relatively small - perhaps as low as 8,000 to 9,000 overall - despite its high profile.
But few expect the group to be wiped out completely.
"It's a setback, not a defeat for Isis," said one diplomat.
Some believe it will fall back on territory around Deir al-Zour in the east, adjacent to the Iraqi border and the group's strategic hinterland in western Iraq.
Much will clearly depend on how it fares there, as Baghdad tries to restore its authority.
There are fears Isis may also intensify the wave of suicide car bomb attacks on other rebel groups that it has already unleashed.
"It's going to be really scary," said an opposition source.
So far there has been little sign that, distracting though the struggle may be for the rebels, it has made them a softer target for regime attacks.
The overall result seems likely to be that ISIS will end up with its wings clipped, and perhaps having to modify its approach of trying to monopolise power and impose its own stark vision of Islamic statehood.
A template for that might have been reached in the north-eastern province of Hassaka, where Isis and four other groups, including the Islamic Front, announced on Thursday that they were setting up a joint Sharia authority and would co-ordinate all their military activities.
In terms of Western and Russian concerns about the flourishing of Islamist radicalism within the opposition, the potential curbing of Isis may not make a massive difference.
The al-Nusra Front, which is actually the official al-Qaeda franchise in Syria, remains deeply embedded in rebel ranks and has proven much more attuned to operating and co-operating in the Syrian environment than the outsider-led ISIS.
Al-Nusra has been designated a terrorist organisation by the US, the UN and others.
There are many other hardline Islamist groups within the rebel movement, many of them gathered in the Islamic Front. Unlike Isis, most have opted to shelve their visions of a post-Assad Syria until after regime change.
Hardly any of the major fighting factions on the ground support the idea of going to Geneva and negotiating a settlement with the regime.
So the possible eclipse of Isis in Syria will have a limited impact on the talks in Switzerland, assuming they go ahead.
But it may have the effect of slightly strengthening the hand of the National Coalition, if it does end up attending, in a political environment where much attention internationally and in Syria itself is on the need to combat "terrorism".
"With the regime talking about the need to fight terror, and even arguing that Geneva II should be dedicated to that, the real fight against it is being done by the opposition," said one coalition official. "So it's a good thing to invest in and put on the table."