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And in a sign of the complexity of the violent power struggles in both countries, ISIL is being confronted not only by its prime enemy in each country — the regime of President Bashar al-Assad in Syria and the government of Prime Minister Nouri al-Maliki in Iraq — but also by rival anti-Assad and anti-Maliki forces.
In northern Syria, a coalition of competing militias grouped under the banner of the Islamic Front, as well as secular fighters of the Free Syria Army, Kurdish militias and concerned citizens, has attempted to roll back ISIL’s domination.
While its prowess in battle and access to arms and funds had carved out a leading role for the Al-Qaeda-linked group among those fighting Assad, ISIL's harsh imposition of extreme interpretations of Islamic Sharia law has alienated many anti-Assad Syrians on the ground.
A combination of the more chaotic environment created by the Shia-dominated Maliki government's alienation of Iraq's Sunni minority and spillover from the Sunni-dominated anti-Assad rebellion on the other side of the border, has fueled a revival of Al-Qaeda's fortunes in Iraq too.
A steady stream of bomb attacks on mostly civilian targets made 2013 the deadliest year in Iraq since 2008, and the recent attempt by fighters from ISIL to hold ground in Fallujah and Ramadi amid heavy fighting reflects a significant escalation.
The Iraqi forerunner of ISIL — Al-Qaeda in Iraq — emerged in the same geographic areas shortly after the U.S. invasion, but was pacified by 2008 after its methods alienated many Sunni Iraqis.
A large number of Sunni insurgents who had fought against U.S. forces later made common cause with the occupier in order to root out Al-Qaeda from their communities. Amid the fighting in Fallujah and Ramadi, it appears that local tribal fighters are once again tackling the extremists.
Despite the major sectarian antagonisms in each country, the recent fighting in both Syria and Iraq represents an intra-Sunni conflict — local Sunnis fighting Al-Qaeda in their midst, even if the movement has managed to attract some local followers and support.
Assad may be delighted to see his enemies fighting one another, as the Al-Qaeda element in the Syrian rebellion has been a major factor discouraging Western powers from providing stronger support for his enemies.
For his part, Maliki — a key backer of Assad's regime — is counting on tribal Sunni forces to take on ISIL. Meanwhile, Washington has promised Hellfire missiles and drones to boost Maliki’s army.
Despite the major [Shia-Sunni] sectarian antagonisms in each country, the recent fighting in both Syria and Iraq represents an intra-Sunni conflict — local Sunnis fighting Al-Qaeda in their midst.
Complex political divisions are further exacerbated by the current violence, and there have even been reports that some Nusra forces have fought against ISIL — while other factions have effectively merged with the organization, and still others maintain a separate Al-Qaeda franchise.
It is difficult to know whether the two main branches of ISIL have coordinated escalating military action during the past week in Syria and Iraq. Their command structure and order of battle are not centralized, as with conventional armies, said Brian Jenkins, an analyst at the RAND Institute.
But the emergence of the organization on both sides of the border is a portent of a deeper shift: “Syria as an actual modern state has ceased to exist," Jenkins told Al Jazeera. "Instead we’re going to have mosaic of enclaves, with the increasing merger of eastern Syria and western Iraq — with low population and low economic activity.”
And, in all likelihood, a low-intensity war against both Damascus and Baghdad that could continue for years.