As the extremist fighters of the Islamic State in Iraq and the Levant (ISIL) insurgency press toward Baghdad, they leave at least 1,300 dead in their wake, according to the U.N. And the Sunni fighters whose lightning offensive has ruptured the region’s geopolitical balance are proud of that tally; online, ISIL propaganda releases gloat about summary executions of Iraqi police, soldiers and Shia civilians as they consolidate their astonishing takeover of swaths of northern Iraq.

ISIL explicitly seeks to eradicate the modern Iraqi state, both by effectively erasing the European-drawn border that separates Iraq from Syria and by replacing the institutions of both states with their own radical brand of Islamic sharia law — as they’ve already done in parts of Syria. Yet despite the movement’s extremist underpinnings, its Iraq offensive of just a few thousand fighters has apparently relied on the unlikely partnership of Sunni militias and a degree of tacit support or consent among Sunnis who’ve previously driven the extremist group out of their country.

And many Syrian Sunni rebels warn that their own experience shows that in making common cause with ISIL against a government they detest, Iraqis are making a grave mistake.

Much like in Syria, where ISIL has wrested control from other rebel factions over a large swathe of the war-torn north, the Al-Qaeda breakaway group has identified a power vacuum in Sunni regions of Iraq — and has deftly manipulated rising resentment of the country’s sectarian Shia Prime Minister Nouri al-Maliki — to dramatically expand its domain.

“It’s an old story,” said Aron Lund, an expert on Syria’s opposition and a contributor to the Syria in Crisis blog, a project of the Carnegie Endowment for International Peace. He pointed to Al-Shabab in Somalia or the Taliban in Afghanistan in the mid-’90s as examples of extremist groups that managed to convince a critical mass of war-weary citizens that their harsh brand of sharia law could bring about stability.

“The lesson here is that chaos breeds movements that will try to exploit it to their advantage,” said Lund. “It creates a situation where a well-armed movement with some level of internal discipline will be able to do a lot.”

Controversial though its policies may be, ISIL has demonstrated a capacity to secure and administer its vast holdings across Syria and Iraq. In Raqqa, the first major city it captured in Syria, ISIL has proved more effective than a previous rebel administration, opening administrative offices to run schools and overseeing the provision of essential services such as water and electricity. It has even cracked down on petty crime, according to some residents.

There are some in Syria’s rebel camp who bristle at the suggestion that allowing ISIL to take control in rebel held areas was a mistake. “Life is better here,” said a citizen journalist in Raqqa who only gave his first name, Abdulrahman.

Still, many analysts believe ISIL cannot foster an economically viable state and that its harsh governance will inevitably alienate the local population. In fact, attempts by Al-Qaeda in Iraq (which spawned ISIL) to impose its ideology on captive populations sparked the Sunni “Awakening” rebellion that ultimately drove the extremist group out of Iraq in 2007-8.

“Just because they can control Sunni territory doesn’t mean they can get an economy running in these areas,” Lund said. “They don’t have the infrastructure of the Iraqi or Syrian states, and there’s no one to trade with. There’s some oil they can smuggle or sell, but it’s not like they can provide for the population, [and that] makes them vulnerable.”

ISIL’s greatest asset, currently, is momentum. “People tend to like a winner,” says Lund, “so as long as the Islamic State continues to be winning they’ll rally a lot of local support.”