All eyes have been on ISIS as the jihadist group, in a matter of days, cut Iraq in half and declared its own state in the cities it captured. With fewer than 10,000 fighters ISIS forced the retreat of the better-armed Iraqi army forces many times its size. Their incredible success on the battlefield has fed into a growing lore about the group: the small band of fanatics that can take down a country. The truth is more basic and it’s something ISIS doesn’t want to admit—they weren’t acting alone.

It wasn’t having God on their side that let ISIS conquer Mosul and Tikrit with hardly a fight, analysts say. It was the other Sunni insurgent groups that were there alongside them, unacknowledged partners in the coalition. Those groups have deep organizational roots and were instrumental in the takeover but have been largely overshadowed by ISIS.

The standoff in Iraq isn’t between a single militant group and the government. There is a broad coalition of Sunni groups—both nationalist and Islamist—who had been plotting against Iraq’s Shia government for years before ISIS's rise provided the chance to strike. ISIS and its partners are unnatural allies. Maintaining their unity was the key to their early success, and is the only way they can hold the ground they have taken, but that incentive may prove to be weaker than the force of their natural hostilities.

“ISIS control in Mosul is contingent on political alliances they have made with the Baathists and the tribal groups,” said Brian Fishman, a fellow at the New America Foundation, who has been following ISIS since the group’s early days during the Iraq war.

“This alliance marching on Baghdad is not a natural one,” Fishman added. “We can understand how it was put together in opposition to the government but what exactly is holding it together, and how sturdy it is, is an open question,” he said.

The anonymity of the non-ISIS members in the anti-government faction wasn’t by choice. Some have used social media to broadcast their war exploits and document their control of conquered territories. It could be an early sign of fissures in the coalition that beat back Baghdad’s army.

(All images used in this story were taken with permission from the twitter account of Middle East scholar Aymenn Al-Tamimi.)

If the rebel groups begin fighting against ISIS and each other, even as they remain at war with the government, it could lead to something like Syria’s war of all against all. Iraq may not descend into the kind of protracted conflict that has ground up Syria and its people, but the days ahead will invariably be filled with bloodshed.

Hassan Hassan, an analyst at the Delma institute in Abu Dhabi, was one of the first observers to point out ISIS reliance on cooperation with Iraqi insurgent groups.

“Non-ISIS groups played a central role in the takeover,” Hassan said.

Tactical details from the past week’s offensive are hard to come by. But Hassan says the groups that cooperated with ISIS include: “The Sufi-Baathist militia known as the Army of the Men of the Naqshbandi Order, which has former members of the Iraqi army during Saddam Hussein’s reign. The Al Qaeda-originated Ansar al-Islam, and provisional tribal councils, many of the which are actually front groups for the Naqshbandis,” according to Hassan.

The coalition seemed—initially, at least—to have tempered ISIS’ severe approach to governance, which traditionally relied on public execution as a staple of justice.

“The involvement of these groups can be felt through the way the fighters treated the local population fairly well, compared to the usual notoriously brutal behavior of ISIS,” Hassan said.

It’s a point echoed by Fishman. “There has been genuine learning from this organization about how to interface with populations,” Fishman said, referring to ISIS’ experiences in Iraq and the areas it currently controls in Syria.

Because Mosul fell so quickly, with little fighting from the Iraqi army, the city appears to have been relatively unscathed by the assault that wrested it from government control. With ISIS and other anti-government insurgent factions long entrenched in Mosul they seem to have been able to quickly restore some basic services in the city. One photo captured by Middle East scholar Aymenn al-Tamimi, who collects tweets from Iraqi insurgent groups including ISIS, shows garbage trucks collecting trash in eastern Mosul on Thursday.

Another shows fuel tankers providing gas to a line of cars.

For all its genuine learning and operational acuity, ISIS’ conception of itself as the embodiment of God’s will is hard to reconcile with practical compromise.

The relative moderation evidenced early on may be necessary to maintain the population’s support and preserve the coalition, but ISIS has begun to impose its fundamentalist approach.

Already, the group has declared itself the sole authority in Mosul and released a set of religious laws for the people of Nineveh province. The laws laid out are no one’s idea of moderate:

- “For women, dress decently and wear wide clothes. Only go out if needed.”- “Our position on Shrines and graves is clear. All to be destroyed basically.”- “Gatherings, carrying flags (other than that of Islamic State) and carrying guns is not allowed. God ordered us to stay united.”- “For the police, soldiers and other Kafir institutions, you can repent. We opened special places that will allow you to repent.”- “No drugs, no alcohol and no cigarettes allowed.”

There are more rules on the list, but that gives a pretty good sense of their severity.

Hassan, who has been observing ISIS said, “It is expected that ISIS would try to impose itself, considering that it brands itself as a state and that it alone has the legitimacy to rule.”

“The group’s propensity for imposing its will regardless of the consequences will likely lead to confrontation and clashes with other groups,” he said. “The offensive was planned together but that does not mean the old rivalries will not come up again. It is very likely that rivalry will lead to clashes.”

War can be a centrifugal force, pulling together disparate groups who share a common enemy. It’s after the heat of battle when it comes time to divvy up the spoils and work out power sharing arrangements, that yesterday’s allies turn on each other.

“That’s where jihadi groups have always failed in the past,” said Fishman. “ISIS has built a military coalition,” he said “at this point they haven’t fully demonstrated it can be a governing coalition. If this crumbles in the near term it won’t be because Baghdad gets it act together but from the coalition coming apart.”

Whether ISIS can maintain the partnerships that allowed it to get this far is uncertain. What is certain, a grim and unavoidable reality at this point, is that Iraq is once again at war, and has a ruthless fundamentalist power lodged in the heart of the country.