With the call to arms today by Iraq's most revered Shiite cleric, the crisis in Iraq threatens to become an even broader regional sectarian conflict. But it could also bring about unusual cooperation between the US and Iran, who have a mutual interest in stemming Sunni militants' lightning advance across Iraq this week.

In a statement read at Friday prayers, Grand Ayatollah Ali Sistani called upon all able-bodied Iraqi Shiite men to fight, saying they were duty-bound “to bear arms and fight terrorists” in defense of their people and holy places. Those who died in the fight would be “honored” as martyrs, he said.

Within an hour volunteers were gathering and being sent to Samarra, 70 miles north of Baghdad, where the Islamic State of Iraq and the Levant (ISIS) clashed with the Iraqi Army this week. In February 2006, the destruction of the Askari shrine there ignited a vicious sectarian war in Iraq that left tens of thousands dead.

Even then, Ayatollah Sistani never issued such a call to arms. But the disintegration of Iraqi Army units, exposing the ineptitude of Prime Minister Nouri al-Maliki’s Shiite-first leadership and deepening Iraq’s ethnic and religious divisions, makes this instance more grave for Iraqi Shiites.

The blitzkrieg by ISIS – backed by angry Sunni tribes disenfranchised by Mr. Maliki’s rule, and even Saddam-era officers – has upended political calculations from Washington to Tehran, where the presidents of both the United States and Iran separately pledged support.

“They’ve all been playing with fire, and now they have a bonfire. And I don’t see any one of them having enough water to put it out,” says Rouzbeh Parsi, an Iran-Iraq specialist now at Lund University in Sweden.

The result may be a “bizarre” situation in which the US and Iran find themselves on the same side, due to the acuteness of the crisis.

"If they want to save Iraq they are going to have to work together,” says Mr. Parsi.

“The US is not going to put any troops on the ground. When it comes to whatever troops are on the ground, it’s going to be the Iranians who are going to help out, to make sure [Iraqis] stay where they are supposed to stay, and shoot in the right direction,” he says. But the US will likely help with air strikes and intelligence, so “one way or another, [the US and Iran] are going to have to get in touch," Parsi adds.

“We can work with the Americans to end the insurgency in the Middle East,” a senior Iranian official told Reuters. “We are very influential in Iraq, Syria and many other countries.”

US, Iran make moves

The Pentagon today announced it was moving an aircraft carrier into the Persian Gulf, although President Barack Obama said the US would not take military action without an Iraqi plan for national reconciliation. The State Department said today the US “is not talking to the Iranians about Iraq."

Meanwhile, Maj. Gen. Qassem Soleimani, commander of the elite Qods Force branch of Iran’s Revolutionary Guard, was reportedly in Baghdad in recent days, coordinating with Iraqi officials. The Wall Street Journal reported that Iran has already deployed two Qods Force units to Iraq, citing Iranian security sources. Iranian officials publicly denied those claims. Other reports from Iran suggest that Iranian drones have also been deployed.

ISIS spokesman Abu Mohamed al-Adnani said the Sunni militants would march on the “filth-ridden” shrine city of Karbala, and the “polytheism” city of Najaf – both among the holiest sites in the world for Shiite believers – in order to “settle our differences” with Maliki.

The threat to Samarra and anti-Shiite rhetoric from ISIS comes as Shiites mark the birthday of the Shiite Messiah, the 12th Imam called Mahdi.

Prior to Sistani’s call to arms today, influential Shiite cleric Moqtada al-Sadr – whose Mahdi Army militia once fought US forces before it was disbanded – said a Shiite “peace brigade” should be formed to defend holy shrines.

Questionable unity

The ISIS advance may have unified long-bickering Shiite factions in Iraq, many of them unhappy with Maliki’s rule. But it has also driven deeper the divisions between Shiites, disgruntled Sunni Arab tribes, and ethnic Kurds. With Sunni militias moving south toward Baghdad, and Shiite militias and volunteers moving north, it is not clear how this transformative week in Iraq will play out.

“Unifying the Shiites is easy, that’s not the art,” says Parsi. “The art lies is unifying Iraq. And so far they haven’t managed to produce a politician who is capable of that.”

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Militias “can come into the fray and perhaps mobilize people to fight. But that was never a difficulty in Iraq – there was always someone to raise a flag…and get a couple people to tag along with their Kalashnikovs,” says Parsi.

“The question now is, can any of these people present a picture that looks like co-existence afterwards?”