Shiite cleric Moqtada al-Sadr speaks during a news conference in the holy Shiite city of Najaf on April 30. (Haidar Hamdani/AFP/Getty Images)

When Moqtada al-Sadr called on his followers to fight against the presence of U.S. troops in Iraq following the 2003 invasion, they obliged to devastating effect. His militiamen launched thousands of attacks on U.S. soldiers. Their notorious roadside bombs killed and maimed.

Now, he has turned his sights to Iraq’s corrupt halls of power, and the dramatic storming of Baghdad’s fortified Green Zone last week by his followers has shaken Iraq’s political system.

It has dramatically ramped up pressure on Prime Minister Haider al-Abadi to implement reforms he has promised.

But it has also caused a backlash against the rabble-rousing cleric — some politicians accuse him of holding the country to ransom and using his call for reform for the purpose of increasing his personal influence.

It has deepened divides in Iraq’s Shiite community, and new blast walls were erected on bridges around the Green Zone on Friday by a premier desperate to prove his mettle after the breach that undermined his credibility.

Iraqi protesters climb a pole with their national flags in Baghdad’s heavily fortified Green Zone on May 1. (Haidar Mohammed Ali/AFP/Getty Images)

Meanwhile, the cleric has riled Iranian-backed Shiite militias in the country, creating a new level of unpredictability to a country already so fragile.

“The big question is the potential for things to get ugly, and we’ve already seen glimpses of that,” said Hayder al-Khoei, an Iraq analyst at the London-based Chatham House think-tank.

Sadr has long been an agitator of the West, but he also has a thorny history with Iran, despite studying there and at points taking its money to fund his militias.

He espouses a strong Arab nationalist platform, and presents himself as a champion of the downtrodden impoverished Shiite masses, from where he draws most support.

Although Iraq is deeply divided along Sunni-Shiite lines, the tension within the Shiite community is driving the current crisis.

“Out, out Iran,” Sadr’s supporters had chanted as they gathered in the Green Zone, after ransacking parliament. They had also turned on Maj. Gen. Qasem Soleimani, the Iranian commander who has led efforts against the Islamic State, both in Iraq and Syria.

“America and Iran have split the cake of Iraq between them, and each is having their share,” said Salah Hassan, a 33-year-old fighter with Sadr’s militia, the Peace Brigades. “People say the Shiites are loyal to Iran, but we are only loyal to Sadr.”

As Sadr’s followers stormed the Green Zone, his militiamen took position around its perimeter. In response, the Khorasani Brigades, an Iranian proxy militia, and others closer to Tehran, deployed heavily on the streets of Baghdad. Other militias close to Iran pulled fighters back from the country’s conflict zones.

“It’s not outside the realms of possibility for them to turn their guns on each other,” Khoei said.

The Peace Brigades, a reformation of the notorious Mahdi Army that waged war against U.S. soldiers in 2004, is at the heart of Sadr’s power. Its members are die-hard and have clashed with the state in the past.

In 2008, then-Prime Minister Nouri al-Maliki launched the “Charge of the Knights” against his Mahdi Army in the city of Basra, in an attempt to wrest control of it from the militia. Then, it was Iran that eventually brokered a cease-fire agreement, and some hope that Iraq’s neighbor can step in to rein in Sadr once more.

“Iran has the same point of view as America, that Iraq should be stable. Stability is very important now,” said Salah Abdul Razzaq, a Shiite politician and former governor of Baghdad. Iran should play a mediation role, he said.

A day after his supporters packed up and left their short-lived sit-in, Sadr got on a plane and left for Iran.

His office said it was a personal trip to visit an Iranian shrine, despite coming at a time when thousands of Shiites stream to a shrine in Baghdad to pay their respects on the anniversary of the death of an 8th-century imam, a pilgrimage he missed.

The Iraqi rumor mill swirled into action, with some politicians speculating he was summoned by a furious Tehran.

“I think they are angry, maybe they blame him for what happened,” Abdul Razzaq said.

Five days later, he has still not returned, and before his departure, he had announced a two-month spiritual retreat. His supporters have remained stoic.

“Everyone should be comforted that even if Sadr stayed in Iran for half a century, he would not be subjected to any pressure or do anything against Iraq’s interests,” Dia al-Asadi, a politician with Sadr’s political bloc, said in a statement last week.

But he distanced Sadr from the anti-Iranian chants, saying the cleric was against them.

There have been other small signs of conciliation from Sadr’s camp, most notably on Friday, when a directive from the cleric’s office ordered protests to be held locally around mosques after prayers, rather than in a mass gathering in Baghdad.

The apparent downscale gives some breathing space for Abadi, who is desperately trying to steer Iraq out of its crisis. At the center of Sadr’s demands is a new technocratic government, a policy of Abadi himself. But despite embarking on a radical reform program last summer to cut government waste and corruption, Abadi’s success has been limited. Change involves upsetting a status quo that Iraq’s political elite currently benefits from, and few expect them to let go easily.

Abadi was already seen as a weak leader, and Sadr’s actions have undermined him further, with members of parliament incensed by the breach of their fortified inner sanctum.

As he attempts to regain control, and credibility, he has pledged to prevent another breach and fired the head of Green Zone security, who kissed Sadr’s hand as the cleric entered the area in March.

New blast walls have been erected on the bridges that cross the Tigris to the Green Zone.

But the Sadrists have promised to escalate and call for the prime minister’s resignation if a new technocratic government is not created. With parliament splintered, even getting enough members together to hold a session and vote will be a challenge. The Kurds are refusing to attend, while a group of rebel parliamentarians refuse to recognize the legitimacy of the speaker.

Just how long Sadr will wait before ratcheting up pressure remains to be seen.

“Iran will try to rein him in, but just how successful they’ll be is anyone’s guess,” Khoei said.

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