The Iraqi military launched a major campaign to take back a key city from the self-proclaimed Islamic State over the weekend—a move that caught the U.S. “by surprise,” in the words of one American government official.

The U.S.-led coalition forces that have conducted seven months of airstrikes on Iraq’s behalf did not participate in the attack, defense officials told The Daily Beast, and the American military has no plans to chip in.

Instead, embedded Iranian advisers and Iranian-backed Shiite militias are taking part in the offensive on the largely Sunni town, raising the prospect that the fight to beat back ISIS could become a sectarian war.

The news is the latest indication that not all is well with the American effort against the terror group. On Friday, U.S. defense officials told The Daily Beast that a planned offensive against the ISIS stronghold of Mosul had been indefinitely postponed. Over the weekend, an American-backed rebel group in Syria announced that it was dissolving, and joining an Islamist faction.

Then there was the unexpected battle for Tikrit. Over the weekend, a reported 30,000 troops and militiamen—mostly Shiites —stormed the Sunni dominated city of Tikrit, former Iraqi President Saddam Hussein’s hometown and the symbolic birthplace of his three decades of repressive practices against the majority Shiite population.

U.S. officials were largely left in the dark of the planning and timing of the operation, defense officials said. The Pentagon said Monday it was not conducting airstrikes in support of the Tikrit offensive because the Iraqi government did not ask for such help.

The U.S. had seen the prospect of strikes in Tikrit for a while but the timing and nature of the attack “caught us by surprise,” one government official explained to The Daily Beast.

Perhaps the most telling evidence that the coalition was not involved in planning for a potential Tikrit campaign could be found in the coalition’s air campaign against ISIS. It has been weeks since coalition forces struck Tikrit. Meanwhile, U.S. warplanes have conducted nearly daily strikes in the Mosul area in preparation for an eventual military campaign.

The depth of Iranian involvement and the dearth of U.S. engagement in the battle for Tikrit suggested the coalition-led campaign did little to weaken Iranian influence on Iraqi security. Two U.S. defense officials told The Daily Beast that Iranian troops were firing Iranian artillery “in the vicinity of” the Iraqi military campaign.

And there were several reports that Major General Qassem Soleimani, the shadowy commander of the Iranian Revolutionary Guard’s overseas operation arm, is also on the ground near Tikrit.

The Iraqi decision to cut out the U.S.-led coalition turned the war against ISIS in Iraq into a dual track approach—one carried out by the U.S.-led coalition another directed by the Iranians. Each has its own military strategy.

“As long as the Iranians perceive that what we’re doing comports with their objectives—which is eliminating ISIL—we’re on a parallel course there,” Director of National Intelligence James Clapper told an audience at the Council on Foreign Relations in New York on Monday.

U.S. officials have said taking back cities from ISIS will take months, in part, to minimize civilian casualties and destruction of communities. In addition, U.S. advisers are training Iraqi brigades that include at least some semblance of Sunni representation.

But the Iranian-led approach the clearing of Tikrit is largely sectarian—with Shiite militias reviled and feared by Sunni residents. Rather than a deliberate military campaign, the forces appear prepared to pound Tikrit, hard. And perhaps because of that, there is no need for an air campaign.

There are already fears that the Iraqi effort, backed by their Iranian supporters, will decimate parts of the city, defense officials said. Such actions would have great symbolic effect and make increasingly unlikely the mending of sectarian tensions between the minority Sunnis and their Shiite-dominated government.

An adviser to the U.S. government tasked with monitoring and engaging with Iraqi officials told The Daily Beast, “I think there is a great deal of joy about going into the city that fought Iran for a decade,” referring to Tikrit’s role in the seven-year war against Iran. “Imagine Qassem Soleimani is in Tikrit directing Iraqi forces in the destruction of the symbol of the former regime and the Sunni resistance,” the adviser added.

Because of that, Pentagon officials are watching carefully how the Iraqi forces carry out their campaign to rid Tikrit of ISIS, though they concede the signs are not promising.

“This is a real bellwether,” said a second defense official. “If this becomes a sectarian battle, we will shift to simply counter terrorism, and away from training Iraqi forces. And the coalition will come apart.”

ISIS has been in Tikrit since June, just after it stormed Iraq’s second-largest city, Mosul, and made it the local capital of its caliphate. Iraqi forces had tried at least three times to wrest control of the city from ISIS, without much success. In one instance, ISIS released photos claiming to have killed 1,700 troops in and around Tikrit.

But never has Iraq sought to seize the city with so many troops and with so much help from Iran, making this campaign it best chance of reclaiming the city.

“I don’t know if this operation is going to succeed. But I know the size and configuration has been successful in the past,” said Sinan Adnan, a research associate at the Washington, D.C.-based Institute for the Study of War.

The forces face significant challenges, however: namely a well-established ISIS that will fight to retain Tikrit, its last grip in Saladin province.

“It is going to be a fight,” the adviser said.

Should the Iranian-backed forces win back Tikrit, it would mark a major psychological victory and lay the groundwork for an eventual campaign to Mosul, as Tikrit sits on the supply line route between Baghdad and the north, Adnan said.

And for some at the Pentagon, it raised fears that its train-and-assist program would be applied to the destruction of Sunni towns, not the elimination of the ISIS threat.

“This is an erosion of Iraqi independence and sovereignty that endangers our interests in the region,” the adviser said. “And there won’t be much of Tikrit left.”

As of January 30, the U.S. and coalition had spent $1.5 billion on the campaign against ISIS, striking roughly 2,500 targets. U.S. Central Command, which is leading the American effort, referred all questions about the Tikrit operation to the Iraqi government.