The American-led coalition is now launching airstrikes to support Iranian-backed militias and Iraqi troops in the key city of Tikrit, a U.S. official tells The Daily Beast. Those forces had previously kicked off their operation to reclaim Saddam Hussein’s hometown from the self-proclaimed Islamic State without informing the U.S. military. But when that campaign stalled, they turned to American airpower.

A U.S. defense official told The Daily Beast that the U.S. conducted 15 strikes beginning at 3:15 p.m. EDT, targeting weapons storage facilities, barracks and roadblocks set up by ISIS. All the targets were “pre-planned,” the official said, suggesting an operation planned before the Iraqis formally sought approval.

Two U.S. officials told The Daily Beast that upon receiving a formal request from the Iraqi government, the attacks began almost instantly. “The coalition has demonstrated the ability to rapidly respond to conditions on the ground,” one of the officials noted.

As the situation deteriorated in Tikrit for Iraqi forces, the U.S. military quietly laid the groundwork for expanding coalition airstrikes into the central Iraq city, the U.S. officials told The Daily Beast. They moved assets and began crafting military plans for striking ISIS targets entrenched there.

By Wednesday, Iraq time, an Associated Press reporter in Tikrit reported hearing warplanes overhead—and multiple explosions below.

In an interview with Reuters that was published early Wednesday, Iraqi President Fouad Massoum said airstrikes would begin soon.

“The Iraqi government along with residents of the area wanted an active contribution from the international coalition,” Massoum told Reuters.

An American airstrike campaign in Tikrit marks an important shift in the ISIS war. Iraqi officials did not engage their American counterparts before they launched the offensive on Tikrit on March 1, with Iranian generals and tanks by their side. And the American military has long insisted that it wouldn’t coordinate too closely with the Iranians, even as both forces fight a common enemy in Iraq: ISIS.

The Tikrit campaign was launched with a patchwork force of 20,000 Shiite militiamen, 3,000 Iraqi troops, and a bevy of Iranian troops, tanks, weapons, and missile strikes. And in the early days of the campaign, General Qassem Suleiman, leader of the Iranian Quds force, was on the ground in Tikrit.

As forces quickly made their way to Tikrit in those opening moments, there were hopes that Iraq would get its biggest win against ISIS within days. Iraqi officials boasted that they were moving surprisingly fast onto the city, which is ISIS’s biggest stronghold in Iraq’s Saladin province. But in the last week, the campaign has stalled as Iraqi forces and militiamen confronted a city laden with explosives. In addition, a key bridge over the Tigris River leading to the city was destroyed, complicating Iraqi troop and militia movement into Tikrit.

As Iraqi troop and militia deaths rose, more Iraqi politicans suggested they needed U.S. help. On Saturday, after receiving an official request from the Iraqi government, the U.S. and the coalition began providing videos and other intelligence learned from surveillance flights to the Iraqi military, as The Wall Street Journal reported and Army Col. Steven Warren, a Pentagon spokesman, confirmed Wednesday.

So far, the U.S. had conducted 5,314 strikes against ISIS targets in Iraq and Syria through March 15, according to Pentagon statistics. U.S. officials told The Daily Beast that the heaviest resistance to expanding the American involvement to airstrikes will not be from the Obama administration—which has long shrugged off the idea of cooperating militarily with Iran—but from some Shiite militia leaders who have said they can reclaim the city without American help.

“The Shiite militia leaders have been saying ‘We don’t need American airstrikes,’ so they have been pushing back on this idea. So there is going to be an internal debate within the Iraqi state,” the adviser said.

In his interview with Reuters, Massoum acknowledged that friction, but said: “The Iraqi government alone decides and no other force decides,” referring to the Shiite militias.

The expansion of the U.S air war into Tikrit was met with mixed feelings inside the Pentagon. While some feared the implications of coming to the rescue of a failed Iranian-led effort, still others welcomed the opportunity to let both Iraq and Iran know that the war in ISIS cannot be won without U.S. help.

“If this leads to the Iranians forced to concede defeat, that would be a satisfactory outcome,” one defense official expanded to The Daily Beast.

Perhaps the only comparable campaign to a potential air campaign over Tikrit was the Iraqi offensive in the northern Iraqi city of Amerli. Iraqi forces, Kurdish forces —known as peshmerga — and Shiite militiamen broke ISIS grip over that city last fall, with the help of U.S. airstrikes.

Of course, the U.S.-led coalition was helping the Tikrit campaign—albeit inadvertently—even before it started sharing intellige nce. American airstrikes outside of the campaign prevented ISIS from moving in reinforcements to Tikrit. But coalition air attacks were noticeably absent from the Tikrit fighting itself. Instead, the strikes happened in the restive Sunni province of Anbar and the northern city of Mosul, Iraq’s second-largest city and the biggest city ISIS holds in that country.

Either way, the request for American airpower suggests the end of hopes that a ground force alone could defeat ISIS in Tikrit. Ahead could be a very different phase of the ISIS war.

UPDATE 4:31 P.M.: This story has been updated to note the Iraqi government’s formal request for airstrikes, and the beginning of American air operations over Tikrit.