Skepticism about the U.S. and Iraqi military plans for the next phase of the ISIS war begins inside the Pentagon.

Less than 24 hours after U.S. military officials publicly detailed their plans for a spring offensive on ISIS-held Mosul, Iraq’s second largest city, many within the Pentagon privately questioned whether that timetable was plausible. They said that they were dubious that their partners in the Iraqi military—the troops supposed to lead the offensive—would be capable of conducting such a campaign by then.

“I really doubt it is going to happen that soon,” said one military officer who, like several others, served in Iraq between 2003-2011 and spoke on condition of anonymity. “And if it does, it will take months.”

The largely Shiite troops of the Iraqi army are unlikely to risk their lives to win back a Sunni dominated city, several U.S. military officers told The Daily Beast on Friday. Indeed, when ISIS stormed the city last June, Iraqi forces walked away, leading the U.S. and 60 other nations to form a coalition against the terror group.

Even if the Iraqi troops do stand up and fight the self-proclaimed Islamic State, having a Shiite force move in and potentially ravage a major Sunni city in a bid to save it could have adverse affects on the Sunnis in Iraq and broader Sunni Arab world. Sectarian tensions, particularly in Iraq, run that deep.

“I cannot believe that Shiites would fight for Mosul,” one officer who served in the restive Sunni province of Anbar during the Iraq War told The Daily Beast.

So far, there is no evidence of a strong Sunni-majority Iraqi Army brigade, and U.S. Central Command has said it will take at least eight brigades to win back the city.

In the absence of such a force, it is not clear that the Sunni-dominated city would welcome those troops. Many Sunnis feel betrayed by Iraq’s Shiite-dominated central government, and all indications are that Shiite militias are becoming increasingly powerful in Iraq as the war against ISIS drags on, only confirming Sunni residents fears.

Critics inside the U.S. defense community note that the battle for Mosul could be much harder than the coalition’s fights so far to reclaim cities from ISIS. It took 112 days for a capable Kurdish ground force and U.S.-led air campaign to win back the small northern Syria city of Kobani.

In many ways, Kobani was one of the easier fights the coalition could’ve picked. ISIS wasn’t particularly well-entrenched there. And the city had been largely abandoned when the ISIS attempted to take it. In other words: the coalition’s airstrikes could be relatively indiscriminate without risk of civilian casualties.

Mosul, on the other hand, is arguably the capital of ISIS’s Islamic caliphate in Iraq. ISIS’s fighters have moved in and out of the city for the last decade, first as members of al Qaeda in Iraq.

“They will fight for Mosul. This is not like Kobani, which was peripheral,” one U.S. military official told the Daily Beast.

Mosul is a heavily populated city, where ISIS forces have already built trenches and barriers. ISIS reportedly maintains security forces, collects taxes, and controls government buildings there. Where Kobani was aspirational for the group, Mosul is key.

“They will fight to the last drop of blood defending Mosul, and for them this battle could define their existence. Losing Mosul means a final defeat for Islamic State in Iraq,” a retired army general living in Mosul told Reuters last month.

Before the Syrian civil war in 2011, Kobani had roughly 45,000 people. Around that time, there were roughly 1.5 million souls in Mosul. Kobani was all but destroyed in the aftermath of the ground and air campaign. The broader Sunni Arab world would likely not accept the same fate for a city as important as Mosul.

“The outrage in the Arab world if you do to Mosul what you did to Kobani, primarily with Shiite and Kurdish forces, would create a firestorm. The integrity of the city needs to be protected,” said Derek Harvey, director of the University of South Florida Global Initiative for Civil Society and Conflict, and a former advisor to former Iraq commanders Gens. Dave Petraeus and Raymond Odierno.

Just last month, Kurdish Prime Minister Nechirvan Barzani said he did not think the Iraqi army would be ready before the fall to take back Mosul. In an interview with Reuters, he said the two best Iraqi divisions are currently protecting the capital and there were not sufficient sources to replace them should a Mosul offensive began.

Asked by Reuters last month about plans touted by Iraqi and U.S. officials for an offensive by June on Mosul, Barzani said: “March, definitely not. June, also I doubt it.”

On Thursday, a U.S. CENTCOM official briefed reporters and telegraphed the upcoming Mosul operation. The official, who would not be named as a condition of the briefing, said an Iraqi force of as many as 25,000 troops could launch an offensive as early as April or May. The forces, which would be made up, in part, of six Iraqi army brigades and three Kurdish peshmerga units would take on an ISIS force of as many as 2,000, the official said.

The official called it an Iraqi plan that the U.S. will assist with. But he would not say how the American forces would help.

Defenders of the war plan announcement noted that ISIS has been anticipating a counteroffensive since June 10, when its forces moved in, faced relatively little counterattack, claimed the city, and seized much of the Iraqi army’s U.S.-provided weapons and equipment, including tanks and Humvees.

The longer the U.S. and Iraqi forces wait, the more entrenched ISIS becomes in Mosul.

“The stronger the defenses get to be, the stronger their caliphate becomes in Mosul,” the defense official said.

There was little cost to telegraphing the operation, this official added. ISIS has already dug trenches and bolstered their forces. Announcing that a counter offensive is imminent does not change what ISIS already is doing. In the last month, U.S. and coalition air strikes have increasingly focused on Mosul. There have been airstrikes every day in the last week, striking at least 19 targets, according to coalition press releases. There were just six airstrikes during the first week of the year.

Defenders of CENTCOM were quick to dismiss concerns of sending a Shiite dominated force to Mosul, calling the military an “Iraqi one, not a Shiite Iraqi Army.”

Mosul has been perilous for U.S. and Iraqi forces from the earliest days of the U.S. invasion. In July 2003, Saddam Hussein’s sons, Uday and Qusay, were discovered hiding in Mosul. The city quickly became a hotbed for al Qaeda in Iraq, which would eventually become ISIS. The U.S. launched its first campaign to take back the city from Sunni extremists in 2004, and then again in 2008, along with Iraqi forces. The fighting lasted for several months, on and off.

The CENTCOM official told reporters Thursday that if the Iraqi Army was not ready, they would move the date back. But Harvey said there already were costs to announcing the operation.

“The worst thing you could is telegraph it, go after it and fail,” Harvey said. And neither [the peshmerga nor the Iraqi security forces] is good at this kind of fighting.”