The U.S. military is typically loathe to talk about future operations, but early this week a senior official laid out pretty detailed plans for an assault on Mosul for a roomful of reporters.

The somewhat unusual briefing outraged Sens. John McCain (R-Ariz.) and Lindsey Graham (R-S.C.) who complained Friday in a letter to President Obama that the "disclosures not only risk the success of our mission, but could also cost the lives of U.S., Iraqi, and coalition forces."

The two members of the Senate Armed Services Committee wanted to know whether the White House had signed off on the disclosures and demanded an investigation. "Those responsible have jeopardized our national security interests and must be held accountable," the lawmakers wrote.

So why did a senior military official spill the details of the war plan to rout the Islamic State from Mosul?

The answer: The Pentagon and Iraqi military commanders are hoping to avoid a massive, bloody battle in Iraq's second largest city. "We want Mosul to look a lot more like the liberation of Paris than Stalingrad or Fallujah," said a senior military official familiar with the planning. In other words, U.S. and Iraqi commanders are hoping that they can convince most of the Islamic State fighters to leave the city before the big battle.

So far it remains unclear whether the strategy will work. The senior military official who briefed reporters on Thursday said that the assault on Mosul would likely begin in April or May and include between 20,000-25,000 troops. American war planes, and possibly helicopters, will provide air support to the Iraqi forces on the ground.

Most military officials believe that there are more than 2,000 Islamic State fighters currently in Mosul. It's unclear how the outnumbered enemy will react to the news of the coming assault by Iraqi forces. For now, though, it appears that the Islamic State fighters are determined to dig in for a hard fight, said military officials.

White House spokesman Josh Earnest told reporters Friday that he didn't know that U.S. Central Command officials would be briefing reporters. But the news of the disclosures didn't seem to bother him too much.

"I guess if I did sign off on these background briefings, then I'd be accused of ...micro-managing the Department of Defense, and I certainly wouldn't want to be accused of doing that," Earnest said. "Right?"