WASHINGTON — In the battle to retake Saddam Hussein’s hometown, Tikrit, from the Islamic State, the United States and Iran have found a template for fighting the Sunni militancy in other parts of Iraq: American airstrikes and Iranian-backed ground assaults, with the Iraqi military serving as the go-between for two global adversaries that do not want to publicly acknowledge that they are working together.

The template, American officials said privately this week, could apply in particular to the looming battle to retake Mosul, Iraq’s second-largest city. Given that President Obama has ruled out the use of American ground troops in Iraq, and that the Iraqi military remains ill-trained for urban warfare, the fight for Mosul will require some combination of American air power, Iranian-backed Shiite militias, Iraqi military forces and perhaps Kurdish pesh merga fighters.

“You can see where this is going,” a senior Pentagon official said, speaking on the condition of anonymity. “Are the Iraqi forces ready yet? I would say no.”

That means, the official said, that any campaign to retake territory from the Islamic State may have to include the Shiite militias, no matter how much American officials proclaim otherwise.