Video of the firefight, obtained exclusively by CBS News, was recorded by a camera mounted on a U.S. soldier's helmet.
American big guns arrive. Attack helicopters and Bradley fighting vehicles.
Mosul has become "insurgent central" in Iraq - al-Qaeda's base of money and foreign fighters.
In recent weeks, much of Iraq - especially Baghdad - is quieter, safer. Insurgents have cut and run.
Not in Mosul. Five-thousand U.S. troops sweep a scared city of almost 2 million scared people. And five U.S. troops within recent days.
Cptn. Hunter Bowers's Lightning Troop patrols the city's West Side. Its worst side.
"This city is just so unpredictable now," Strassmann said.
"Yes, sir," Bowers said. "It's very unpredictable. One area can be fine one day. And the next it can be the worst area you've ever been in."
Fighting could last for months.
Bowers and this Iraqi Army commander now see them surfacing here.
"So right now, it is a capital for terrorists right now. So I think there will be a final battle very soon," a soldier told Strassmann.
The potential threat could be anywhere
"They'll put a machine gun in here, flip the back seat around and fire out the back," TK said.
One U.S. soldier said, in Mosul, there are no good areas. Only degrees of bad.