President-elect Donald Trump will soon inherit America’s wars against ISIS.

The U.S. is training and assisting Iraqi and Kurdish forces in the liberation of Mosul, the last major city in Iraq controlled by ISIS.

The operation is six weeks old.

Lieutenant General Stephen Townsend served in northern Iraq during the U.S. occupation 10 years ago, and now he’s back, as the commander of the U.S. coalition against ISIS.

A new generation of Americans in Iraq, this time aiding Iraqi troops who are doing most of the fighting on the front-line.

“This fight would challenge any army,” said Townsend. “This fight would challenge the United States Army.”

But then Townsend broke off our interview, because his men had ISIS in their sights.

There are now around 6,000 U.S. troops in Iraq, supporting over 40,000 Iraqi fighters in the battle for Mosul, an offensive criticized by Mr. Trump for its lack of secrecy.

“Whatever happened to the element of surprise?” he asked during a debate.

Is a battle that size something Townsend can keep a secret?

“No, I don’t think so….it’s hard to move 40,000 troops and all their tanks and trucks and artillery into position and it not be noticed by somebody,” he said.

Lieutenant General Stephen Townsend speaks with CBS News’ Holly Williams. CBS News

Mr. Trump’s stated solution to ISIS is: “Bomb the hell out of them.”

But at this command center -- where they launch airstrikes on ISIS -- every strike must be approved by senior officers, it can take minutes -- or several hours -- and sometimes they’re refused permission.

Captain Matt Lyles explained to CBS News why it is done this way.

“We need to get it right for us. We need to get it right for them,” he said.

They want to avoid killing civilians at all costs. He said they don’t want to destroy a house if they don’t need to.

American air power is playing a decisive role in beating back ISIS -- but the simple truth is that there are no easy solutions to ISIS, and the battle for Mosul will be long and deadly.