Donald Trump orders
Pentagon officials are already meeting about what to do if they get illegal orders from Donald Trump. Photo by Chip Somodevilla/Getty Images

America's military leaders are engaging in informal discussions about how they would respond if President-elect Donald Trump issued orders to use active-duty U.S. troops against civilians and aid his mass deportation plans, according to a report.

Trump talked on the campaign trail about using military forces to quell protests in cities and said that one of his first actions upon entering the White House again would be to begin mass deportations of illegal immigrants.

During his campaign he characterized protesters as "enemies from within" who may have to be "handled" by the National Guard or military. He later also included his Democratic critics, such as former House Speaker Nancy Pelosi and Rep. Adam Schiff, as the "enemy from within" and somehow more "dangerous" than China or Russia.

Of particular concern is Trump issuing an unlawful order, especially if his political appointees in the Pentagon fail to push back.

"Troops are compelled by law to disobey unlawful orders," a defense official told CNN.

"But the question is what happens then – do we see resignations from senior military leaders? Or would they view that as abandoning their people?" the official said.

How the military's top brass would react to those orders is part of ongoing discussions at the Defense Department.

"We are all preparing and planning for the worst-case scenario, but the reality is that we don't know how this is going to play out yet," another defense official told CNN.

U.S. military leaders are having informal discussions about how they will respond if President-elect Donald Trump issues controversial orders when he enters the White House. Alex Wong/Getty Images

Trump had a tense relationship with the military officials in his first administration, including with former Marine Gen. John Kelly, who served as the White House chief of staff.

Kelly accused Trump of being a "fascist" and claimed that he said Adolf Hitler "did some good things, too."

Retired Army Gen. Mark Milley, the former chairman of the Joint Chiefs of Staff, said Trump was "fascist to the core."

Trump responded by calling the military leaders "woke" and "weak."

"The relationship between the White House and the DoD was really, really bad, and so ... I know it's top of mind for how they're going to select the folks that they put in DoD this time around," a former defense official told CNN.

American troops could be deployed in cities to help carry out Trump's plan for mass deportations, a former defense official told CNN.

Local law enforcement departments "don't have the manpower, they don't have the helicopters, the trucks, the expeditionary capabilities" that the military brings, the official said.

But a decision to send active-duty troops into American cities must be looked at seriously.

"You can never water that down, you can never say with a straight face that it's not a big deal. It is a big deal," the official said. "But it's the only way to address issues at scale."

"We have the outside enemy, and then we have the enemy from within and the enemy from within in my opinion is more dangerous than China, Russia." He said the National Guard or military would have to deal with these Americans.

He later specifically named Pelosi and Schiff as "enemies from within."