In the middle of Tuesday’s rambling speech at Marine Corps Base Quantico in Virginia, President Trump told hundreds of the country’s military commanders his latest thinking on where they should next set their sights.

Not Poland, or Romania, or Estonia or Denmark, all NATO allies where Russian drones have in the past month violated airspace in a challenge to the alliance’s borders.

The president chose San Francisco. Chicago. New York. Los Angeles.

“We’re going to straighten that out one by one, and this is going to be a major part for some of the people in this room,” Mr. Trump told the generals, admirals and enlisted leaders, referring to what he has described as crime-filled urban hellscapes.

“It’s a war from within,” he said.

In that moment, the president again pitted himself against the wishes of the country’s founding fathers, historians and former military leaders say.

Mr. Trump’s suggestion that “we should use some of these dangerous cities as training grounds for our military” is in tension with a core principle that the country’s armed services have long sought to preserve — that the military should be nonpartisan.

This principle, with its deep roots in American democratic traditions, is meant to ensure that the standing army initially feared by the country’s founding fathers serves the nation as a whole, and not one political party or leader.