In the spring of 2020, Mark T. Esper, the defense secretary, was alarmed to learn of an idea under discussion at a top military command and at the Department of Homeland Security to send as many as 250,000 troops — more than half the active U.S. Army, and a sixth of all American forces — to the southern border in what would have been the largest use of the military inside the United States since the Civil War.

With the coronavirus pandemic raging, Stephen Miller, the architect of Mr. Trump’s immigration agenda, had urged the Homeland Security Department to develop a plan for the number of troops that would be needed to seal the entire 2,000-mile border with Mexico. It is not clear whether it was officials in homeland security or the Pentagon who concluded that a quarter of a million troops would be required…

Mr. Esper declined to comment. But people familiar with his conversations, who would speak about them only on condition of anonymity, said he was enraged by Mr. Miller’s plan. In addition, homeland security officials had bypassed his office by taking the idea directly to military officials at Northern Command. Mr. Esper also believed that deploying so many troops to the border would undermine American military readiness around the world, officials said.

After a brief but contentious confrontation with Mr. Miller in the Oval Office, Mr. Esper ended consideration of the idea at the Pentagon.