But in the last furious days before the midterm elections, photographs of a group of soldiers, clad in full military kit and flak jackets, were presented at Trump rallies and shown in the news media. Mr. Trump himself lauded the stringing up of “beautiful barbed wire” at a rally in Montana.

Still, there is political theater and there is real life.

Two days after the midterms, on Thursday, a platoon of Army engineers in Hidalgo, Tex., who were stretching bands of concertina wire on the American side of the Rio Grande had ditched the body armor. The decision to wear only their uniforms, canteens, gloves and helmets was simple: It was too hot to wear the armored vests, and the soldiers knew they didn’t need them. And some had already suffered heat exhaustion, just days into their new mission.

Roughly 15 miles away, some 500 troops — a medley of medical units, military police officers and engineers — were settled into a routine at Base Camp Donna. It was named after the adjoining Texas town, which Border Patrol agents believe is one of the most likely entry points into the United States for the migrant caravan, should it arrive.

Wedged between a four-lane highway and the American-Mexican border wall, the base is reminiscent of those found in Afghanistan and Iraq in the early 2000s.