Being Texans, though, many of the people who live between the fence and the river say they don’t count exclusively on the government to keep them safe. One recent afternoon, a retired refrigerator repairman wearing camouflage clothing strolled south of the wall with a handgun in a holster on his hip. Mr. Monsees was even more heavily armed as he spoke outside his home while sitting in his pickup truck.

“This is a single-shot .410, and that’s a single-shot .22, and I keep the pistol between the seats,” he said.

In another buffer-zone community along the river about 50 miles northwest of Brownsville, on a muddy dirt road outside the town of Donna, Arturo Munoz, 72, sat in the carport of his house just south of the border fence, sipping a Bud Light. His dog Max lay at his feet.

“It’s just about the same as on the north side,” said Mr. Munoz, a retired welder and Vietnam veteran, though he conceded that he has no mail service, no landline telephone and no cable television.

He said he bought the property nearly 15 years ago because he grew up nearby and feels a connection to the area. When it came time to build the border fence, he said, the government made him an offer he could refuse, and he did. He stayed put, and the wall went up just north of his white-brick home, right behind a line of trees in his back yard.

“It’s a lot safer than in town,” he said. “I lived in town. I was renting, and got broke into three times.”

He described riding his bicycle near his house one day and seeing a few migrants walking along. He said he waved to them and pedaled on. They had crossed the river on their journey to America, and had found it. Sort of.