Mr. Trump returned to the same section of the border that he visited in March 2018 to inspect prototypes of his new wall. This area has also been a flash point for another hotly disputed policy enacted by the administration. The policy forcing migrants seeking asylum to remain in Mexico for the duration of their case began at the San Ysidro port of entry this year, forcing hundreds of migrant families to wait in shelters in Tijuana.

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The policy has since been extended farther east to more dangerous Mexican cities like Nuevo Laredo, where migrants have been subjected to kidnappings and violence. More than 40,000 migrants have been returned to Mexico under the policy, which the administration says is needed to weed out fraudulent asylum claims and ease the burden on a detention system that experienced extensive overcrowding this year.

The president was joined by Kevin K. McAleenan, the acting secretary of homeland security, and other top officials who are also still serving in an acting capacity, underscoring the extensive turnover at the department as the president has repeatedly fired officials who disappointed him in cracking down on immigration, including its general counsel, who was dismissed just this week. “I wasn’t happy with the job they were doing,” Mr. Trump said, but added that he would announce permanent appointments soon.

Mr. Trump said he still expected to get nearly all of the 500 miles completed by the time he stands for re-election next year and said more was not needed because of natural barriers along stretches of the border. While Mexico is not paying for the wall, he noted that it is paying for its soldiers to stop migrants in a more aggressive campaign begun in recent months under pressure from Mr. Trump.

The characteristically money-conscious president repeatedly noted that this was the most expensive possible variant, saying he had originally wanted a simple and less costly concrete barrier but was convinced by Border Patrol professionals that it would be better to have a slatted barrier that they could see through to observe any potential threat on the other side. “That’s the Rolls-Royce version,” he said in a chagrined tone.

Mr. Trump said the steel slats would be wired — the Army general accompanying him declined to elaborate — and added that they would absorb heat so that they would be too scalding for potential migrants to even touch, much less scale. “You can fry an egg on that wall,” he said.

Mark Morgan, the president’s acting commissioner of Customs and Border Protection, called the new barriers “a game changer” that has reduced illegal crossings.