President Trump wouldn’t need to make an emergency declaration to have the Pentagon build at least some of his border wall, a top Defense Department official told Congress Tuesday, though he said no final decisions have been made.

Defense Undersecretary for Policy John Rood told Congress that under at least one provision of federal law, the Defense Department already has powers to build roads and fences on the border to block drug-smuggling corridors in support of homeland security.

And Mr. Rood assured Rep. Mo Brooks, Alabama Republican, it wouldn’t take an emergency declaration to trigger.

“No,” he told the congressman flatly, adding that they are prepared to follow through if Mr. Trump orders them. “If we judge it to be a lawful order, yes sir. And I assume it would be.”

Mr. Rood was testifying to the House Armed Services Committee, which called its first hearing under the new Democratic majority to examine the Trump administration’s commitment of troops to the border and to look ahead to future moves such as tapping the Pentagon for fence construction.

Mr. Rood and Vice Admiral Michael Gilday, director of operations at the Joint Chiefs of Staff, said 2,300 active-duty troops are at the border, down from 5,900 at the peak of the deployment late last year. About 2,000 additional National Guard troops are there.

The defense officials said the decision to deploy the troops was made in response to the migrant caravans, which demanded quick action to harden the border in November and December.

Troops laid 70 miles of razor wire, Admiral Gilday said.

Mr. Rood said the troops are still needed with the government tracking three new caravans, one of them with an estimated 12,000 members, making their way north.

Democratic lawmakers questioned the tradeoffs of deploying troops to the border, given other competing demands on the military.

“We’ve got other needs in the world,” said committee Chairman Adam Smith, Washington Democrat. “This is not primary to our mission and if we start down this road … and say DOD is all about the border, where does it stop?”

The defense officials said they can handle the current deployment, though Admiral Gilday said the one area where the military is “a bit pinched” in providing support is in helicopters, which are highly sought in war theaters overseas.

He said, though, that while some of the deployment is a drain, in other respects it’s beneficial because it gives troops a chance to train and build readiness.

Mr. Trump has repeatedly looked to the military to step in and solve border problems with two separate deployments of troops.

Now he’s eyeing using the Pentagon to build his border wall, should Congress fail to come up with the $5.7 billion he has requested for some 230 miles of new and replacement barriers.

The White House has signaled he could claim emergency powers to have the Defense Department lead the construction.

The Pentagon officials said they have done some “prudent pre-planning,” but declined to talk about which options they have studied or to predict costs or tradeoffs should Mr. Trump follow through.

And they said what powers they have to build will depend on which part of the law the president would choose to flex.

“The president has a range of different authorities that he can invoke. And each of those authorities are tied to laws that have specific requirements that would dictate the determination, the calculus the department would go through, to determine whether or not you could justify using those funds to build a barrier,” Admiral Gilday said.

“We would have to show DOD benefits — whether the argument would be we no longer have to deploy 5,900 people to the wall. We have to take a look at that more deeply to see if that’s a justifiable cause-effect,” he said.

Even without wall-building, some active-duty troops have been authorized to be on the border through Sept. 30.

But their mission will change from assistance at the ports of entry to helping Border Patrol agents police the miles between the crossings.

Admiral Gilday said the military is preparing to assist with surveillance, using vehicles with cameras and sensors to detect looming incursions and to direct agents where to go.

“We bring a skill set with respect to detection and monitoring that I think is going to be very valuable” for Customs and Border Protection, the admiral said.

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