(CNSNews.com) - On the way to McAllen, Texas on Wednesday, Defense Secretary Gen. James Mattis said he was going to the southern border because "I want to see how the troops are doing."
One thing he doesn't expect them to be doing "at this time" is having direct contact with the thousands of foreigners heading this way.
"Border security is part of national security," Mattis told reporters traveling with him. "Our units are in a position to enable the Border Patrol's law enforcement operations. We determined that the mission was absolutely legal, and this was also reviewed by Department of Justice lawyers."
Mattis called it a "moral and ethical mission" to support the Border Patrol.
"The Department of Defense missions do not involve military personnel at this time directly participating in any law enforcement. Law enforcement is left in the hands of the Customs and Border Police, who have the statutory authority to carry that out. At the present, I do not anticipate military personnel coming into direct contact with migrants."
The troops are "very highly disciplined," he said, and the soldiers laying barbed wire are not armed. "They don't need their weapons, the engineers, to lay the barbed wire." Mattis said armed MPs are providing force protection for the military engineers.
So far, he said, "We have provided planning and engineering support, equipment and transportation support and temporary housing for Customs and Border Patrol personnel and their medical teams."
5,900 Troops are there now; number may rise
"As of Nov. 13, about 5,900 troops are deployed to support the Border Patrol," Mattis said. He added that the number will fluctuate as troops rotate in and out.
"I anticipate right now it could go around 7,000 although I prefer to say that we'll just keep you updated daily with how many troops are there."
Mattis could not give an estimated cost for the support mission at the border, but in terms of readiness, he said it's actually helping, because it's good training for deployments to actual war zones.
A reporter asked Mattis why the name of the mission changed from "Operation Faithful Patriot" to "Operation Border Support."
"When the name of the mission first came in, I had given instructions, I do not want to put this mission in some arcane military terms. If what we're doing is laying wire, don't talk about implementing a barrier plan...I want to talk to the American people because this is a highly politically visual issue, and I want you tell them what we are doing.
"I want you to tell them we are operating in support of Customs or of Border Police. Do not say we are supporting a federal agency. Tell them what we're doing."
Mattis said he told his people to "quit using military terms...that are subject to misinterpretation."
"I said talk in terms that people understand. It's their country, it's their border."
Mattis also told reporters the military mission is not unprecedented.
"There's nothing new under the sun," he said, noting that President Wilson deployed the army to the Southwest border over a century ago to stop Pancho Villa's cross-border raids.
Mattis also noted that four different administrations of both political parties, including those of Clinton, George W. Bush, Obama and now Trump -- all have sent troops to the border to assist law enforcement. (The longest period of U.S. military support at the border happened under Obama, he said.)
Homeland Security Secretary Kirstjen Nielsen requested active-duty military support in addition to the National Guard "due to an unprecedented situation with multiple large-scale caravans en route to the Southwest border," Mattis said.
One of the caravans overran the checkpoint between Mexico and Guatemala, clashing with Mexican police.
Television footage that aired on Wednesday showed some illegal immigrants sitting on top of a border fence in California, taunting law enforcement.