This picture taken 26 December 2011 shows the Pentagon building in Washington, DC.  The Pentagon, which is the headquarters of the United States Department of Defense (DOD), is the world's largest office building by floor area, with about 6,500,000 sq ft (600,000 m2), of which 3,700,000 sq ft (340,000 m2) are used as offices.  Approximately 23,000 military and civilian employees and about 3,000 non-defense support personnel work in the Pentagon. AFP PHOTO (Photo credit should read STAFF/AFP/Getty Images)
STAFF/AFP/Getty Images
This picture taken 26 December 2011 shows the Pentagon building in Washington, DC. The Pentagon, which is the headquarters of the United States Department of Defense (DOD), is the world's largest office building by floor area, with about 6,500,000 sq ft (600,000 m2), of which 3,700,000 sq ft (340,000 m2) are used as offices. Approximately 23,000 military and civilian employees and about 3,000 non-defense support personnel work in the Pentagon. AFP PHOTO (Photo credit should read STAFF/AFP/Getty Images)
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While President Donald Trump has been relatively silent on the threat posed by what he had termed an “invasion” of migrants heading toward the southern border prior to the midterm elections, Defense Secretary James Mattis traveled to Texas on Wednesday with a simple message for US troops deployed to the area: Ignore the news and focus on the mission at hand.

He also made it clear that their mission will not involve fending off a hoard of dangerous migrants as Trump insisted in his pre-election rhetoric.

Accompanied by Secretary of Homeland Security Kirstjen Nielsen, Mattis appeared to take the lead when addressing troops at Donna Base Camp, using the opportunity to encourage those present to block out any outside noise related to politics, and he emphasized the importance of serving as “confidence builders” for Border Patrol officers.

“Now, there’s all sorts of stuff in the news, and that sort of thing. You just concentrate on what your company commander, your battalion commander tells you. Because if you read all that stuff, you know, you’ll go nuts,” Mattis said. “You know what your mission is here. You’ve had to deploy on short notice to a nontraditional location and do your jobs. So you focus on doing that.”

“We were asked by the secretary, due to the number of people coming this way, to back them up. What does that mean? It means that her people do all the work, but we’re standing behind them as a confidence builder,” he added, referring to the request from the Department of Homeland Security to provide support.

While touring the base camp, Mattis and Nielsen saw the living conditions of the troops and were given a briefing on the camp’s laundry service.

They also spoke to several service members about their deployment experience thus far, including one woman who told Mattis that her husband was serving overseas and their young child was being cared for by the grandparents.

A stark contrast

The visit by Mattis and Nielsen stands in stark contrast to Trump’s relative silence in recent days on the migrants traveling to the US from Central America, after characterizing their approach as an “invasion” prior to the midterm elections.

Since November 6, Election Day, Trump hasn’t mentioned the so-called caravan in a tweet. He has used the word “border” a single time – in a tweet on Friday in which he included a link to a “Presidential Proclamation Addressing Mass Migration Through the Southern Border of the United States” that said, essentially, that he was trying to push people entering the country illegally to specified ports of entry.

On Wednesday, Mattis sought to distance the US military from the criticism that the President used the deployment to rev up his base prior to the election, emphasizing that the border mission is not unique in a historical context.

“There’s nothing new under the sun,” he said.

“I think many of you are aware that President Wilson 100 years ago, a little over a 100 years ago, deployed the US Army to the Southwest border. That’s over a century ago. The threat then was Pancho Villa’s troops, a revolutionary raiding across the border into the United States, New Mexico, in 1916 and there’s a more recent history of DoD support on the border: It spans four administrations and both political parties,” Mattis added.

He defended the legitimacy of the mission, particularly the task of backing up Border Patrol officers ahead of the migrants’ arrival.

“We determined the missions as absolutely legal and this was also reviewed by Department of Justice lawyers. It’s obviously a moral and ethical mission to support our border patrolmen,” Mattis told reporters prior to visiting the base camp.

While the troops in Texas have largely been tasked with reinforcing border crossing points, largely with concertina wire, Mattis suggested that the deployment serves as a valuable training opportunity.

When one service member said they were planning a field exercise when they got the call to deploy to the border, Mattis laughed before saying: “We just gave you the best field training exercise possible.”

But when asked about the long and short term plans for the operation, Mattis was able to offer few details.

“Short term right now, get the obstacles in so that the border patrolmen can do what they’ve got to do … longer term, it’s somewhat to be determined,” he said before encouraging troops to “keep the faith in each other, listen to your NCOs and you do what your officers tell you.”

Nielsen under fire

While Mattis repeatedly made clear that the border operation is being led by DHS, Nielsen, for the most part, appeared to take on a secondary role as the two Cabinet secretaries toured the base camp and engaged with troops, seeming to take her cues from Mattis before chiming in.

When she did speak, Nielsen focused her on thanking the troops for their service and partnership in the border operation.

“We thank you for your partnership always. We greatly, greatly appreciate it,” she said. “We are right there with you, we know you’re with us and we greatly appreciate it. So thank you, each and every one of you.”

Nielsen’s trip to the border comes amid indications that she could potentially be out of a job soon.

Trump could ask Nielsen to resign in the coming days, multiple officials familiar with the matter predicted, describing the President’s continued frustration at her handling of his signature issue: immigration and border security.

Mattis has faced a steady wave of rumors in recent months suggesting he too could be replaced but his name has not been mentioned lately as reports circulate that the President has been eyeing potential replacements for several senior positions in his administration – both inside the West Wing and across the Cabinet.

CNN’s Chris Cillizza, Ryan Browne, Michael Conte and Barbara Starr contributed reporting