Mattis pushed back on critics who have said that the 5,800-troop deployment — ordered by President Trump late last month ahead of the Nov. 6 midterm elections — was weighing on the military's combat readiness.
“In terms of readiness, it’s actually, I believe, so far improving our readiness for deployments,” Mattis told reporters en route to visit troops in Texas at the U.S.-Mexico border.
He added that he’s heard from military officers that the deployment has been “very good training,” as it serves as practice for wartime activities such as loading and unloading aircraft.
The former Marine Corps general also likened the deployment to former President Wilson’s 1916 border deployment to counter the late Mexican revolutionary Gen. Francisco “Pancho” Villa.
Wilson deployed tens of thousands of National Guard and active-duty troops to the border to prevent a Mexican military raid into the United States.
“That’s over a century ago, and the threat then was Pancho Villa’s troops — revolutionary raiding across the border into the United States,” Mattis said.
He also pointed out that former Presidents Obama and George W. Bush had ordered National Guard troops to be used in border missions.
However, neither president ordered the missions shortly before an election.
Trump directed that the Pentagon deploy the troops in anticipation of a caravan of several thousand Central American migrants making their way through Mexico to the U.S.
Democrats and immigration rights advocates have accused Trump of using the deployment as a political stunt designed to stoke anti-immigrant fears ahead of the midterm elections. Critics have also said the deployment is detrimental to the U.S. military, calling it a waste of time and resources.
The Pentagon has been unable to provide a cost estimate for the deployment, but estimates from outside organizations puts the figure somewhere between $42 million and $110 million if Trump follows through with threats to send up to 15,000 service members.
Roughly 2,100 National Guard troops were already at the border as part of Operation Guardian Support, which began in April. That operation is estimated to cost $182 million, the Pentagon said in May.
Mattis would not provide an estimate of how much the current mission will cost, saying that preliminary figures are “not anywhere near right.”
He added, “very quickly we’ll know the real cost. So we’ll keep you posted as the real costs come in.”
Trump has said little about the border deployment since last week’s elections.
Mattis said that within 10 days the 5,800 service members at the border will have finished all tasks initially requested by U.S. Customs and Border Protection. Additional tasks, however, may be added, with the Pentagon and the Department of Homeland Security working those out.
Deployments are scheduled to last until Dec. 15, but Mattis did not say how soon the mission might end.
He also deflected questions on whether he thinks there is a security threat at the border that justifies the use of the active duty troops, deferring instead to Homeland Security Secretary Kirstjen NielsenKirstjen Michele NielsenUS to restart 'Remain in Mexico' program following court order Far-left bullies resort to harassing, shaming Kyrsten Sinema — it won't work Ex-Trump official: 'No. 1 national security threat I've ever seen' is GOP MORE, who visited the border with him.
In speaking with senior U.S. commanders and addressing troops, Mattis said their mission is to “back up” Customs and Border Protection. “Right now that’s our role and that’s all our role is,” he said.
“The eyes of the world right now — certainly all of the Americans — are on you,” Mattis said. He added that they are part of a “nontraditional” mission.
“We’re here because of the number of illegals who say they are going to illegally try to cross into our country,” he said.
Asked by a soldier what the long-term plans for the military mission are, Mattis said it is “somewhat to be determined.”
“When you’re in something like this it’s dynamic, it’s unpredictable," Mattis said. "We’ll have to see.”
He told reporters that the mission was reviewed by Justice Department lawyers and determined as legal.
Pentagon officials have said the U.S. troops will not directly interact with migrants approaching the U.S., and will instead assist Border Patrol staff to fill in gaps in physical barriers along the border.
“It’s obviously a moral and ethical mission to support our border patrolmen,” Mattis said.