One soldier asked Mattis what are the short- and long-term plans for the military mission.
Mattis said the short-term objective is to get sufficient numbers of wire and other barriers in place along the border as requested by Customs and Border Protection. The longer-term objective, he said, is "somewhat to be determined."
"When you're in something like this," Mattis said, "it's dynamic, it's unpredictable. We'll have to see" what develops with the potential for Central American migrants to try to cross into the U.S. illegally.
In an interview with reporters traveling with him to Texas, Mattis took issue with assertions by critics that the deployment undertaken shortly before the Nov. 6 midterm elections was costing the military in preparedness for combat. He said military officers have told him that the deployment has been "very good training" because it amounts to a rehearsal for the kind of logistical demands — such as loading aircraft — that must be met in wartime.
"In terms of readiness, it's actually, I believe, so far improving our readiness for deployments," he said.
Mattis said the mission, which does not include performing law enforcement tasks, was reviewed by Department of Justice lawyers and deemed a legal undertaking. "It's obviously a moral and ethical mission to support our border patrolmen," he said.
In making the argument that the military has often been used on the U.S.-Mexico border, Mattis said it is in line with missions dating back to early in the 20th century. He noted that President Woodrow Wilson deployed tens of thousands of National Guard and active duty troops to the border in 1916 in response to Mexican military raids into the U.S.
"That's over a century ago, and the threat then was Pancho Villa's troops — revolutionary raiding across the border into the United States," he said.