WASHINGTON — Defense Secretary Jim Mattis said on Wednesday that American troops stationed at the southwest border would not be armed with guns to confront incoming migrants, despite a White House directive that aims to protect border security officials by pairing them with military forces.
The White House memo seeks to expand the mission of the troops at the border to also include duties such as crowd control and temporary detention. But Mr. Mattis said it left the final decision on what American soldiers and Marines could do — and could not — to the defense secretary.
Mr. Mattis mentioned one possible instance in which troops might act: defending a border agent who was hit by a rock, and detaining the migrant who threw it. But asked whether such a situation might call for the American soldier to be armed, the defense secretary unequivocally said it would not.
“No,” he said. “Not with a firearm.”
Sent to the Pentagon on Tuesday night, the memo was signed by John F. Kelly, the White House chief of staff, not President Trump. It is the latest in a series of White House directives to prod active-duty troops toward a more aggressive role at the border against an approaching caravan of thousands of migrants from Central America.
The Pentagon has resisted some of those efforts by raising the specter of the Posse Comitatus Act, a law that dates back to the Reconstruction era, which bars American forces from engaging in law enforcement activities within the borders of the United States.
American troops may be outfitted with riot gear for the border mission, Defense Department officials said.
Still, “we are not doing law enforcement,” Mr. Mattis insisted. “There is no arrest authority under Posse Comitatus for U.S. federal troops.”
The White House push for a more aggressive military role was undercut by an internal Homeland Security Department document, dated Nov. 17, that described the probability of American border guards facing violence at the southwest border as “minimal.” The border intelligence analysis contrasted with White House statements stoking fear about migrants overrunning official ports of entry and attacking border guards to enter the United States.
Troops on the border were already allowed to defend themselves with force and protect border agents. The White House memo restated that authority, but also opened the door to sending Marines and soldiers to accompany and defend border agents who might come under attack, administration officials said.
Asked how he planned to prevent a repeat of a 1997 episode in which a Marine deployed to the border shot and killed a teenager who was herding his family’s goats, Mr. Mattis dismissed the question. “I’m not going to dignify that,” he said. “They’re not even carrying guns, for Christ’s sake.”
Since Mr. Trump announced that he wanted the American military to repel the migrant caravan that he has characterized as an “invasion,” the White House and the Pentagon have presented contrasting images of what the deployed troops can do.
Mr. Trump described a “wall of people” to stop the caravan in an ABC News interview and called for up to 15,000 troops to defend the border; the Pentagon reluctantly sent 5,900. Mr. Trump spoke of telling the military to respond to migrants throwing rocks as if they were rifles; Pentagon leaders quickly walked back those comments.
Mr. Mattis said that under special circumstances, he could direct troops to detain migrants. But he said such a detention would be measured in “minutes, not even hours.” He also said he would order detentions only after a specific request from the Homeland Security Department.
No decision has been made on whether to expand the border mission beyond Dec. 15, when it is scheduled to conclude, Mr. Mattis said.
More than 3,200 migrants from the caravan have reached the border city of Tijuana, Mexico, where they might have to wait for months if they decide to seek legal entry. Officials estimate that the number could reach more than 10,000. An additional 3,000 migrants are in the border city of Mexicali, about 100 miles east of Tijuana, according to reports from Customs and Border Protection.