The order, issued in response to President Trump's call for using troops to stem illegal immigration, specifies that the National Guard will assist the Department of Homeland Security along the border in the Southwest but not perform law enforcement missions and will be armed only when necessary for self-defense.
"Tonight National Guard troops are deploying to support border security missions along the U.S. southwest border," Mattis and Homeland Security Secretary Kirstjen Nielsen said in a joint statement late Friday.
The troops will be under state control, but the cost of deploying them will be paid out of the Defense Department budget, according to the order, which was released by the Pentagon. The order did not say where they will be deployed along the 1,954-mile border.
The order did not specify which Guard units would be used. But state officials in Arizona and Texas announced Friday that they would mobilize reservists for the border mission, and Nevada has volunteered to send National Guard soldiers to assist the mission.