WASHINGTON, -- Defense Secretary James N. Mattis last night authorized up to 4,000 National Guard troops to deploy to the U.S. border with Mexico to support the Department of Homeland Security border security mission there.National Guard troops began deploying after the announcement of the authorization.In a joint statement, Mattis and DHS Secretary Kirstjen M. Nielsen said DHS worked closely with border-state governors and identified security vulnerabilities the National Guard could address."We appreciate the governors' support and are dedicated to working with them to secure the national borders," Mattis and Nielsen said.DoD and DHS are committed to "using every lever of power to support the men and women of law enforcement defending our nation's sovereignty and protecting the American people," they said, adding that they will continue to work with the governors to deploy the necessary resources until the nation's borders are secure.President Donald J. Trump authorized the National Guard, with the affected governors' approval, to enhance its support to U.S. Customs and Border Protection along the southern U.S. border. In a presidential memorandum April 4, he said a "drastic surge of illegal activity on the southern border" is threatening national security.NATIONAL GUARD TROOPS DEPLOY IMMEDIATELYMattis authorized the use of Title 32 duty status and DoD funds for up to 4,000 National Guard personnel to support DHS's southern border security mission while under the command and control of their respective governors through Sept. 30. Title 32 status is full-time duty other than inactive duty performed by a member of the National Guard. It allows the governor, with the approval of the president or the secretary of defense, to order a member to duty for operational homeland defense activities.The troops will not perform law enforcement activities or interact with migrants or other individuals detained by DHS without approval from Mattis, according to the DoD memo. Arming will be limited to circumstances that might require self-defense, the memo says.Air Force Gen. Joseph L. Lengyel, the chief of the National Guard Bureau, said in a Twitter post last night that up to 500 troops were deploying immediately in support of the mission. "Always Ready, Always There! Moving up to 500 #NationalGuard troops immediately on the SW border security mission. Vehicles, equipment & helicopters on the way tonight," the general tweeted.The National Guard's efforts will include aviation, engineering, surveillance, communications, vehicle maintenance and logistical support, chief Pentagon spokesperson Dana W. White said in a news briefing April 5."We look forward to continuing our partnership with DHS to ensure the defense of our nation and the security of the American people," White told reporters.