President's Authority to Use the National Guard or the Armed Forces to Secure the Border [April 19, 2018] [open pdf - 310KB]
"President Trump's recent announcement and memorandum to the Attorney General and the Secretaries of Defense and Homeland Security regarding the possibility of deploying the military to help guard the U.S.Mexican border against aliens entering the country unlawfully as well as to stop the flow of drug and gang activity raise questions regarding his statutory or constitutional authority to do so. The answer may depend on which troops are sent and the role they will play. This sidebar addresses the legal authorities for the President to use the National Guard or the armed forces to assist in securing the southwestern border."
CRS Legal Sidebar, LSB10121
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html