The new “cabinet order” was signed by White House Chief of Staff John Kelly, not President Donald Trump. It allows “Department of Defense military personnel” to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary” to protect border agents, including “a show or use of force (including lethal force, where necessary), crowd control, temporary detention. and cursory search.”

There are approximately 5,900 active-duty troops and 2,100 National Guard forces deployed to the U.S.-Mexico border.

Some of those activities, including crowd control and detention, may run into potential conflict with the 1898 Posse Comitatus Act. If crossed, the erosion of the act’s limitations could represent a fundamental shift in the way the U.S. military is used, legal experts said.