The White House has reportedly issued a memo that would empower U.S. military personnel along the border with Mexico, along with National Guardsmen operating there under federal authority, to directly engage in law enforcement activities and use lethal force if necessary. The decision to employ the military domestically in this way could violate the Posse Comitatus Act, which imposes major restrictions on such activities, but may still be legal through exemptions and other legislation. Military Times’ Tara Copp was first to report that President Donald Trump’s Administration had actually pursued this course of action, with White House Chief of Staff John Kelly said to have signed a “cabinet order” authorizing the plan late on Nov. 20, 2018. CNN and other outlets had previously reported that Trump might issue an executive order to this effect.

The order gives “Department of Defense military personnel” the authority to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary” to support Customs and Border Protection’s mission along the United States’ southern border, according to Military Times. These tasks could include “a show or use of force (including lethal force, where necessary), crowd control, temporary detention, and cursory search.” In October 2018, U.S. Northern Command began deploying thousands of personnel from all of the services to provide what is called “Defense Support to Civil Authorities,” or DSCA, along the boundary with Mexico. These missions, which you can read about in more depth here, typically include a host of support functions, including surveillance, construction projects, vehicle maintenance, and static site security.

US Army A convoy of vehicles belonging to the US Army's 87th Engineer Company prepares to move out in Arizona in support of the border support mission in November 2018.

The ostensible reason for this new operation, initially nicknamed Faithful Patriot, was to help bolster security on the border in advance of the arrival of a large group of migrants from Central and South America. These individuals, who are fleeing violence and high crime in their home countries, had very publicly stated their intention to make their way to the United States and seek asylum. There is no assurance that those claims would be successful and that they would be allowed to remain in the country. At present, there are approximately 5,900 U.S. military personnel, together with some 2,100 members of the National Guard acting in a federal capacity, conducting DSCA missions on the border. The emplacement of concertina wire along portions of the border, including in areas that already have physical barriers, has been, by far, the most visible of those activities. “Short term, get the obstacles in,” is how Secretary of Defense James Mattis described the goals of the operation during a visit with troops deployed on the border on Nov. 14, 2018. “Longer term...it is somewhat to be determined.”

The long-term mission now appears to encompass the authority to conduct actual law enforcement activities, including detaining immigrants who cross illegally and employing potentially lethal force if necessary. U.S. Northern Command does have standing Concept of Operations Plans (CONPLAN) outlining how it conducts DSCA missions and how it would proceed with more active missions. CONPLAN 3501 covers DSCA tasks specifically, but notes the prohibition on troops taking an active law enforcement role “except as otherwise provided.” CONPLAN 3502, Civil Disturbance Operations, sets out the basic parameters for responding to the breakdown of law and order during a major domestic crisis.

NORTHCOM Relevant portions of CONPLAN 3501 covering support to law enforcement activities.

CBP Customs and Border Protection officers guard the San Ysidro border crossing during a temporary shutdown on Nov. 19, 2018, to install additional protective measures.