The U.S. military and the southern border
The last three U.S. presidents have deployed military personnel to the southern border region. The stated purpose is to help maintain border security. But there are a number of legal barriers preventing the armed forces from performing what are, at root, police functions.
The use of military forces to perform domestic functions — such as constructing barriers along the US-Mexico border — could pose fundamental legal, policy and administrative challenges.
“Military forces operating freely within civilian jurisdictions risk upsetting the constitutional balance between civil authority, the military, and the private sector,” the Army said this week in a newly updated manual.
Therefore, “Army leaders must ensure that even in a catastrophic event, Army support remains within the boundaries of constitutional principles, U.S. laws, DOD policies, and Army regulations,” the manual said. See Defense Support of Civil Authorities, Army Doctrine Publication 3-28, February 11, 2019.
As a practical matter, the Army publication said, “Commanders should begin by viewing each domestic operational environment as an assortment of civil authorities, each with primacy in its jurisdiction.”
Fair enough. But what about proposals for using Defense Department funds to help build a physical wall along the border? The Congressional Research Service offers some insights. A key finding? Presidents Bush and Obama used national emergency declarations to "carry out otherwise unauthorized construction with military construction funds."
The entire CRS report is worth reviewing to get a sense both of the legal issues involved with the border, as well as a history of the U.S. Army's activities along the Mexican border.