Rosamond Press

“They’ve been using military metaphors and talking about invasions — in that sense, it’s not a surprise, but when you move from political rhetoric to actual policy, I don’t see a legal way to actually do it,” Meade said. “The U.S. Armed Forces are far too professional and risk-averse and rule-bound to accept some vague cowboy mission down there. That’s just not what they do.”

The Posse Comitatus Act, which passed after the Civil War to keep federal troops from policing the South, limits federal troops’ deployment on U.S. soil and forbids using them to enforce domestic laws.

The president can deploy troops if there’s an insurrection or invasion on U.S. soil. Congress later gave the military the ability to provide equipment and personnel for certain drug enforcement operations. The Coast Guard is exempt from the act’s restrictions.

The president can deploy troops if there’s an insurrection or invasion on U.S. soil. Congress later gave the military the ability to provide equipment and personnel for certain drug enforcement operations. The Coast Guard is exempt from the act’s restrictions.

The estimated 1,200 migrants in the caravan would not count as an invasion, Meade said. Border Patrol has also apprehended significantly fewer unauthorized crossers since arrests peaked in 2000, according to data from the agency, which Meade said would also argue against sending military.

Can the National Guard move in?

Governors can deploy the National Guard in their states to respond to emergencies. Previous presidents have, with governor signoff, used the National Guard to supplement Border Patrol agents during hiring pushes.

Yet as California has taken a firm stance as a force of opposition to the Trump administration’s immigration policies, Meade speculated that California’s governor would be unlikely to give Trump access to the California National Guard. The governor’s office did not respond to a request for comment in time for publication.

During George W. Bush’s administration, in 2006, under Operation Jump Start, about 6,000 members of the National Guard from across the border region were sent to help Border Patrol agents. Because they weren’t allowed to work as law enforcement, they couldn’t arrest anyone.

Instead, they worked in support roles like monitoring cameras and building and repairing roads and fencing. The operation’s goal was to allow more Border Patrol agents to work in enforcement roles at the border and give the agency time to hire and train 6,000 new recruits by 2008, according to Customs and Border Protection.

Between fiscal 2006 and 2008, Border Patrol staffing grew from 12,349 to 17,499, an increase of 5,150, according to data from CBP.