Early reports of the caravan of immigrants from Central America in October led President Trump to tweet about calling up the military and closing the border in response. Kristine Huskey, a professor at the University of Arizona James E. Rogers College of Law, says the president can legally close ports of entry, but it's less likely he could stage troops at the border to act as law enforcement.
“There’s an 1878 act that prohibits basically our U.S. military troops from engaging in domestic law enforcement in the U.S.," Huskey said. "By law enforcement function we mean things like apprehension, arrest, search and seizure, interrogation and detention." According to Huskey, the military can act in a support role to law enforcement and federal agent along the border.