>"Posse Comitatus Act”
Migrants arriving at the border will now see a sizable U.S. military presence even though their mission will be largely a support role.
That's because the military is bound by the Posse Comitatus Act, a 19th-century federal law that restricts military participation in law enforcement activities. Unless Congress specifically authorizes it, military personnel can't have direct contact with civilians, including immigrants
https://www.militarytimes.com/news/your-military/2018/10/31/federal-law-limits-what-us-troops-deployed-at-the-border-can-do/