I think a case can be made that at least some elements crossing the southern border are part of an organized military (or military-like) force that pose a grave danger to the country. Al Qaeda certainly did its bit while it was here. Iran has sleeper cells awaiting orders to strike. Hezbollah and other terrorist organizations, too. Even the drug cartels qualify for military interdiction by our armed forces, imo. Of course, not everyone should be greeted with a gun in his face. But, in order to be effective, everyone has to go through the same screening process and woe be to he who resists. I’d love to see the libs try to stop Trump from militarizing the border in that sense. In the courts or in their press. I don’t care. They would lose. Just my man-in-the-street opinion.
“I think a case can be made that at least some elements crossing the southern border are part of an organized military (or military-like) force that pose a grave danger to the country... terrorist organizations... drug cartels”
That case has been successfully made, and the Military (and Intelligence Community) has those missions.
There has been a long-standing counter-Narcotics mission, and standing Joint Task Force (JTF-Bravo) out of US SOUTHCOM, since 1983 (they famously participated in countering Pablo Escobar and the Cali Cartel). They still operate, and have received powerful additional impetus to do so against Mexican Cartels from Executive Orders issued by President Trump.
There is an annual budget line within the DoD budget for Counter-Narcotics (called section 284), that specifically authorizes its use for building border fencing, roads and lighting. That has provided rock solid legal basis for the President to spend billions of DoD dollars on the Wall, last year, and again this year - Congress has previously authorized it.
Anti-terrorism is also an accepted Military mission, and the post 9-11 Authorization for the USE of Military Force (AUMF) gives the Special Operations Command worldwide authority to pursue and engage terrorists.
Basically though, the Military does its thing outside our borders, and other elements of Government deal with internal Law Enforcement.