Posse Comatitus
US Military cannot be used for police action within US borders.
ReSearch and read to readily answer such a simple question -
https://www.militarytimes.com/news/your-military/2018/10/31/federal-law-limits-what-us-troops-deployed-at-the-border-can-do/
Depends entirely on how the action is described. The President could declare Martial Law on the border then your objection would be moot.
Ok Thanks. Seems strange to limit a military in such a manner. If I read it right it is to protect a citizenry against an unruly army. But surely regarding your southern border it would be protecting your citizenry against unruly non citizens.
Military can be used in defense against a declared invasion - the cops can mop up the survivors ... or not.