Unfortunately, whatever military force we put on the border is limited by law to monitoring, intelligence, construction, etc.
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Aka Posse Comitatus. However, should DJT declare we are being invaded, then all bets are off and the full use of military force or some sort of ROE can be used. Used once and wisely will send a message that the border is closed; the MSN will howl at the deaths of so many innocent immigrants - now invaders.
Actually there is precedent to an end-run around posse comitatus which can be used to federalize National Guard: Pres. Trump can cite the Insurrection Act - a pretty good fit for foreign nationals infiltrating and intentionally causing border chaos.
Then he'd be late to the party.
Many FReepers have been; rightly; calling them Border Invaders for years now.