Activating the military would violate The Posse Comitatus Act. It limits the powers of the federal government in using federal military personnel to enforce domestic policies within the United States.
National Guard I think has to requested by the individual states.
That really puts us in a bind. If we cannot stop this enemy column, then there will be many more and larger and nothing else will matter. The enemies without and within will simply overwhelm by numbers those of us who still want to defend the USA.
We declare a State of Emergency and the PC Act is null. If that don't strike your fancy then you seize a 50 mile section of Northern Mexico and declare it to be a DMZ. Cross it and die.
...and no, I'm not joking. Mexico has waged war on US for decades. Be glad I'm not in charge. The International Community would be mourning the loss of Mexico City.