Well, I can think of one possible solution to your issue of US Military being unable to "arrest" invaders under Posse Commitatus (which I opine needs to be revisited by the SCOTUS). But I very much doubt that it will ever happen.
Don't you ever wonder about that statement that these "Zeta" guys are all "deserters" from the Mexican military?
I've got to wonder if that's just a cover for a covert tactical military deployment under the full, albeit secretive auspices of the FOX Regime for US border penetration and creating internal chaos in preparation for a more significant invasion. Since they've been helping Jihaddists get into the US, don't we suppose that Islamic Jihad might reciprocate by assisting Mexico in it's ambitions?
Didn't Osamma BL consider drugs to be another weapon to be used against Europe and America? Did all those poppy fields in Afghanistan have anything to do with AlQuida setting up HQ there?
Are we seeing a certain connection here?
What we might need is a serious dust-up between Zetas and Federal Officers - which they seem to be reasonably adept at avoiding for some reason.
If Zeta casualties or captives could be directly linked to Mexican military, or that they had any kind of military support, we could then declare war on Mexico in response to their preemptive invasion of soverign US territory.
Then any "illeagals" - at least those caught under arms - could be considered "Enemy combattants"/POWs.
But you're right; that's a pipe dream.
The corruption probably goes a lot higher up the US administrative ladder than we would even care to consider.
What's more likely is that terrorists will blackmail the US with a nuke into ceding a good part of the SW US back to Mexico.
Then our undefended border will be West of Louisiana and Arkansas, and somewhere South of Nebraska.
Won't that be fun!?
Then extension of the Caliphate throughout "restored" Mexico will be deferred to another day; they have bigger fish to fry first.