I can think of only one thing that will happen, another mexican-american war, with the border states declaring it, and being at the forefront of it. The feds would be powerless to do anything about it, or more likely, would side with the mexicans and kill their own citizens. Their actions as of late would suggest the latter scenario would be the most likely one. I would suggest for California, New Mexico, Arizona, and Texas to tell the feds to get f!@#?d, if they refuse to do the job they’re paid to do, then they’ll do it for them, and tell the feds they won’t get a single nickel from them until they live up to their Constitutional obligations. That’s how it works, if the feds won’t uphold their end of the bargain, then they are deemed useless, and the states should act accordingly.
Can you imagine the reaction of the American people if they saw our troups firing on Americans defending our border?