What pray tell is more central to the fundamental duties of the military than protecting our borders from an invasion?
Is this an actual, honest-to-God invasion by a hostile power? If so, then invade Mexico, get rid of the offending government, and create one more suited to our interests.
If it's just keeping people out of the country who shouldn't be here, then use the duly sworn civilian law enforcement agencies to do the job.
Make up your mind. Either do it right, or not at all.
Why can't we secure the borders of our own country?
After all, they have murdered us HERE and will continue to murder us HERE.