Cipher (who I have on ignore but I saw this anyway) wrote:
" The armed forces cannot be used to enforce civilian law. "
That is true, but Posse Comitatus deals only with the military being used in "law enforcement" against American citizens.
I doubt it has ANY application whatsoever regarding a border invasion by aliens -- whether those aliens are dressed in uniform of if they are wearing civilian garb.
To go further, Posse Comitatus has no effect when it is the "National Guard" of the states doing the actions. I believe federal law specifically EXEMPTS NG troops from the restrictions of PC.
Quick solution:
President should mobilize all NG troops in America, and send them for border enforcement. That should hold up against any court challenge based on PC...