Unless the entirety of III Corps, XVIII Corps, and 1st Army is mobilizing to head to the border, it is a moot point.
Also throw into that mix Marines and a carrier battle group parked off each coast of mexico.
5-10,000 people on foot? no weapons (possibly a few).
How many will get across where they try to cross the border? Disperced, it could happen. In even small groups, very doubtful success.