You are wrong. The Mexican border is an international border and the President can militarize it any time he wants to. He can use regular army troops also.
“The Mexican border is an international border and the President can militarize it any time he wants to....”
Our borders are first and foremost our perimeter. Anything beyond the perimeter is potentially hostile. Infantry 101 says when you set in, first secure your perimeter. Our perimeters are not secure.