Why not send the National Guard?The military is going down there as a logistics support group. Border patrol agents, even some desk bound ones will be in the field enforcing US law.
The military has zero enforcement powers. They can't touch, detain, or do anything even if they come across the borden en masse other to defend themselves by law.
Obama sent the military to the border during his tenure as well. Trump is just doing it much faster and in a grander style.