U.S. Constitution, Article 4 Section 4:
"The United States shall guarantee to every State in this Union a Republican Form of Government,
1. The act of invading; the act of encroaching upon the rights or possessions of another; encroachment; trespass.
Hi, Travis. Good to see you back.
How about building 2 walls, with the space between having 20-foot tall steel plates pounded into the ground to discourage tunnelling?
My original idea was to salt the border with high-level nuclear waste, but that would be a really hard sell.