I don't understand why a wall, sort of like the iron curtain except taller, can't be constructed to keep them out. There must be some design that is impervious to ladders and such. All you'd need is guard tower every mile or so with either cameras or men with night vision capabilities.
The DMZ in Korea does not keep all of the commie infiltrators out, but it sure slows 'em down considerable.
It's about a mile of no-man's land between two fences, which is a "free-fire" zone. Artillery can and does fire live rounds at random at all hours of the day and night in there, and anything that moves between those fences without prior authorization can be shot or bombed on the spot. Land mines?
Oh yeah; lots of 'em.
They say that no amount of military presence on our border can stop the illegal invaders from Mexico; well, that may be true, I suppose.
But like wrapping barbed wire around the bannister to keep Grannie from sliding down in the Morning...
It might not stop 'er, but it'll damnsure slow her down!