Which is impossible. Even if you go full tilt Nazi/Stalinist/Stassi, there’s always the water. Are you going to build a wall along every inch of shore line and the entire Canadian border as well as the Rio Grande?
Unless you're Jesus Christ, they're not going to simply run across the water—and if Jesus is going to make his Second Coming that way, we have a lot more problems on our hands than any defense system will ever achieve.
It took Diana Nyad five tries to swim across the strait between Florida and Cuba, and that's only 90 miles. Waterways are a much slower and more easily patrolled form of entry than land. For that, use the Coast Guard.