> Have nanotech magic-growth fences been invented and I didn't notice?

The border was fairly tight for most of US history. Don't pretend this is a technology problem. This is a corruption and lack of will problem. It could be solved in short order given the will. As I alluded to, Ike responded to a growing problem and effectively dealt with it in the space of about three months. It remained solved pretty much until the early 70s.