Curious example... since the Great Wall didn't work. Neither did the Maginot Line. Both of those were also a mere fraction of our 22,000 miles of border.
It's easy to say "seal" the borders, and certainly more effort at border patrol should be (and believe it or not, has been) done, but it is useless without a general hardening of the country as a whole. It is perhaps even more important to make it difficult for them to move and communicate if they do get in. Because they will get in nomatter how high the wall.
Not true. Israel shut down terrorist infiltration from Gaza with a secuity fence.