The great wall of China was primarily built to keep the Mongols out, a task it comprehensively failed to do. And do you know how the Mongols bypassed it? They just bribed the gatekeepers to open the doors. Simple as that. The Chinese government almost bankrupted their country building this immense wall, and then paid the guards peanuts.
As we have moved on in this discussion to consider, a great wall on the southern border will be useless without the political will to continually man it, and the sense of cultural identity and solidarity that makes the society that built it want to protect itself. The left has outflanked us. Their domination of academia and the media has left too many Americans questioning their very identity.