Neither the great wall of China, nor the other great walls, were built with the idea of keeping out invaders. What they were intended to do was create an obstacle to raiders, quickly riding in, robbing and looting, and fleeing back home before they could be confronted and killed. The walls just slowed them down.
And in this, they worked as intended.
So what will a US wall do? It will *not* stop “all” illegal crossings. But that is not its intent. It will just add an obstacle to crossing, which alone may *reduce* the numbers of illegals crossing.
Right now, illegals move north in corridors. Passable areas without terrain obstacles, and near sources of water and roads. If *just* those corridors are blocked, it may reduce the number of illegals by 60%.
This is *very* cost-effective. But putting fence over land where crossings don’t often happen reduces things even further, perhaps another 15%, taking away the motivation to build roads on the Mexico side to facilitate crossing.
But it costs more, and has less effect, so it is less cost-effective. But 75% fewer illegal aliens are crossing.
The next step has been proposed by Donald Trump. This is to *tax* the money earned in the US which is then wired to Mexico. So Mexico does, in fact, “pay for the wall”.
This is a very good idea, because in effect it lowers the wages of Mexican nationals working in the US, precisely to wire back to Mexico their earnings. This makes working in the US a less attractive proposition.
And it they think they can hand carry it out of the US to Mexico, it will attract attention as being drug proceeds.
The next step is a peculiar one. Non-Mexican foreigners, including terrorists, will pay a premium to get into the US. The way to stop this cold is to offer a *bounty* on them, payable (anonymously) to Mexicans who inform on non-Mexican crossers.
Oddly enough, it does not have to be a great sum of money, and only works if the US catches the illegal non-Mexican crosser. Say $100 for just an ordinary illegal crosser. $500 if they are a “person of interest”, and $1,000 if they are a wanted terrorist or spy.
This is a good enough deal that even the ‘mules’ who would help them across the border would turn around and sell them out. Otherwise, every foreign crosser would be observed by a thousand hungry eyes.
Making our border airtight, at least to non-Mexicans.
The “Trump Wall “ is a methaphorical construct. One element will be strong law enforcement on the American side. A second will be strong effort on thew Mexican side.
The deal might also include an airborne invasion of a cartel village wiping it all out.