I don't know what Trump is trying to achieve with this.
It's not so much about what Trump is trying to achieve, more what the establishment figures behind him want. That's right I said it, Trump's anti-establishment credentials are a charade; he's got deep ties to the establishment class in the US, and has done for a while, likely his whole life.
To me, it's a psychological thing. W Bush was the "idiot president", Obama the "aspirational president" and Trump is the "wild-man president", but all these are just semi-characters, as near enough all public speaking is scripted for US Presidents. I know Trump is genuinely shooting from the hip on stage, but let's not forget he's the real professional
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So what's the psychology behind the wall? Well, it's a political acknowledgement/manifestation of the issue that this is all leading towards: the borderless nature of the internet. There's no way that Trump is stupid enough to think this is gonna work, just remember that these campaigning politicians are playing the role of the modern-day preacher man. Anything he says that looks like he's using it to get a big crowd reaction should be looked at that way, he wants the modern day equivalent of the rapt congregation extolling "praise be" and so on.
To sum it up, not sure what the specific political card to be played is, but this proposal isn't serious. Trump knows that. It's all to do with demonstrating to the public what the 21st century really means, and presumably, how that is intended to be stopped. Or, just maybe, to demonstrate the opposite: that it can't be stopped.