Ancient Romans knew how to handle the border towards " the barbarians" (latin: limes) otherwise they wouldn’t have lasted for over 2000 years in one form or the other.
People think that the Roman border was a physical separation wall protected by fearless legions hermetically sealing one of the greatest civilizations from the bearded trouser wearing savages (note: wearing trousers was considered the ultimate depravity by the Romans, alongside human sacrifice, cannibalism and eating while sitting)
The truth was very different - Roman legions did provide security on the border, but their major tasks were collecting tax and custom duties from the lively trade between “barbarians” and Rome. That’s why marvels of Roman defensive engineering such as the Hadrian wall had gates for barbarian ox carts to facilitate and control (tax) cross-border trade.
Romans knew that in order to increase the security of their frontier, they needed to increase the economic well being of barbarian tribes just across their border. These barbarian lands provided a market for Roman products (Germanic barbarian tribes were buying massive quantities of Roman wine at exorbitant prices, leading to the spread of the new invention called barrels) while the barbarians provided agricultural products and luxury items such as tin and amber and provided what you’d call migrant artisan workers for Roman garrison towns on the frontier, not to mention manpower for the Roman legions.
The major source of livelihood for Germanic tribes close to the Roman border were, beside subsistence agriculture, funds sent by their tribesmen in Roman service, working either as artisans, traders or soldiers.
What’s the lesson for the modern-day Roman Empire called the USA? Help Mexico improve it’s economic situation, The better off Mexico is, the less people will come to the US. Despite @therajraj’s racist fantasies US-Mexico relations are not a zero sum game and migration is a combination of push and pull factors affecting population transfers.
To illustrate this point, in the 70ies and 80ies African migrants constantly tried to enter apartheid South Africa, despite it’s abhorrent record of treating Africans not to mention extensive border fortifications and walls. It’s just that the pull factors (South African need for semi-skilled manual laborers) as well as push (horrible state of their home economies such as Nigeria and Zaire/DRC) outweighed the migrant’s abhorrence towards racial laws.
Studies made in the EU show that immigrants are highly susceptible to economic changes both in their host and home countries, the prime example being Poland. As soon as Polish economy started growing at higher than-EU average rates, Poles started coming back to Poland proper.
Of course, another option is for Trump to destroy the American economy and the US in general, thereby reducing it’s attractiveness as a destination for potential immigrants. Maybe there’s a method in his madness after all…