The US could have used the money to set up “safe zones” in industrial zones within Mexico (and LatAm) to ensure worker safety and the people would have stayed there instead of coming north. They mainly come for safety / to work.

Before people start saying “but it’s their problem”, the US has a horrible track record of destroying and destabilizing their nations: https://en.m.wikipedia.org/wiki/Operation_Condor , along with the C I A nurturing the cartels in the 1980s onward to create a supply chain so they could profit: https://oig.justice.gov/sites/default/files/archive/special/9712/ch01p1.htm

At this stage it’s evident there’s a corrupt faction at the top (intel agencies with no oversight) of people who DON’T work for the public, yet take public tax dollars, and the public still has to foot the bill for the shit resulting from the destabilization of other countries.