This was the map in 1794:
In 1794, Mexico included the territory of what would be the Louisiana Purchase. This Louisiana Territory had belonged to France until 1762, when, doing badly in the Seven Years' War, France signed it over to Spain. Then in 1800, Napoleon, doing quite well militarily, induced the King of Spain to cede it back to France, then sold it to Jefferson in 1803. Then for 45 more years the US kept taking more territory from Mexico. Apparently many people aren't nearly as linguistically sensitive as I am, so that place-names from Texas to California to Colorado, From San Antonio to Albuquerque to Los Angeles, don't constantly scream to them that they were places in Mexico before they became places in the US.
After we stopped taking land from them, we started taking water: the appropriately-named All-American Canal, described as the largest canal in the world, diverts water from the Colorado River just north of the Mexico border, and leads it into Imperial County, California, widely touted as the largest irrigated area in the world, where many of the crops, I would imagine, are harvested by undocumented workers from Mexico, although it's hard to get exact statistics for such things.
All of that water diverted by the All-American Canal into Imperial County has left less in the Colorado River, which continues south into Mexico. Most maps still show the river as continuing south of the US-Mexico border for about 50 miles and emptying into the Gulf of California, but the truth is that now, for at least part of the year, the Colorado goes completely dry before it reaches the Gulf. Boats stranded where there used to be a Colorado River in Mexico silently testify about how things have changed:
Of course, it's not solely the All-American Canal, or even mostly the canal, which is responsible for this transformation of a region of Mexico: upstream, up north, quite a few dams take a lot of water out of the river before it gets to the canal, the biggest of which is the Hoover Dam, which keeps a lot of lawns green in and around Las Vegas.
Then there's the war on drugs and the wars against Mexican drug cartels, and the constant depiction of those cartels, in US media and entertainment, as being unimaginably ruthless and bloodthirsty. Prohibition of alcohol, as we learned in the 1920's, benefited above all gangsters, and corrupt politicians who co-operated with those gangsters, and got a lot of people killed. And of course, the prohibition of marijuana and cocaine and meth is doing exactly the same thing.
Diversion of water and the drug wars. And how many more major examples could be named? I came up with these in 10 minutes.
Fewer people would want to escape from Mexico if the US screwed Mexico over less.