Appoint a military governor like Patton, MacArthur or Black Jack Pershing. Secure their narrower southern border. Round up and shoot their drug cartel gang leaders.
Within a generation, this part of Mexico would be a place that people wanted to come instead of flee. If the other 75% of Mexico still doesn't get the idea, then rinse and repeat.
The would just about cover the cost of the quarter of the population of mexico currently squatting in the US, at US taxpayer expense.
The only people who would object (vehemently in fact) to this idea would be the US cronies, and their buddy mexican cronies, both of which profit handsomely off of the current border chaos status quo.