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With a POD anytime after 1900, turn Mexico into a stable, prosperous, developed industrial nation with one of the highest standard of living in the world and whose close political, economic & industrial ties to USA make it viewed as “USA’s hat” (USA’s beard? ). Meanwhile Canada is to become a developing nation with high crime rates, endemic political corruption and gang warfare. By 2000s large areas of the country are controlled by organized crime cartels and many Canadians enter USA illegally in search of a better life only to end up working low wage medial jobs.
Now if you find this too challenging, you can use a 19th century POD if you want as long as it doesn’t butterfly away OTL territory of Mexico and Canada (so, for example, you can’t butterfly away the Mexican-American war and have Mexico keep the territory it lost in the Treaty of Guadalupe Hidalgo).