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According to wikipedia, back in 1930 and 1940, almost 90% of USA's population were white americans.
So your challenge is to make that percentage be the same in 2013, in a USA with the same borders, and see how that changes the evolution of American society and economy.
The percentage can go down inbetween, but no less than 75%.
Also, the rest of the population can be African, Asian, Hispanic, or Native-American in any proportion that combined is not over 10% of total population.